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Distribution and abundance of particulate matter
P'og Oceanog Vol Ig. pp 373--.~)[, lg87 Pnnted in Great Britain. All rights reser',ed
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S U M M E R U P W E L L I N G ON T H E S O U T H E A S T E R N C O N T I N E N T A L S H E L F
OF THE U.S.A. DURING 1981
Distribution and Abundance of Particulate Matter GL:STAV-ADOLF PAFFE'~H6FER" and THO~I~,S N. LEE" "Sktdaway ht~'tttate of Oceanography. Smlannalt. G,4 3140r~-1)687, I_' S.A. "'R~sensttel Schuol of Martne and ,4ttrtosp/tertc Sciettce. L'mver~ity qf ,%hamt. 4fi00 Rickenhacker Causeway. Mtami. FL 33140. U S.A,
ABSTRACT This paper reports
on the three dimensional coverage of Gulf Stream upwelling events to
determine the d i s t r i b u t i o n and abundance of p a r t i c u l a t e matter and chlorophyll a over time. This approach resulted in a description of size, shape and longevity of phytoplankton patches The strongest upwelling event occurred in June (patch I) July to e a rl y August (patches 2 and 3). frontal
eddy and northward winds occurred simultaneously at a time when the Gulf Stream
was in an onshore p o s i t i o n .
Patch I was i d e n t i f i e d by temperature anomalies and concentra-
t i o n of chlorophyll and p a r t i c u l a t e matter. when i t it
followed by weaker events during
These upwellings were observed when a Gulf Stream
was displaced by southward flow.
I t remained on the shelf from June I0 to July 7, Patch 2 was observed from July 13-29 af t er which
was advected northward at mid-shelf from our study area.
27 to August 16 when i t
stranded at the 20m isobath.
Patch 3 was studied from July
Patches 2 and 3 were distinguished
by high concentrations of chlorophyll a and p a r t i c u l a t e matter. of chlorophyll
a increased throughout the study period,
af t e r an i n i t i a l
increase.
In patch 2 concentrations
whereas in patch 3 i t
decreased
A l l three phytoplankton patches were induced by physical events.
Their shape was governed by bathymetry.
Their size and longevity was to a large extent
a function of the i n t e n s i t y and period of onshore and offshore advection.
CONTENTS 1.
Introduction
374
2.
Methodology
374
3.
Results
4.
3.1
Sequence of Major Intrusion Events
375 375
3.2
Individual Intrusion Events and Phytoplankton Patches
378
3.3
Vertical D i s t r i b u t i o n of Temperature, N i t r a t e , Chlorophyll a and P a r t i c l e Volume
386
Discussion
396
Acknowledgements
400 400
References 373
OIIA " P~,FFE'-HOFERand T
~ --~
I.
\
L_~
INTRODUCTION
One of the main goals of b i o l o g i c a l oceanographic research has been to determine the production and fate of organisms and t h e i r byproducts. of organisms
(school
of fishes,
To accomplish this goal a d i s t i n c t group
cohort of zooplankton) or a d i s t i n c t
observed over appropriate periods of time.
water mass must be
Pelagic populations may be tracked using drogues,
current meters or standard CTD-methods. Advection shear and d i f f u s i o n usually modify patches, the dimensions of which sometimes can be so large that they cannot be sampled at s u f f i c i e n t l y frequent
intervals
to
systematically determine production and f a t e .
One of the keys to
describing variables is to determine patch size and contents within adequately short intervals of time.
This has been accomplished only on rare occasions (SAVAGE and WIMPENNY, 1936).
The scarcity of information and the s i g n i f i c a n c e of the time f a c t o r was pointed out by HAURY, McGOWAN and WIEBE (1978)
who stated:
"Perhaps the most important time factor of pattern
is the tenq}oral persistence or l o n g e v i t y of i n d i v i d u a l patches on the micro- to coarse scale. Much has been inferred but l i t t l e
is known about this problem."
Patches of stranded upwelled
water on the southeastern shelf are e i t h e r on a coarse-scale of I to lOOkm maximum dimension (ATKINSON, O'MALLEY, YODER and PAFFENHOFER, 1984) or reach mesoscale dimensions (I00
to
lO00km). Our goal was to i d e n t i f y masses of upwelled water which stranded on the southeastern continental shelf (intrusions) and to sample them repeatedly to determine the development and fate of t h e i r phyto- and zooplankton.
In this paper we present information on the abundance
of chlorophyll a and particulate matter and i t s size d i s t r i b u t i o n , in relation to n i t r a t e concentration and temperature, a time series.
Repeated hydrographic coverage of the same area provided
Displacementsof patches were related to water mass displacement as determined
from moored current meters (see LEE and PIETRAFESA, 1988).
A review of past research on upwelling studies on the southeastern shelf
is presented by
PAFFENHOFER, SHERMAN and LEE (1988) and ATKINSON, LEE, BLANTON and PAFFENHOFER (1988) and will
not be repeated here.
Almost a l l
earlier
b i o l o g i c a l oceanographic research on the
southeastern shelf was 2-dimensional (mostly z and x) and included only one short time series (YODER, ATKINSON, BISHOP, HOFMANN and LEE, 1983).
Repeated 3-dimensional coverage of
a
s u f f i c i e n t l y large area was considered essential to provide information necessary to describe the development and f a t e of planktonic organisms and patches of phytoplankton. SON, BISHOP, BLANTON, LEE and PIETRAFESA (1985) in Gulf Stream intrusions during summer 1981.
YODER, ATKIN-
presented data on phytop]ankton dynamics
Their results dealt mostly with one- or two-
dimensional analyses over time as compared to the three-dimensional coverage discussed here.
2.
METHODOLOGY
Our approach to study stranded intrusions of upwelled Gulf Stream water on the shelf o f f F lorid a and Georgia was based on information from previous studies. the dimensions of
intrusions
Knowing the range of
and the p r o b a b i l i t y that they could remain on the shelf f o r
Particulate m a t t e r
longer than 2 weeks, we decided to: alongshore transects at
(I)
3-5
locate water with temperatures less than 20°C from
zne 25 and 40m isobaths with a towed temperature sensor, and (2)
cover the area i d e n t i f i e d with at least 3 onshore-offshore transects at a6 to 55km intervals with stations spaced 13km apart.
This sampling strategy was repeated at least every week.
The area with the highest ~robability that intrusions would occur on the eastern Florida shelf is between 29° and 3i°N (BLANTON, ATKINSON, PIETRAFESA and LEE, 1981) during early June to mid-August when north to northeastward winds favor the formation of intrusions (WEBER and BLANTON, 1980).
Therefore,
the studies described here extended from mid-June to late
August 1981.
From mid-June to the end of duly 1981, using the R/V
Blue Fin,we determined v e r t i c a l temper-
ature p r o f i l e s with an Expendable Bathy-Thermograph
(XBT) using Sippican probes.
This was
followed by Niskin b o t t l e casts to determine concentrations of n i t r a t e (ATKINSONet a~.1988), chlorophyll with a T A I I
a /
phaeopigments
(YENTSCH and MENZEL, 1963) and p a r t i c l e
size d i s t r i b u t i o n
Coulter Counter (PAFFENHOFER, 1983) at 3-5 selected depths, depending on the
depth of the water column.
We then collected zooplankton from d i f f e r e n t depth ranges (see
PAFFENHOFER, SHERMANand LEE, 1988).
During August 1981 we used the R/V Cape FlorYx~equipped
with a CTD on a rosette sampler f o r measuring physical variables and to c o l l e c t water. While we ran a transect about every 15 hours,
i r r e s p e c t i v e of the time of day on the R/V
8~ue Fi~, transects on the R/V Cape F~orida were r e s t r i c t e d to daytime.
The same transects/stations close to several of the current meter moorings were occupied repeatedly over the 2 month study period (Figs.
I and 2).
Large phytoplankton c e l l s f or
measurement of c e l l width were obtained from zooplankton tows with a 103um mesh net.
Widths
were measured at 320 x with an inverted microscope. The methodology used to c o l l e c t and process current meter data is presented by LEE and PIETRAFESA (1988).
3.
RESULTS
We w i l l present a chronology of intrusion events as observed from current meters sited at the 30 and 40m isobaths (Fig.3, LEE and PIETRAFESA, 1988).
Because we did not obtain data
from near-bottom current meters at moorings 8 and 14, we present data from current meters at 17m depth which were usually within or s l i g h t l y below the thermocline.
Subsurface intru-
sions are indicated in current meter records as a cold onshore flow persisting for several days to weeks in the lower layer.
3.1
Sequence of Major Intrusion Euents
We observed 2 major intrusions during the 2 month study period.
Onshore, near bottom flow
occurred from June I0 through 27, 1981 at Mooring 2 and from June 18 through 27 at Mooring 9 (Fig.3, Intrusion I, ATKINSON e t a l .
1988). This upwelled water was displaced southward
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C o n t i n e n t a l s h e l f o f G e o r g i a and n o r t h e r n the l o c a t i o n s o f c u r r e n t m e t e r s .
Florida
showing
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FIG.3.
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AUG
JUL
1981
.-) Io
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~4
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JUL
r--r'
r
AUG
Current d i r e c t i o n s and v e l o c i t i e s of the f o l l o w i n g current meters: 8-I (17m depth), 9-2 (Tm), 9-5 (37m), 14-I (17m). The f i r s t number represents the array ( F i g . 2 ) , the second the r e s p e c t i v e current meter on the array. Mooring l o c a t i o n s can be found on f i g u r e 2.
and o f f s h o r e between June 28 and July 7.
From July 20-30, there was a continuous, onshore
near bottom f l o w at Moorings 2 and 9 ( i n t r u s i o n
2, ATKINSON et a~. 1988).
displaced southward and o f f s h o r e from J u l y 31 to August 3. f l o w from August 5-11.
Therefore,
most of
intrusion
I n t r u s i o n 2 was
This was f o l l o w e d by onshore
2 remained on the s h e l f
during our
study period. 3.2
Indiuidua~ Intrusion Euents and Phytop~ankton Patches
Events and patches w i l l nitrate,
chlorophyll
(Equivalent Spherical below 16°C at i t s
be described by presenting near bottom d i s t r i b u t i o n s
a and p a r t i c u l a t e matter, the l a t t e r Diameter).
Intrusion
center at m i d - s h e l f
in the size range from 2-128~m ESD
I was stretched
(Fig.4).
of tB~peratur~
along-shore with
temperatures
The northernmost p a r t of i n t r u s i o n
I (20°C
isotherm) was not covered. N i t r a t e in the northern p a r t of the i n t r u s i o n was l a r g e l y exhausted
Paniculate matter
81
79
80
379
79 ~32
9C
91
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.,
;..-
31
/
i! L,f< Botlom
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Te~ature(oc) June23-26 ~98~
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<
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\ ",~'|" ~l~'~" ] Part~cleVolurne } june23-26
% 81
FIG.4.
80
7g
"-, 81
i :,, 80
79
Near bottom d i s t r i b u t i o n of temperature, n i t r a t e , chlorophyll a and p a r t i c l e volume between June 23 and 26, 1981. Patch I is i d e n t i f i e d with Intrusion I.
Chlorophyll a (patch I) was highest at mid-shelf increasing from north to south, while particle volume changed l i t t l e along the same isobaths (alongshore). Chlorophyll ~ and particle volumes were highest where temperatures were lowest. Chlorophyll concentration was probably directly related to the amount of nitrate available (ATKINSON, O'MALLEY, YODER and PAFFENHOFER 1984). Our plan to study intrusion I in a time series was not achieved because of a major breakdown of the R/VB~e F~n after completing 2 transects one week l a t e r . JPO
19:3/4-K
.~;~(j
G.-A. P~,FFE',;HL~FERand T x,:. LeE
8]
8D
7g
91
80
FS ::::~-//,~/ :' 05: "~'
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,-. -o~ /
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ti:.i\\
Temoera,u,e
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Bottom
(mg/m 3)
\ ~'~
& 81
FIG.5.
80
7g
i
Botto
i, ;
(mm3/O
-,-.: i \ 81
80
Near bottom d i s t r i b u t i o n of temperature, n i t r a t e , c h l o r o p h y l l a and p a r t i c l e volume between July 13-16 1981.
Our coverage of u p w e l l i n g was resumed from J u l y 13-16 ( F i g . 5 ) . lowed the bottom topography and c o l d , n i t r a t e - r i c h Concentrations t h e r e of c h l o r o p h y l l patch 2 ( F i g . 5 ) . It
seemed to
near
and O.5um n i t r a t e
from a f r o n t a l
bottom
isopleth,
s h e l f near the 30m i s o b a t h . ~)
and i t s
a and p a r t i c l e
By t h i s time isotherms f o l -
waters were located along the 40m i s o b a t h . volume were high and formed phytoplankton
This patch was not associated w i t h i n t r u s i o n s 1 ( F i g . 4 ) or 2 (Figs 6 and 7)
originate
One week l a t e r
79
center
isotherms
eddy ( f r o n t a l
eddy # 6,
had moved toward shore
LEE and PIETRAFESA, 1988). (Fig.6):
which were at the 40m isobath by J u l y 13-16, Patch 2 had increased in area ( l u g
1- I
The 20°C isotherm were now at mid-
i s o p l e t h of c h l o r o p h y l l
had been d i s p l a c e d about 25km west-northwestward towards shore
(Current
81
"I j
,./
FIG.6.
80
.,.) /:
/
)
;
A
7g
,
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,;
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i i,
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,." | v,.""' i Bo,Io~ '",' I" "?l' i ~°'"~'° Vo~.~
.....
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80
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81
79
29
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79 32"
Near bottom distribution of temperature, nitrate, chlorophyll a and particle volume between July 22-24, 1981.
81
..-
80
......i
," ,',
7g
(°C) July22-24,198'
i
i,
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on
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Near bottom distribution of temperature, n i t r a t e , chlorophyll a and particle volume between July 27-29 1981. -Patch 3 starts to develop in Intrusion 2 on the middle shelf.
gJl
,>]
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~ {l ,Jul,~ / ~ .~'.
~0
:' ~ :'
~ ~
FIG.7.
2~
dl
32
#31
2~_
G-A PCFFE~HOFERand T N LEE
Meter 9-5 of Fig.3). of
intrusion
At the ~Om isobath between 29°30 ' and 30°N we observed the leading edge
2 (Fig.6).
One week l a t e r
(July 27-29, Fig.7)
17°C water was at mid-shelf
at 30°N. This indicated the presence of i n tr u s io n 2, which extended somewhat beyond 30°30'N. This conclusion was supported by the n i t r a t e d i s t r i b u t i o n .
From July 23-29 water displace-
ment at current meter 9-5 was about 48km north- to northwestward and at meter 14-I was 41kF~ northward (Fig.3).
Therefore, patch 2 was now located f a r t h e r north, as shown by the d i s t r i -
bution of chlorophyll ~ and p a r t i c l e volume (Fig.7), the past week.
both of which had increased during
South of 30°30'N, where n i t r a t e was high and chlorophyll a and p a r t i c l e
volume lower, phytoplankton patch 3 was s t a r t i n g to develop (Fig.7). Ten days passed before the next transects on August 6-8 (Fig.9). on the shelf was f i r s t for
4 days.
After
displaced northward u n t i l
During that period water
July 30, followed by a current reversal
that flow was again towards shore (Fig.3).
For current meter 8-I
we
calculated that water displacement was 22km north to northwestward (July 29-30), then 47km southward (July 31 to August 3), and eventually 18km north- to northwestward (August 4-6). Because there were no current meter data f o r near bottom flow at the 20 and 30m isobaths (except at Mooring 2),
and because knowledge of water flow at these isobaths is important
in late July and August, we attempted to develop a flow scenario from near bottom temperature data (Fig.8) 14-I
and compare them with the water displacement data from current meters 8-I and
(Fig.3).
A decrease of
near bottom temperatures indicated strong onshore/northward
flow between July 21 and 29 at Moorings 2, 8, 13, 14 and 15, which were located on the middle and inner shelf (Fig.8). 1988).
This signature showed the presence of intrusion 2 (ATKINSON et a~.
Warming from July 30 or 31 to August 4 indicated pronounced southward and offshore
flow at Moorings 2, 8, 9, at Moorings 9,
13 and 14.
This was followed by a marked temperature decrease
13 and 14 from August 5 on (Fig.8~ i n d i c a t i n g onshore- to northward flow.
These observations agree with the flow data from current meters 8 - I , It
9-5 and 14-I (Fig.3).
can be concluded that the water encountered on mid-shelf at 30°N on August 6-8 (Fig.9)
had been in the area since July 28. By August 6-8 most of the middle shelf was covered with intrusion 2 water (Fig.9). rations of n i t r a t e ,
Concent-
compared to July 27-29, had decreased and those of chlorophyll a and i
p a r t i c l e volume had increased. 30m isobaths at 30°N. intrusion 2 (Fig.7).
These maxima represented phytoplankton which originated mainly from
This patch contained the large chain-forming diatoms Stephanopyxis
(Fig. 10), O~inaz'dia f~aoof~a onshore flow. 8-12 ( F i g . l l ) .
Maxima of these 2 variables occurred between the 20 and
and Rhizoso~enia
sp.
s p p . After August 6-8 there was further
The 20°C isotherm was located further north, at the 20m isobath, by August The increase in nitrate was caused by another intrusion (Fig.11).
Now we
could distinguish between one (chlorophyll a / particle) maximum along the 20m isobath (patch The nearshore patch was characterized by Stephanopgx~s sp. (near 100um cell width) (Fig.10), which was abundant at stations where particle
3) and another along the 40m isobath. concentrations
were highest.
The same water remained on the shelf during 13-16 August,
despite the i n i t i a t i o n of weak southward flow by August 14 (Fig.3).
Southward flow was
also shown by the more southward position of the 19° and 20°C isotherms on the middle shelf (Fig.12).
Nitrate was not exhausted at mid-shelf, and patch 3 was s t i l l
positioned along
Paruculat~ m a u e r
383
29,, . ii ma • 27 " ' " "
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July
FIG.8.
~o~ oo O O oo32
~o
.
25
.
°o .
30 1
.
5
.
10
August
1'5
1981
Near bottom temperatures from moorings 2, 3, 8, 9, 13, 14 and 15. Mooring locations are shown on figure 2.
the 20m isobath (Fig.12).
Concentrations of chlorophyll a and p a r t i c l e s at mid-shelf remained
low despite abundant n i t r a t e .
The offshore maximum of phytoplankton remained near the 40m
isobath and was probably of d i f f e r e n t origin than those in patch 3 because the average width of Stephanopwx~s there was much narrower than nearshore (Fig.t0).
O~
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oo,,_
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8-12,
1981.
Near bottom distribution of temperature, nitrate chlorophyll ~ and particle volume between August
~ , ,1{I, / ~' '/~" !
81
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...,~...'~
89
"['~ I 1 Temperature ['C] ~8 !August 8-12, 1981
,:.t"
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,.i.,
,t,~, '
~:." .:~. ...... .,: %'""
G1
'31 I~ "II. _'5 "i~:;I",i"')''//,,'
I
79
a9
al
,,,
~l
7~
Near bottom distribution of temperature, nitrate chlorophyll a and particle volume between August 13-16, 1981.
\
,
~-" ,.~'~,. ,-"':'? ;'/"
~,
IIb:-. ~
!
i
,.' /
90[\I,~"i'/.." : ~o, om~,.._. b ", i (n~j/n a)
a:
2
'7/ /'
:Y f""
,::...... ,:
80
I \ .:-.x..::':..i,.,,-~,~o ,~,
~
..4 ;~"""
<:
.or t"~'<-J
31
~
81
P,
.~0
3.3
O ",-'%.. Pa,FFE'~.HOFER .~nd T. N L~.E
Ver~ica~ D~s~rib~vton of E~mper~re,
Intrusion I
(patch I ) ,
extended h o r i z o n t a l l y and v e r t i c a l l y over most of the middle shelf,
as indicated by a shallow upper mixed layer. cline
(Fig.13a).
.Vt=r~e, Ch:~ro?hg~ ~ ~nd ?arv!cie ~ o ~ e
N i t r a t e was abundant in and below the thermo-
High concentrations of chlorophyll a and p a r t i c l e volume throughout much
of the water column indicated l i t t l e
grazing a c t i v i t y (Fig.13b).
One week l a t e r the thermo-
c l i n e was deeper and n i t r a t e was exhausted, but chlorophyll a and p a r t i c l e s were not s i g n i f i c a n t l y reduced (data not shown). Patch 2 was observed over 3 successive weeks (Figs.
14, 15 and 16).
mixed layer (UML) consisted of r e l a t i v e l y cool water (<28°C, Fig.14a).
Initially
the upper
Concentrations of
chlorophyll a and p a r t i c l e volumes were lowest in the UML and increased with depth (Fig.14b). By July 22-24 the UML was warmer and upwelled water had moved towards mid-shelf as seen from n i t r a t e concentrations (Fig.15a). in upwelled water of 20°C or less.
N i t r a t e concentrations of >I~M were usually observed
Concentrations of chlorophyll a and p a r t i c l e s had inc-
reased in most of the water column along the 2 northern transects (Fig.15b, patch 2) and were f a i r l y
low throughout the water column along the transect with the strongest upwelling
(Stations 405 through 410). shore (Fig.16).
By July 27-29 upwelled water had been advected f a r t h e r towards
Then differences in the v e r t i c a l d i s t r i b u t i o n of chlorophyll ~ and p a r t i c u -
late matter between transects were obvious. at the same depths at a l l
stations,
were in and below the thermocline.
In the northern transect,
the thermocline was
and higher concentrations of p a r t i c l e s and chlorophyll Along the middle transect ( i n t r u s i o n 2) the thermocline
domed at Stations 506 and 507 and the isopleths of chlorophyll a and p a r t i c l e volume f a i r l y closely followed the isotherms. We chose the center of patch 2 (Stations 313, 414 and 515) during the 3 successive weeks to f o l l o w changes in the patch and o~erlying water column (Table I ) .
Largest changes occurred
in the bottom layer where chlorophyll a increased more than p a r t i c l e volume. increased from July 23-29.
Phaeopigments
We divided p a r t i c l e volumes into 3 size ranges of which the
middle represented sizes which can be ingested at high rates by most crustacean plankton. The largest percentage of p a r t i c l e concentration occurred in the middle size range in the bottom layer throughout the study period. From August 6-16 the bottom layer of much of the middle and part of the inner shelf consisted of intrusion 2, i t s n i t r a t e l a r g e l y depleted. from a new i n t ru s io n to the mid-shelf during the f o l l o w i n g 4 days (Fig.19a),
Onshore advection at 30°N introduced n i t r a t e
(Fig.18a,
Stations 634-636) which was not taken up
Phytoplankton patch 3 which we described e a r l i e r
(Figs. 9-12), was not l i m i te d to water below the thermocline as the Imm3 1-I isopleth reached the surface at almost a l l stations (Figs. 17, 18b, 19b).
At most of the same stations con-
centrations >O-5ug 1-I of chlorophyll a occurred in part of the UML. As in the case of patch 2 we compared concentrations of chlorophyll a, phaeopigments ~ and p a r t i c l e volume from stations near the center of the patch s t a r t i n g on July 27-28 (Table 2). Concentrations of the 3 variables increased in the thermocline and intruded water through
~ 2 ~ '
AI .. ?3 24 I'~ll
N
L
L~I~R[
-
I
,,%
I
,
l
~
I
--~..
I]ISIAN~f ~ F S I ~
2,) ICJBI
23 24 19Sl
iv,M)
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r~)l
J J ~, z5 VJ~I
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q~M)
(fiN)
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I
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I
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(it
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,
JU St) /U UP~IANII {~l',}W~l
~i~ 2% I'JHI
C.~L a
gU (~,N)
I LU
"7
~
V,A.,.,
"~
and p a r t i c l e volume from June 23-26,
1981
'~[j
L"
\
x
](j
"T
Dp, IAI~(I L~I',IIUffl
JO
ItllllJ/ Ij
]lJ
u ~i.
~Ctl, 2~ i,m J
____.~
t , . . . ~IH
Vertical distribution of (a) temperature and nitrate, and (b) chlorophyll a
(KN)
'-,~ ~', -
FIG.13
t}Ibl^Nli
, o ~
t~
lfl
('~2J
'°~~!J I0
B~
~j(j
_ .
(~N~
. :Z
I Ill
F¢ B c~
T
10
5~
I/)
10
_
,
,
I
Io('l
I
,
,
,
I
I
I
,
,
,
I
I
DISMN~E ~FSHi}R~
I
I
15 I'RII
,
I
,~Jy
(N:}
I
. J~iy 15 IG l(~llt
i
.
i
FIG. 14.
(~N)
,
,
I
NO3
(~I,11
I
I
13-16,
,
I
I
i
,
I
z ~ _ _
,
I
"--.
I
'
II0
b(l
I0
\
i
i
,
{~,lu/,,, J
CHL a
I
CHL ~
\..
i
,
i
l I
,
DISI^~[ ~ f ~
i
I
5O _ July t5116 19BI
i
I
i
I
i
l w I L - - -
(KM)
I
1981.
Jill k~ 14 r J ~ (IIWII
(.wn3/0 .wJy 15 16 IgBI
\ DISIANCE ~FS~UWE (~M)
:G?71,
_~ - ~ ' - . ~ ,
distribution of (a) temperature and nitrate, chlorophyll a and particle volume from July
71,) ~ DISI^N~[ OFISIIOR~ (~M}
I
July I~ l~tll
NO~
I
Vertical and (b)
I
15 16 I~I
10
~-.\
/I f
fro
"Z
Z I O-
>
C~
K
'\
511
I0
i~
FIG.15.
3U ~J 70 IJISIANC[ O~tSt~R[
I'
Jl.~y 24 191~1
,oF\
51)
,
i
I
IO
L._
,
,
I
,
I
,
]0 SO 70 90 DISIAN£E {~-FSb~)RE (l~M)
l
J~lly ,';' IItU I
lib
1
-~ _
,
llO
--L
I0
5O
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a
,
T
,
T
( I1)l,l/m 3 )
i
T
,
;
i
7
T
T
i
;
,
~
I
7___7 .
.
.
.
.
.
.
,
(lllniJ/i)
iiiiipj/i)
i
[llltl)/l)
.
.
and (b) c h l o r o p h y l l
DI$1ANCE OFFq~IOR£ (KM)
Im,iJ/m :1) Jl~y 22 ~:J 1981
CllL
i
(fl~tlll j )
V e r t i c a l d i s t r i b u t i o n of (a) temperature and n i t r a t e , and p a r t i c l e volume from J u l y 22-24, 1981.
90 O',M)
.
I
24 lull1
J~l~ ,'2 / a P,tI~ I
NO 3
i\.
r
,~
T .
.
,
.
.
i ~
.
.
.
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a -
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~~-~_
Y_~_7__i~ ~ - , f - -
1
.
-io
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x~ ~2
u
ILl
,
I
to
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:tJ
l
,
-
'
,
,
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1
+
I
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l
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FIG. 16.
9U tlxM+
,
,
_'2.\_
I
i
flu
I
t+oj
10
,~+IA+
"It,
DI',IAN~i O~f';+~tJRi
JO
~O
M~MMM
./tll+ +++J I~)+~ 1
II+M'
t,~O I
(pMt
~]~t)
oo4~
~':
.
I lU
,
,
h
.
~
~
....
.+
h
'
,
~
,I+
(~M)
q
\
l
',~
cc
,
OISIAPdLE UtlbtlOlq
,; ~h''~b
, .q.,
I
(n,,),m ~I
u,~ u
+
L i l t ,+ ,l,q +,+fl ~.++, '+, q J d l .
~,
,
,;0
I
L__
~,
t ~IU
I
,
/I
]LJ
I
(ltlSlltJRl
r
''
,
,
?'
~w)
_ +J"
~'7
UIS/,~NI[
JU
t
~ . , i r l . h' ' W . I , W , . '
-.
Vertica! distribution of temperature, nitrate, chlorophy|l a and particle volume from July 27-29, 1981.
.
-.c. 'i-I
"k'% ~
I
+t~ ',~; 'tJ t,l+,l+.'lll [,+t*,+It)lil
I
I
ltl Xl, '+, '1 I
I
..............
~uU~
'"" ~ ;
,,,I
I
-
u+
,
t'2
1
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m m
Z
-t
m
m
7
o
I0
I
....
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FIG. 17.
~
<',L'
7°
IlU tO
~J ~3 70 OI$1ANtl L~lS~il~
~
IJO ~P,M)
I I0
o • ~
~.~
'\ \
~lSIA~k ~ F ~
~
V e r t i c a l d i s t r i b u t i o n of temperature, n i t r a t e , volume from August 6-8, 1981.
~1o
~
~-
.
S
J._~
~
.
l
~,,,L~,.:~:~ i., ~ -,;,,
I
c hlo r o p h y ll a and p a r t i c l e
¢KWI
t
E
SO
tO
I0
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iO
5O
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I0
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1
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I
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,
A~jusi
,
,
,
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t
t
t
,
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FIG. 18.
I ~3 (KW)
,
,
~-~1~
,~ ~ ~
~__1
,
I _~ _J , t 3U ~3 70 DIMANC[ LWFS)~[
I
ll) i u ~ i
• | O, lll~)l.th, u ~,llj)
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t
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t
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,
,
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,
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,
%.f.
t
I
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30 50 70 DISIANC[ O~FSHORE(KW)
,'f
t
AuguSl 11 19{}1
NOj
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,
110
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5~
50
ID
5O ~
,
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AuguSl II 19~I
1(3
.
,
.
1
.
......
.
e
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IT:T.
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C)¢ d
I
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,
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,
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h
(n~rPn) A u ~ s l It )9~)
I
(.I,~I0
Vertical distribution of (a) temperature and nitrate, and (b) chlorophyll a and particle volume from August 8-12, 1981.
i I I0
i
I
I
A uijust I~,i9~ t
I0
L ~ l
I
,--~-_
r" tn m
p~
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7
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f~UrdhWe
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FIG.19.
NO 3
,
I ~
,
.o
.o ,
o~
l ,
' I
DISIAN~E OFFf~IOR£ (WW)
Augu~l 13.198 1
\o,
Aul)usl 14 1981
NO,I
AUC~U51 15 19BI
NO3
I
Au~,51 16,1981 ,
I
5O
IU
I0
50
I0
a
*
I
,
,
I
.
~
I
,
l
I-
I
,
,
,
\
I
I
I
,
,
,
I
L
I
30 50 OlSI^NEE OFFSH(]~[ (KM)
CHL a (mg/,n~) Au~st i~ 19~i
I
(.w~/l,(~} Al;~llc.lFl I')III
I
A~(Ju~I 15 19Ul
OL (.~.#)
l
(.~vm3)
O1
Z
,
,
--L_
L
/
I0
(f1|n3/LJ
and p a r t i c l e
volume from August 13-16,
1981.
\
\
#J b~l 70 ~tJ [}ISI^N~[ UI}~K~E (~H}
PJI, ~ V,~J,w, (o.iJ.l}
/'
P~ t~b: wa.i~ (lla~i]/1)
P~V~,~
Vertical distribution of (a) temperature and nitrate, and (b) chlorophyll
I10
L._>' 1
I
I
DISI^NCE L~FS~edRE (KM)
30
,' ~'~,'
IoC) _AuQU;I t4,1901
I
lempe~at~e {%) 15.Bill
I
[oc) ~ J S l ~6,1981
I0
110
F¢
3
414
514
23 July 1981
29 July 1981 0.17 0.30 12.00
T I
9,05
I U
0.19 0.31
U
5,23
!
T
0.23
T
4.10
0,19
0.12
0.32
0.01
0.07
0,99
0.13
0.08
lJg I - I
ug 1- I
0.19
Phaeo.a
Chl.a
U
Depth Range
*ESD = Equivalent Spherical Diameter
I = Intrusion
T : Thermocline
U = Upper Mixed Layer
313
Station
5.42
0.55
0.42
5.55
0,50
0.28
4.61
0.66
0.35
Total Particles mm3 1- I
0.43
0,09
0.08
0.43
0.13
0.07
0.43
0.14
0.07
2-8
3.26
0.27
0.21
3.06
0.23
0.14
2.26
0.25
0.17
8-32 mm3 1- I
1.73
0.19
0.13
2.06
0.14
0.07
1.92
0.27
0.II
32-128
Concentrations in size ranges (~m ESD*)
8
16
19
8
25
26
9
21
21
2-8
60
49
51
55
46
48
49
38
47
8-32
32
35
30
31
28
27
42
41
32
32-128
% t o t a l volume w i t h i n s i z e ranges
Patch 2: Concentrations of chlorophyll a, phaeopigments a, and p a r t i c u l a t e matter at patch center; and concentrations and percentage distributTon of p a r t i c u l a t e matter within size ranges.
15 July 1981
Date
TABLE I :
rn m
p-
C~
rn l
0
r
3.13
I
3.03
I
674
*ESD = Equivalent Spherical Diameter
I
T 2.69
0.27
1.59
T
640
665,666, U
0.40
633,634, U
U = Upper Mixed Layer T : Thermocline I : Intrusion
15 August 1981
I I August 1981
0.66
0.32
1.60
I U
0.37
T
T
0.12
U
602
-I
600,601
~g 1
Chl.a
6 August 1981
Depth Range
501,502
Station -I
0.59
0.13
1.64
0.57
0.16
1.15
0.38
0.16
0.95
0.09
0.06
~g 1
Phaeo.a
6.68
0.53
4.49
1.24
1.02
2.55
1.38
I.II
1.36
0.74
0.33
Total Particles 3 -I mm 1
0.27
0.12
0.50
0,28
0.22
0.31
0.26
0.20
0.27
0.24
0.II
2-8
0.79
0.29
1.32
0,54
0.61
I.I0
0.72
0.64
0.58
0.33
0.15
mm3 1-1
8-32
5.62
0.13
2.67
0.42
0.18
1.14
0.40
0.27
0.51
0.17
0.07
32-128
Concentrations in size ranges (~m ESD*)
4
22
11
22
22
12
19
18
20
32
33
2-8
12
54
30
43
60
43
52
58
43
45
45
8-32
84
24
59
34
18
45
29
24
37
23
21
32-128
% t o t a l volume w i t h i n size ranges
Patch 3: Concentrations of chlorophyll a, phaeopigmentsa, and particulate matter; and concentration and percentage distribution of particulate matter within size ranges
27-28 July 1981
Date
TABLE 2:
396
G-A. P~FFE%H,~FEK and T. N LEE
August 71, except that p a r t i c l e volume began to decrease in the UML and t~e~mocline after
August 6.
By August 15 a l l
intruded water.
variables except p a r t i c l e volume had decrease~ in the ~ L
and
Division of p a r t i c l e volume into 3 size groups showed that "arge parzicles
(32 to 128um ESD) increased in concentration in the i n t r u s i o n , throughout the study period, whereas small
and medium sized p a r t i c l e s decreased between August 11-15.
were the diatoms G. f ~ a c ~ a ,
Rhtzoso~enia
4.
Spp. and Stephanopyx~s
_~rge p a r t i c l e s
Sp.
DISCUSSION
Subsurface intrusions of Gulf Stream water reach the middle shelf o f f
Flori~
and Georgia
during summer when winds are approximately northward during the passage of a Gulf Stream frontal
eddy (LEE, ATKINSON, LEGECKIS 1981) at times when the Gulf Stream is in an onshore
position (LEE and PIETRAFESA, 1988). While f r o n t a l eddies occur at an average frequency of one per week, major intrusions occurred at the rate of one per month during the summer of 1981.
We repeatedly observed that i n t r u -
sions of cold, n i t r a t e - r i c h Gulf Stream water onto the southeastern shelf led to the development of patches of phytoplankton. cesses.
This implies that patches were induced by physical pro-
Here we compare the t r a j e c t o r i e s ,
longevity and dimensions of intrusions and discuss
changes in chlorophyll a and p a r t i c u l a t e matter over time. 4.1
Trajectories
Patch 1 moved onto the shelf near 29°N between June 10-27 (Fig.3, LEE and P!~TRAFESA, 1988) and was displaced northward as a tongue at mid-shelf
along isobaths.
A current reversal
resulted in i t s offshore displacement between June 28 and July 7 (Fig.3) near the same geographic location.
Patch 2 was displaced onshore at 30°25'N (July 13-22) and then alongshore.
Patch 3(aspart of intrusion 2) which i n i t i a l l y
had the same t r a j e c t o r y as patch I, was displaced
northward from July 20-30 to about 30°30'N about 40km southward and offshore (Fig.3).
(Fig.7,
nitrate),
and from July 31 to August 4
Thus, part of the intrusion which was on mid-
shelf near 30°N (Figs 3 and 10, the l a t t e r showing abundant
Stephanop~xis c e i l s of approxim-
at e ly lOOum width) remained on the shelf forming a well-defined patch (patch 3, Figs. 9 and I0).
Stephanopgxis
used to trace patch 3. it
was abundant within the 2mm31-I
p a r t i c l e volume isoba~h and could be
This patch was displaced towards the 20m isobath a f t e r August 4 where
remained throughout the rest of the study period.
and PIETRAFESA (1985, t h e i r F i g .t 1 ,
our Fig.20),
YODER, ATKINSON, BISHOP, BLANTON, LEE
however, postulated that the phytoplankton
maximum, observed nearshore near 30°N by August 10-13 (patch 3) originated from a maximum located nearshore at 290N on July 27-30 at 29°N and was displaced northward. ing 7 do not support t h e i r findings. followed the same route.
Data from Moor-
Our results show that intrusions I and 2 i n i t i a l l y
P~rtIculatc m3~t~r
81
80
< ."
7g
.~9"
81
w. /
80
i
<{"
¼)
'
~ \ C . L \ \ ~t "
81
',; i
........ " • i. . • ',-'~ \\ ~Ar>--"
,, :~ ,<,.-,,,o r .~,,_= .,<,<>. \"
7g
81
~-~
,; 3
81
FIG.20.
80
2~
7g
7g
,,'t
..A
• ......:,
t;. 'i
;" •
/
C~
:;
' " " 't
a
&~7~. . `
~.;
,
1/
;
I \~._J,-;t
80 •
?
2g~kk,~?.~-'~.',,,
-"-"-'" ' 3 0
eOTTOM
80
< 2
)
t )< 2/
•' ~
,<
7g
Patc
I
V#tV-.k~,~.
~
-~
~ ",
"~L
81
eOTTO,,, /
80
~g
7g
Near bottom d i s t r i b u t i o n of c h l o r o p h y l l a and phaeopigments combined from July 20-23 to A-ugust 10-13, 1981 (modified from YODER et ~ . 1985). Patch 2 could not be i d e n t i f i e d by August 10-13.
395
G.-A. P~FFE~HOFERand T N LEE
The l i f e t i m e of patch I on the shelf was 27 days from a r r i v a l to departure over the middle shelf.
Patch 2 was observed fo r at least 21 days (Fig.5 was the i n i t i a l
observation) and patch 3 f o r 27 days.
and Fig.20 the f i n a l
Because patches 2 and 3 were s t i l l
on the shelf when
we terminated observations, the average longevity of a summer upwelling on the middle shelf probably exceeds 25 days.
This time is s u f f i c i e n t f o r a phytoplankton bloom to develop and
for zooplankton populations to respond to enrichment (YODER e t a~. 1983, 1985; PAFFENHOFER, SHERMAN and LEE 1988). enhanced because (a) no d i r e c t
Persistence of phytoplankton patches on the southeastern shelf is
weak winds induced l i t t l e
vertical
mixing,
(b)
there was limit e d or
influence of the Gulf Stream on the middle and inner shelf (LEE and PIETRAFESA,
1988: HAMILTON, 1988), and (c) large diatoms formed a large part of patches 2 and 3 and were not grazed by zooplankton.
4.2 It
E o r ~ z o m t a l D~mens~om~
was d i f f i c u l t
to accurately determine the horizontal dimensions of patches because of
the spacing of transects and because we could not cover each patch completely. When 1.0mm3l-I of p a r t i c l e volume was used to define the edge of patch I, the maximum width was 63km. shelf,
the length exceeded 140km, and
Patch widths are r e s t r i c t e d by the width of the continental
and thus could not exceed 75km.
Lengths can exceed 180km i f
onshore flow persists
fo r more than 2 weeks, as we observed using bottom temperature d i s t r i b u t i o n to define i n t r u sion 1 (Fig.4).
In this case, the inner, middle and part of the outer shelf were covered
with upwelled water of
less than 20°C over an area exceeding lO,O00km2,
Shorter periods
of upwelling resulted in patches of smaller dimensions.
On July 22-24 patch 2 was about -I 47km wide and exceeded lOOkm in length, using lug chlorophyll a 1 to define i t s l i m i t s (Fig.6).
Later i t exceeded 110km in length and 50km in width, as observed (YODERet aZ. 1985,
Fig.20) on July 27-30 . (Figs. 7,20).
Patches 2 and 3 could not be c l e a r l y separated during July 27-30
The northern part of patch 3 could have contained part of patch 2 (Figs. I I ,
12, Fig.20: July 27 to August 13).
Due to lower layer flow towards shore the width of patch
3 decreased from 52km on August 6 to about 35km by August 13-16 (Figs. 9,12). exceeded 155km on August 13-16.
Its
length
The development of patch 3 was also d e t a i l e d by YODERet
a~. (1985, Fig.20, July 27 to August 13). According to OKUBO (1978)
"...
patch size depends on the balance between the aggregative
process of organism growth and reproduction and the anti-aggregative process of d i f f u s i o n " . Minimum c r i t i c a l sion d-I
patch r a d i i range from 2km at 1 c e l l d i v i s i o n d-I to 40km at 0.1 c e l l d i v i -
(OKUBO, 1978).
On the southeastern shelf the rate of c e l l d i v i s i o n rates should
have exceeded 0 . I d-I f o r a period of >10 days in patches I and 3 (YODER e~ a~. 1983).
This
would allow the patches to maintain themselves as t h e i r minimum dimensions were about 50km.
Particulate m a t t e r
3~9
MACKAS, DENMAN and ABBOTT (1985) note that n u t r i e n t enrich~e~ is o~e source of b i o l o g i c a l variances in the ocean and described 3 factors common to e~richmen~: v e r t i c a l movement of water, strong horizontal heterogeneity of the physical and c~emical environment, and advecr i v e f i e l d s coupled to regional bathymetry. rence of patches at the same location. shelf between 29° and 31°N.
The l a t : e r results in the persistence or recur-
All 3 elements are c h v a c t e r i s t i c of the southeastern
Intrusions 1 and 2 upwelled ont~ the shelf near 29°N and were
displaced northward, mainly directed by bathymetry at mid-shelf, and had strong horizontal heterogeneity.
Concerning the persistence of phytoplankton patches we know only of the study
of SAVAGE and WIMPENNY (1936) who followed a patch of the diatom Rh~zosolenta stg~ifo~nisin the North Sea from June to September 1934. from 8 to 334 1- I . MACKAS et al.
During that period c e l l concentrations increased
Maximum patch dimensions were about 190km length and 90km width.
(1985) found that plankton community patches are usually elongated p a r a l l e l
to local currents
and isobaths, as we also observed on the middle shelf in summer. They
found a 3:1 r a t i o f o r average alongshore:cross-shelf patch dimensions. to species occurrence, the Stephanopyx~s (Fig.10).
Referring s t r i c t l y
patch (patch 3) on August 8-12 had a r a t i o of 4:1
Comparing lengths and widths on the basis of chlorophyll i ,
phaeopigments a and
p a r t i c l e volume, patch dimensions r a t i o s ranged from about 3:1 in patch 2 (YODERet ai. 1985, Fig.20,
July 27-30)
to 6:1 f o r
the nearshore maximum on August 10-13 (Fig.20).
The r a t i o
of 2.5:1 which we obtained f o r patch I is an underestimate because we could not define the northern and southern l i m i t s of the patch (Fig.4). 4.4
Changes in Chlorophyll a and Particulate Ma~ter over Time
Near the center of patch 2 we observed substantial increases in chlorophyll ~ and l i t t l e change in particle volume from week to week (Table I). variable in the UML and thermocline.
There was l i t t l e change in either
We relate the pronounced increase in phaeopigments
in the intruded water to grazing a c t i v i t y since,
in the presence of abundant zooplankton
(PAFFENHOFER et ai. 1988), neither particle volume U or chlorophyll a decreased. This indicates rapid phytoplankton growth rates and not phaeopigment formation due to lack of nutrients. In the UML and intruded waters of patch 3 we observed increases, and then decreases, chlorophyll a, phaeopigments a and particle volume (Table 2).
of
We attribute these decreases
to grazing effects, particularly since the decreases occurred in small and medium-sized particles.
The probability is low that these decreases were due to sinking because the concentr-
ation of large particles, which usually sink faster than small ones, increased nearly 50% during that period.
Total particle volume in the intrusion increased because the large part-
icles were too wide (mostly adult Eucalanus pileatus KNOWLES, 1978).
> 60um) to be ingested by zooplankton other than copepodid and and late copepodid and adult
Temora s t y l i f e r a
(PAFFENHOFER and
While chlorophyll a did not increase between August 6-11, the concentration
of large p a r t i c l e s did.
The small amount of phaeopigments on August 15 indicates that (a)
zooplankton had grazed on d e t r i t a l occurred due to lack of n u t r i e n t s .
and fecal
material
and (b)
little
phytoplankton death
'l)d)
G -A. P-kFFE%HOFER and T N LEE
To determine the development and persistence of a plankton bloom i t
is essenzial
to zrack
the bloom and the water mass in which i t occurs. This was achieved by combining 3-dimensional hydrographic surveys with simultaneous current meter observations.
Our results show that
complete coverage of an intrusion event requires that the study exceeded 4 weeks in time. Because of the lack of s u f f i c i e n t time the fates of patches 2 and 3 were not determined. Earlier observations
showed that mid-shelf water (entire water column) at 30° and 31°N is
slowly displaced during summer in an alongshore direction (ATKINSON, LEE, BLANTON
and
CHANDLER, 1983). Thus, the longevity of patches 2 and 3 should have been considerably longer than the 21 and 27 days, respectively, that were studied.
Our observations lead us to con-
clude that most of the particulate matter in patch I was eventually advected from the shelf. Most of that in patches 2 and 3 remained on the shelf and was eaten by zooplankton, benthos,
or microorganisms.
ACKNOWLEDGEMENTS This research was supported in part with the Department of Energy by contracts 76EV00936 and DE-ASO5-76EV05163, and National 0CE85-00917.
DE-ASO9-
Science Foundation Grants 0CE81-17761
and
The captains and crews of R/V B~ue Fin and R/V Cape Florida cooperated profess-
i o n a l l y and contributed to our success.
Without the assistance of Byron Sherman, J u l i a
C l i f t o n , Carolyn Benson and Thomas Brandt we could not have accomplished our goals. Hagin provided Figure 3.
Dannah McCauley typed the manuscript.
Boyette prepared the figures.
Bill
Suzanne Mclntosh and Anna
Chandler provided base map software.
comments of two anonymous reviewers improved our manuscript.
Michael
The constructive
We would l i k e
to thank a l l
those who supported our e f f o r t s .
REFERENCES ATKINSON, L.P., T.N. LEE, J.O. BLANTON and W.S. CHANDLER (1983) Climatology of the southeastern United States continental shelf waters. Jo~rna~ o f Geophysical Research, 88, 4705-4718. ATKINSON, L.P., T.N. LEE, J.O. BLANTON and G-A. PAFFENHOFER (1988) Summer upwelling on the southeastern continental shelf of the USA during 1981: Hydrographic observations. Progress in Oceanography, 19, 231-266. ATKINSON, L.P., P.G. O'MALLEY, J.A. YODER and G-A. PAFFENHOFER (1984) The e f f e c t of summertime shelfbreak upwelling on n u t r i e n t f l u x in southeastern United States continental shelf water. Journa~ o f Marine Research, 42, 969-993 BLANTON, J.O., L.P. ATKINSON, L.J. PIETRAFESA and T.N. LEE (1981) The intrusion of Gulf Stream Water across the continental shelf due to topographically-induced upwelling. Deep-Sea Research, 28A, 339-405. HAMILTON, P. (1988) Summer upwelling on the southeastern continental shelf of the USA during 1981: The structure of shelf and Gulf Stream motions in the Georgia Bight. Progress in Oceanography, 19, 329-351. HAURY, L.R., J.A. McGOWAN and P.H. WIEBE (1978) Patterns and processes in the time-space scales of plankton d i s t r i b u t i o n s . In Spatia~ Patterns in Plankton Communities, J.H. STEELE, e d i t o r , Plenum Press, New York and London, 277-327. LEE, T.N., L.P. ATKINSON and R. LEGECKIS (1981) Observations of a Gulf Stream f r o n t a l eddy on the Georgia continental shelf, April 1977. Deep-Sea Research, 28A, 347-378. LEE, T.N. and L.J. PIETRAFESA (1988) Summer upwelling on the southeastern continental shelf of the USA during 1981: C i r c u l a t i o n Progress in Oceanography, 19, 267-312. MACKAS, D.L., K.L. DENMANand M.R. ABBOTT (1985) Plankton patchiness: Biology in the physical vernacular. Bulletin of Marine Science, 37, 652-674.
Particulate m a t t e r
40[
OKUBO, A. (1978) Horizontal dispersion and c r i t i c a l scales for phytoplankton patches. In; Spatial pac~ern~ ix ?~ank~o~ communities, J . H . STEELE, editor, Plenum Press, New York and London, 21-42. PAFFENHOFER, G-A. (1983) Vertical zooplankton distribution on the northeastern Florida shelf and i t s relation to temperature and food abundance. Journal of Plankton Research, 5, 15-33. PAFFENHOFER, G-A. and S.C. KNOWLES (1978) Feeding of marine planktonic copepods on mixed phytoplankton. Narine Biology, 48, 143-152. PAFFENHOFER, G-A., B.K. SHERMANand T.N. LEE (1988) Summer upwelling on the southeastern continental shelf of the USA during 1981: Abundance, distribution and patch formation of zooplankton. Progress in Oceanography, 19, 403-436, SAVAGE, R.E. and R.S. WIMPENNY (1936) Phytoplankton and the herring. Part I I . 1933 and 1934. Fisheries Investigations Series II, 15, 1-88. WEBER, A.H. and J.O. BLANTON (1980) Monthly mean wind f i e l d for the South Atlantic Bight. Journal of Physical Oceanography, 10, 1256-1263. YENTSCH, C. and O. MENZEL (1963) A method for the determination of phytoplankton chlorophyll and phaeophytin by fluorescence. Deep-Sea Research, I0, 221-231. YODER, J.A., L.P. ATKINSON, S.S. BISHOP, E.E. HOFMANNand T.N. LEE (1983) Effect of upwelling on phytoplankton productivity on the outer southeastern US continental shelf. Conti~enta~ She~f Research, I, 385-404. YODER, J.A., L.P. ATKINSON, S.S. BISHOP, J.O. BLANTON, T.N. LEE and L.J. PIETRAFESA (1985) Phytoplankton dynamics within Gulf Stream intrusions on the southeastern US continental shelf during summer, 1981. Continental Shelf Research, 4, 611-635.
, , . ' ~ ' , ' , i E ~," $,' q~- 5*) C,~p3r,ght ~ la',,~ Pergamon P-.~, pic
S U M M E R U P W E L L I N G ON T H E S O U T H E A S T E R N C O N T I N E N T A L S H E L F
OF THE U.S.A. DURING 1981
Distribution and Abundance of Particulate Matter GL:STAV-ADOLF PAFFE'~H6FER" and THO~I~,S N. LEE" "Sktdaway ht~'tttate of Oceanography. Smlannalt. G,4 3140r~-1)687, I_' S.A. "'R~sensttel Schuol of Martne and ,4ttrtosp/tertc Sciettce. L'mver~ity qf ,%hamt. 4fi00 Rickenhacker Causeway. Mtami. FL 33140. U S.A,
ABSTRACT This paper reports
on the three dimensional coverage of Gulf Stream upwelling events to
determine the d i s t r i b u t i o n and abundance of p a r t i c u l a t e matter and chlorophyll a over time. This approach resulted in a description of size, shape and longevity of phytoplankton patches The strongest upwelling event occurred in June (patch I) July to e a rl y August (patches 2 and 3). frontal
eddy and northward winds occurred simultaneously at a time when the Gulf Stream
was in an onshore p o s i t i o n .
Patch I was i d e n t i f i e d by temperature anomalies and concentra-
t i o n of chlorophyll and p a r t i c u l a t e matter. when i t it
followed by weaker events during
These upwellings were observed when a Gulf Stream
was displaced by southward flow.
I t remained on the shelf from June I0 to July 7, Patch 2 was observed from July 13-29 af t er which
was advected northward at mid-shelf from our study area.
27 to August 16 when i t
stranded at the 20m isobath.
Patch 3 was studied from July
Patches 2 and 3 were distinguished
by high concentrations of chlorophyll a and p a r t i c u l a t e matter. of chlorophyll
a increased throughout the study period,
af t e r an i n i t i a l
increase.
In patch 2 concentrations
whereas in patch 3 i t
decreased
A l l three phytoplankton patches were induced by physical events.
Their shape was governed by bathymetry.
Their size and longevity was to a large extent
a function of the i n t e n s i t y and period of onshore and offshore advection.
CONTENTS 1.
Introduction
374
2.
Methodology
374
3.
Results
4.
3.1
Sequence of Major Intrusion Events
375 375
3.2
Individual Intrusion Events and Phytoplankton Patches
378
3.3
Vertical D i s t r i b u t i o n of Temperature, N i t r a t e , Chlorophyll a and P a r t i c l e Volume
386
Discussion
396
Acknowledgements
400 400
References 373
OIIA " P~,FFE'-HOFERand T
~ --~
I.
\
L_~
INTRODUCTION
One of the main goals of b i o l o g i c a l oceanographic research has been to determine the production and fate of organisms and t h e i r byproducts. of organisms
(school
of fishes,
To accomplish this goal a d i s t i n c t group
cohort of zooplankton) or a d i s t i n c t
observed over appropriate periods of time.
water mass must be
Pelagic populations may be tracked using drogues,
current meters or standard CTD-methods. Advection shear and d i f f u s i o n usually modify patches, the dimensions of which sometimes can be so large that they cannot be sampled at s u f f i c i e n t l y frequent
intervals
to
systematically determine production and f a t e .
One of the keys to
describing variables is to determine patch size and contents within adequately short intervals of time.
This has been accomplished only on rare occasions (SAVAGE and WIMPENNY, 1936).
The scarcity of information and the s i g n i f i c a n c e of the time f a c t o r was pointed out by HAURY, McGOWAN and WIEBE (1978)
who stated:
"Perhaps the most important time factor of pattern
is the tenq}oral persistence or l o n g e v i t y of i n d i v i d u a l patches on the micro- to coarse scale. Much has been inferred but l i t t l e
is known about this problem."
Patches of stranded upwelled
water on the southeastern shelf are e i t h e r on a coarse-scale of I to lOOkm maximum dimension (ATKINSON, O'MALLEY, YODER and PAFFENHOFER, 1984) or reach mesoscale dimensions (I00
to
lO00km). Our goal was to i d e n t i f y masses of upwelled water which stranded on the southeastern continental shelf (intrusions) and to sample them repeatedly to determine the development and fate of t h e i r phyto- and zooplankton.
In this paper we present information on the abundance
of chlorophyll a and particulate matter and i t s size d i s t r i b u t i o n , in relation to n i t r a t e concentration and temperature, a time series.
Repeated hydrographic coverage of the same area provided
Displacementsof patches were related to water mass displacement as determined
from moored current meters (see LEE and PIETRAFESA, 1988).
A review of past research on upwelling studies on the southeastern shelf
is presented by
PAFFENHOFER, SHERMAN and LEE (1988) and ATKINSON, LEE, BLANTON and PAFFENHOFER (1988) and will
not be repeated here.
Almost a l l
earlier
b i o l o g i c a l oceanographic research on the
southeastern shelf was 2-dimensional (mostly z and x) and included only one short time series (YODER, ATKINSON, BISHOP, HOFMANN and LEE, 1983).
Repeated 3-dimensional coverage of
a
s u f f i c i e n t l y large area was considered essential to provide information necessary to describe the development and f a t e of planktonic organisms and patches of phytoplankton. SON, BISHOP, BLANTON, LEE and PIETRAFESA (1985) in Gulf Stream intrusions during summer 1981.
YODER, ATKIN-
presented data on phytop]ankton dynamics
Their results dealt mostly with one- or two-
dimensional analyses over time as compared to the three-dimensional coverage discussed here.
2.
METHODOLOGY
Our approach to study stranded intrusions of upwelled Gulf Stream water on the shelf o f f F lorid a and Georgia was based on information from previous studies. the dimensions of
intrusions
Knowing the range of
and the p r o b a b i l i t y that they could remain on the shelf f o r
Particulate m a t t e r
longer than 2 weeks, we decided to: alongshore transects at
(I)
3-5
locate water with temperatures less than 20°C from
zne 25 and 40m isobaths with a towed temperature sensor, and (2)
cover the area i d e n t i f i e d with at least 3 onshore-offshore transects at a6 to 55km intervals with stations spaced 13km apart.
This sampling strategy was repeated at least every week.
The area with the highest ~robability that intrusions would occur on the eastern Florida shelf is between 29° and 3i°N (BLANTON, ATKINSON, PIETRAFESA and LEE, 1981) during early June to mid-August when north to northeastward winds favor the formation of intrusions (WEBER and BLANTON, 1980).
Therefore,
the studies described here extended from mid-June to late
August 1981.
From mid-June to the end of duly 1981, using the R/V
Blue Fin,we determined v e r t i c a l temper-
ature p r o f i l e s with an Expendable Bathy-Thermograph
(XBT) using Sippican probes.
This was
followed by Niskin b o t t l e casts to determine concentrations of n i t r a t e (ATKINSONet a~.1988), chlorophyll with a T A I I
a /
phaeopigments
(YENTSCH and MENZEL, 1963) and p a r t i c l e
size d i s t r i b u t i o n
Coulter Counter (PAFFENHOFER, 1983) at 3-5 selected depths, depending on the
depth of the water column.
We then collected zooplankton from d i f f e r e n t depth ranges (see
PAFFENHOFER, SHERMANand LEE, 1988).
During August 1981 we used the R/V Cape FlorYx~equipped
with a CTD on a rosette sampler f o r measuring physical variables and to c o l l e c t water. While we ran a transect about every 15 hours,
i r r e s p e c t i v e of the time of day on the R/V
8~ue Fi~, transects on the R/V Cape F~orida were r e s t r i c t e d to daytime.
The same transects/stations close to several of the current meter moorings were occupied repeatedly over the 2 month study period (Figs.
I and 2).
Large phytoplankton c e l l s f or
measurement of c e l l width were obtained from zooplankton tows with a 103um mesh net.
Widths
were measured at 320 x with an inverted microscope. The methodology used to c o l l e c t and process current meter data is presented by LEE and PIETRAFESA (1988).
3.
RESULTS
We w i l l present a chronology of intrusion events as observed from current meters sited at the 30 and 40m isobaths (Fig.3, LEE and PIETRAFESA, 1988).
Because we did not obtain data
from near-bottom current meters at moorings 8 and 14, we present data from current meters at 17m depth which were usually within or s l i g h t l y below the thermocline.
Subsurface intru-
sions are indicated in current meter records as a cold onshore flow persisting for several days to weeks in the lower layer.
3.1
Sequence of Major Intrusion Euents
We observed 2 major intrusions during the 2 month study period.
Onshore, near bottom flow
occurred from June I0 through 27, 1981 at Mooring 2 and from June 18 through 27 at Mooring 9 (Fig.3, Intrusion I, ATKINSON e t a l .
1988). This upwelled water was displaced southward
3 "~r~
G.-A.
P~,FFE~,I~OFER ~ n J T
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==
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m~[[er
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#1-22 DOE • ND80 meterological buoy • Coastal meterological statiof ~ , Coastal sea level ---Tidal channels
Cape
28
80°
81 ° FIG.2.
C o n t i n e n t a l s h e l f o f G e o r g i a and n o r t h e r n the l o c a t i o n s o f c u r r e n t m e t e r s .
Florida
showing
.:-'~
G.-A. P~FFE'~HOFERand T N L~a
=--" 1981 g-
JUN " =0
-'-
JUL
1981
_
ii'l
==
,°.
II
~L' I •
AUG
-
,
x
i'~
¢4
I
,'~
zz
z~
"
i,
JUN :
,uo
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~
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...\\I,
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i,
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-
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IZ
JUN
FIG.3.
's
AUG
JUL
1981
.-) Io
~
I~
:'.
~4
Z"
JUL
r--r'
r
AUG
Current d i r e c t i o n s and v e l o c i t i e s of the f o l l o w i n g current meters: 8-I (17m depth), 9-2 (Tm), 9-5 (37m), 14-I (17m). The f i r s t number represents the array ( F i g . 2 ) , the second the r e s p e c t i v e current meter on the array. Mooring l o c a t i o n s can be found on f i g u r e 2.
and o f f s h o r e between June 28 and July 7.
From July 20-30, there was a continuous, onshore
near bottom f l o w at Moorings 2 and 9 ( i n t r u s i o n
2, ATKINSON et a~. 1988).
displaced southward and o f f s h o r e from J u l y 31 to August 3. f l o w from August 5-11.
Therefore,
most of
intrusion
I n t r u s i o n 2 was
This was f o l l o w e d by onshore
2 remained on the s h e l f
during our
study period. 3.2
Indiuidua~ Intrusion Euents and Phytop~ankton Patches
Events and patches w i l l nitrate,
chlorophyll
(Equivalent Spherical below 16°C at i t s
be described by presenting near bottom d i s t r i b u t i o n s
a and p a r t i c u l a t e matter, the l a t t e r Diameter).
Intrusion
center at m i d - s h e l f
in the size range from 2-128~m ESD
I was stretched
(Fig.4).
of tB~peratur~
along-shore with
temperatures
The northernmost p a r t of i n t r u s i o n
I (20°C
isotherm) was not covered. N i t r a t e in the northern p a r t of the i n t r u s i o n was l a r g e l y exhausted
Paniculate matter
81
79
80
379
79 ~32
9C
91
? /
.,
;..-
31
/
i! L,f< Botlom
30
~
~Ji"'
It}
I
Te~ature(oc) June23-26 ~98~
f/,ll,
29
i
(.j
o,'
-.:
1/"/ i BottomNO3 '. CuM)
l#/, 1 7 • 3
2"
:
2
30
-26
5i Z
2g
•
/.o..
<
,.:
.,
\ ",~'|" ~l~'~" ] Part~cleVolurne } june23-26
% 81
FIG.4.
80
7g
"-, 81
i :,, 80
79
Near bottom d i s t r i b u t i o n of temperature, n i t r a t e , chlorophyll a and p a r t i c l e volume between June 23 and 26, 1981. Patch I is i d e n t i f i e d with Intrusion I.
Chlorophyll a (patch I) was highest at mid-shelf increasing from north to south, while particle volume changed l i t t l e along the same isobaths (alongshore). Chlorophyll ~ and particle volumes were highest where temperatures were lowest. Chlorophyll concentration was probably directly related to the amount of nitrate available (ATKINSON, O'MALLEY, YODER and PAFFENHOFER 1984). Our plan to study intrusion I in a time series was not achieved because of a major breakdown of the R/VB~e F~n after completing 2 transects one week l a t e r . JPO
19:3/4-K
.~;~(j
G.-A. P~,FFE',;HL~FERand T x,:. LeE
8]
8D
7g
91
80
FS ::::~-//,~/ :' 05: "~'
2
,-. -o~ /
I\
7g
ti:.i\\
Temoera,u,e
,g'';
31
I \ ?:.
i
i
,";"
:!':
31
;~,o,s ! ~ : ' ,/
'd.
30
' o "' ]/
I \ '~"', k,./ ~; i 2~'-,. . . .
':
i
"}'
Bottom
(mg/m 3)
\ ~'~
& 81
FIG.5.
80
7g
i
Botto
i, ;
(mm3/O
-,-.: i \ 81
80
Near bottom d i s t r i b u t i o n of temperature, n i t r a t e , c h l o r o p h y l l a and p a r t i c l e volume between July 13-16 1981.
Our coverage of u p w e l l i n g was resumed from J u l y 13-16 ( F i g . 5 ) . lowed the bottom topography and c o l d , n i t r a t e - r i c h Concentrations t h e r e of c h l o r o p h y l l patch 2 ( F i g . 5 ) . It
seemed to
near
and O.5um n i t r a t e
from a f r o n t a l
bottom
isopleth,
s h e l f near the 30m i s o b a t h . ~)
and i t s
a and p a r t i c l e
By t h i s time isotherms f o l -
waters were located along the 40m i s o b a t h . volume were high and formed phytoplankton
This patch was not associated w i t h i n t r u s i o n s 1 ( F i g . 4 ) or 2 (Figs 6 and 7)
originate
One week l a t e r
79
center
isotherms
eddy ( f r o n t a l
eddy # 6,
had moved toward shore
LEE and PIETRAFESA, 1988). (Fig.6):
which were at the 40m isobath by J u l y 13-16, Patch 2 had increased in area ( l u g
1- I
The 20°C isotherm were now at mid-
i s o p l e t h of c h l o r o p h y l l
had been d i s p l a c e d about 25km west-northwestward towards shore
(Current
81
"I j
,./
FIG.6.
80
.,.) /:
/
)
;
A
7g
,
,!"
,;
./"
0
"i.....
'"
".t /
~ /"
". i
/
" ._)!
2
."t
8!
c::
"I
,
80
i i,
.... ,. I!
,." | v,.""' i Bo,Io~ '",' I" "?l' i ~°'"~'° Vo~.~
.....
i!'~.~(~'P.tch
.'f'i,
',
2
ao,om
¢i L."~"J/ ,l
c.../..<--
...>." J
! i,
"i Int'uilon
,.ii ]
/,.....
80
\tl":'~!f ~(I' .o~
I
f
81
79
29
"'J
31
79 32"
Near bottom distribution of temperature, nitrate, chlorophyll a and particle volume between July 22-24, 1981.
81
..-
80
......i
," ,',
7g
(°C) July22-24,198'
i
i,
i
..""
29[~,
,
:i
.,.:!
"f emperalu~e Bollom
Bl
II
80
3017
"
?"
' ,
'
"-.
.Jl ~ ,'~:~
'
~ ':, ',
.,
~
:
~)LI
h.(irn31 July2Z~)91*J~l
(.II[ d
iJ .......
IHt< I
Y~"
i
td I
\\
* /['
." . <,--
on
,ill '1
," /
D';~,
I(
l
~#
~l
"
',
<
',,
'
{ ll~l ]
.+iulv ?/ .'U
NLJ
j
Itlt I
/ ~J
-'~
g,'
/t?
i,ill i
7-
I ~1~111~111
-T--
j i7
H()
Near bottom distribution of temperature, n i t r a t e , chlorophyll a and particle volume between July 27-29 1981. -Patch 3 starts to develop in Intrusion 2 on the middle shelf.
gJl
,>]
~'J
~ {l ,Jul,~ / ~ .~'.
~0
:' ~ :'
~ ~
FIG.7.
2~
dl
32
#31
2~_
G-A PCFFE~HOFERand T N LEE
Meter 9-5 of Fig.3). of
intrusion
At the ~Om isobath between 29°30 ' and 30°N we observed the leading edge
2 (Fig.6).
One week l a t e r
(July 27-29, Fig.7)
17°C water was at mid-shelf
at 30°N. This indicated the presence of i n tr u s io n 2, which extended somewhat beyond 30°30'N. This conclusion was supported by the n i t r a t e d i s t r i b u t i o n .
From July 23-29 water displace-
ment at current meter 9-5 was about 48km north- to northwestward and at meter 14-I was 41kF~ northward (Fig.3).
Therefore, patch 2 was now located f a r t h e r north, as shown by the d i s t r i -
bution of chlorophyll ~ and p a r t i c l e volume (Fig.7), the past week.
both of which had increased during
South of 30°30'N, where n i t r a t e was high and chlorophyll a and p a r t i c l e
volume lower, phytoplankton patch 3 was s t a r t i n g to develop (Fig.7). Ten days passed before the next transects on August 6-8 (Fig.9). on the shelf was f i r s t for
4 days.
After
displaced northward u n t i l
During that period water
July 30, followed by a current reversal
that flow was again towards shore (Fig.3).
For current meter 8-I
we
calculated that water displacement was 22km north to northwestward (July 29-30), then 47km southward (July 31 to August 3), and eventually 18km north- to northwestward (August 4-6). Because there were no current meter data f o r near bottom flow at the 20 and 30m isobaths (except at Mooring 2),
and because knowledge of water flow at these isobaths is important
in late July and August, we attempted to develop a flow scenario from near bottom temperature data (Fig.8) 14-I
and compare them with the water displacement data from current meters 8-I and
(Fig.3).
A decrease of
near bottom temperatures indicated strong onshore/northward
flow between July 21 and 29 at Moorings 2, 8, 13, 14 and 15, which were located on the middle and inner shelf (Fig.8). 1988).
This signature showed the presence of intrusion 2 (ATKINSON et a~.
Warming from July 30 or 31 to August 4 indicated pronounced southward and offshore
flow at Moorings 2, 8, 9, at Moorings 9,
13 and 14.
This was followed by a marked temperature decrease
13 and 14 from August 5 on (Fig.8~ i n d i c a t i n g onshore- to northward flow.
These observations agree with the flow data from current meters 8 - I , It
9-5 and 14-I (Fig.3).
can be concluded that the water encountered on mid-shelf at 30°N on August 6-8 (Fig.9)
had been in the area since July 28. By August 6-8 most of the middle shelf was covered with intrusion 2 water (Fig.9). rations of n i t r a t e ,
Concent-
compared to July 27-29, had decreased and those of chlorophyll a and i
p a r t i c l e volume had increased. 30m isobaths at 30°N. intrusion 2 (Fig.7).
These maxima represented phytoplankton which originated mainly from
This patch contained the large chain-forming diatoms Stephanopyxis
(Fig. 10), O~inaz'dia f~aoof~a onshore flow. 8-12 ( F i g . l l ) .
Maxima of these 2 variables occurred between the 20 and
and Rhizoso~enia
sp.
s p p . After August 6-8 there was further
The 20°C isotherm was located further north, at the 20m isobath, by August The increase in nitrate was caused by another intrusion (Fig.11).
Now we
could distinguish between one (chlorophyll a / particle) maximum along the 20m isobath (patch The nearshore patch was characterized by Stephanopgx~s sp. (near 100um cell width) (Fig.10), which was abundant at stations where particle
3) and another along the 40m isobath. concentrations
were highest.
The same water remained on the shelf during 13-16 August,
despite the i n i t i a t i o n of weak southward flow by August 14 (Fig.3).
Southward flow was
also shown by the more southward position of the 19° and 20°C isotherms on the middle shelf (Fig.12).
Nitrate was not exhausted at mid-shelf, and patch 3 was s t i l l
positioned along
Paruculat~ m a u e r
383
29,, . ii ma • 27 " ' " "
1
ml•
251 '
~
23"
~--=""~"
1 ]
mlJtmmlw
"'"
21 i ~ 19"
I
• J
T
• ,13 """ ' "
14
~
,~ccc,:~?
:
.
15 .
.
.
.
¢,
2 I~o?-----~
~
191.~°o
OooooO 15
.
.
12 15
.
20
July
FIG.8.
~o~ oo O O oo32
~o
.
25
.
°o .
30 1
.
5
.
10
August
1'5
1981
Near bottom temperatures from moorings 2, 3, 8, 9, 13, 14 and 15. Mooring locations are shown on figure 2.
the 20m isobath (Fig.12).
Concentrations of chlorophyll a and p a r t i c l e s at mid-shelf remained
low despite abundant n i t r a t e .
The offshore maximum of phytoplankton remained near the 40m
isobath and was probably of d i f f e r e n t origin than those in patch 3 because the average width of Stephanopwx~s there was much narrower than nearshore (Fig.t0).
O~
0 ~ 0
. . . . . . . . . . . . . °°
~
CO
.... "'"~-.
~---
~
~-o
CD (-~
CO
77 ~.
:|
,
'~; . : %
,
,":~,~=~,~.. ,=
7.
"',,,
•
~'~
,
D
•
,~
!i
C
F
ig.11.
2g
30
31
32
2g
30
3!
2t2
I
,
k
l
/
""t
....
/"
O0
(mg/m a)
~o,,om C.~ ~
,
:
7r3
os
i
/
('l ~
l :
I.~ !
.o'v"
,~:'?' t \
"?H" ' t",~" {
~u-'~
.
"~
"
al
.o
31
79
Inm~/)
29
Particle yo~rne 3 0
oo,,_
':..----! .:)
i\
'~ .).<'"s ........,:'' /"
~tc.ia
i,
"J
u,:
i,o,tom.o3
:'., ,.:
t ~ ."..ttS:.:::~ ~.
.
80
." i': """" '!, ~:! .......
¢"" • i~"
I Y
81
8-12,
1981.
Near bottom distribution of temperature, nitrate chlorophyll ~ and particle volume between August
~ , ,1{I, / ~' '/~" !
81
7g
...,~...'~
89
"['~ I 1 Temperature ['C] ~8 !August 8-12, 1981
,:.t"
,""
<,L:
81
.o,,..
Fig.12.
->Of
~
.
....
....... .:
...... : ,
.:."
:[::'.: { ~
;'
i ,,,,
/
""~ /
\ ":~I 'X~'~
\"::':/~,
":'
UO
30
!?g
"
I
I
U}I~- , ~T':
:
_
B~,.o,..
,.i.,
,t,~, '
~:." .:~. ...... .,: %'""
G1
'31 I~ "II. _'5 "i~:;I",i"')''//,,'
I
79
a9
al
,,,
~l
7~
Near bottom distribution of temperature, nitrate chlorophyll a and particle volume between August 13-16, 1981.
\
,
~-" ,.~'~,. ,-"':'? ;'/"
~,
IIb:-. ~
!
i
,.' /
90[\I,~"i'/.." : ~o, om~,.._. b ", i (n~j/n a)
a:
2
'7/ /'
:Y f""
,::...... ,:
80
I \ .:-.x..::':..i,.,,-~,~o ,~,
~
..4 ;~"""
<:
.or t"~'<-J
31
~
81
P,
.~0
3.3
O ",-'%.. Pa,FFE'~.HOFER .~nd T. N L~.E
Ver~ica~ D~s~rib~vton of E~mper~re,
Intrusion I
(patch I ) ,
extended h o r i z o n t a l l y and v e r t i c a l l y over most of the middle shelf,
as indicated by a shallow upper mixed layer. cline
(Fig.13a).
.Vt=r~e, Ch:~ro?hg~ ~ ~nd ?arv!cie ~ o ~ e
N i t r a t e was abundant in and below the thermo-
High concentrations of chlorophyll a and p a r t i c l e volume throughout much
of the water column indicated l i t t l e
grazing a c t i v i t y (Fig.13b).
One week l a t e r the thermo-
c l i n e was deeper and n i t r a t e was exhausted, but chlorophyll a and p a r t i c l e s were not s i g n i f i c a n t l y reduced (data not shown). Patch 2 was observed over 3 successive weeks (Figs.
14, 15 and 16).
mixed layer (UML) consisted of r e l a t i v e l y cool water (<28°C, Fig.14a).
Initially
the upper
Concentrations of
chlorophyll a and p a r t i c l e volumes were lowest in the UML and increased with depth (Fig.14b). By July 22-24 the UML was warmer and upwelled water had moved towards mid-shelf as seen from n i t r a t e concentrations (Fig.15a). in upwelled water of 20°C or less.
N i t r a t e concentrations of >I~M were usually observed
Concentrations of chlorophyll a and p a r t i c l e s had inc-
reased in most of the water column along the 2 northern transects (Fig.15b, patch 2) and were f a i r l y
low throughout the water column along the transect with the strongest upwelling
(Stations 405 through 410). shore (Fig.16).
By July 27-29 upwelled water had been advected f a r t h e r towards
Then differences in the v e r t i c a l d i s t r i b u t i o n of chlorophyll ~ and p a r t i c u -
late matter between transects were obvious. at the same depths at a l l
stations,
were in and below the thermocline.
In the northern transect,
the thermocline was
and higher concentrations of p a r t i c l e s and chlorophyll Along the middle transect ( i n t r u s i o n 2) the thermocline
domed at Stations 506 and 507 and the isopleths of chlorophyll a and p a r t i c l e volume f a i r l y closely followed the isotherms. We chose the center of patch 2 (Stations 313, 414 and 515) during the 3 successive weeks to f o l l o w changes in the patch and o~erlying water column (Table I ) .
Largest changes occurred
in the bottom layer where chlorophyll a increased more than p a r t i c l e volume. increased from July 23-29.
Phaeopigments
We divided p a r t i c l e volumes into 3 size ranges of which the
middle represented sizes which can be ingested at high rates by most crustacean plankton. The largest percentage of p a r t i c l e concentration occurred in the middle size range in the bottom layer throughout the study period. From August 6-16 the bottom layer of much of the middle and part of the inner shelf consisted of intrusion 2, i t s n i t r a t e l a r g e l y depleted. from a new i n t ru s io n to the mid-shelf during the f o l l o w i n g 4 days (Fig.19a),
Onshore advection at 30°N introduced n i t r a t e
(Fig.18a,
Stations 634-636) which was not taken up
Phytoplankton patch 3 which we described e a r l i e r
(Figs. 9-12), was not l i m i te d to water below the thermocline as the Imm3 1-I isopleth reached the surface at almost a l l stations (Figs. 17, 18b, 19b).
At most of the same stations con-
centrations >O-5ug 1-I of chlorophyll a occurred in part of the UML. As in the case of patch 2 we compared concentrations of chlorophyll a, phaeopigments ~ and p a r t i c l e volume from stations near the center of the patch s t a r t i n g on July 27-28 (Table 2). Concentrations of the 3 variables increased in the thermocline and intruded water through
~ 2 ~ '
AI .. ?3 24 I'~ll
N
L
L~I~R[
-
I
,,%
I
,
l
~
I
--~..
I]ISIAN~f ~ F S I ~
2,) ICJBI
23 24 19Sl
iv,M)
J~
r~)l
J J ~, z5 VJ~I
*
q~M)
(fiN)
*
I
I
I
=i
5L
IC
bO
I ]0
10
I0
'i1
L
I
It)
(it
~t
,
JU St) /U UP~IANII {~l',}W~l
~i~ 2% I'JHI
C.~L a
gU (~,N)
I LU
"7
~
V,A.,.,
"~
and p a r t i c l e volume from June 23-26,
1981
'~[j
L"
\
x
](j
"T
Dp, IAI~(I L~I',IIUffl
JO
ItllllJ/ Ij
]lJ
u ~i.
~Ctl, 2~ i,m J
____.~
t , . . . ~IH
Vertical distribution of (a) temperature and nitrate, and (b) chlorophyll a
(KN)
'-,~ ~', -
FIG.13
t}Ibl^Nli
, o ~
t~
lfl
('~2J
'°~~!J I0
B~
~j(j
_ .
(~N~
. :Z
I Ill
F¢ B c~
T
10
5~
I/)
10
_
,
,
I
Io('l
I
,
,
,
I
I
I
,
,
,
I
I
DISMN~E ~FSHi}R~
I
I
15 I'RII
,
I
,~Jy
(N:}
I
. J~iy 15 IG l(~llt
i
.
i
FIG. 14.
(~N)
,
,
I
NO3
(~I,11
I
I
13-16,
,
I
I
i
,
I
z ~ _ _
,
I
"--.
I
'
II0
b(l
I0
\
i
i
,
{~,lu/,,, J
CHL a
I
CHL ~
\..
i
,
i
l I
,
DISI^~[ ~ f ~
i
I
5O _ July t5116 19BI
i
I
i
I
i
l w I L - - -
(KM)
I
1981.
Jill k~ 14 r J ~ (IIWII
(.wn3/0 .wJy 15 16 IgBI
\ DISIANCE ~FS~UWE (~M)
:G?71,
_~ - ~ ' - . ~ ,
distribution of (a) temperature and nitrate, chlorophyll a and particle volume from July
71,) ~ DISI^N~[ OFISIIOR~ (~M}
I
July I~ l~tll
NO~
I
Vertical and (b)
I
15 16 I~I
10
~-.\
/I f
fro
"Z
Z I O-
>
C~
K
'\
511
I0
i~
FIG.15.
3U ~J 70 IJISIANC[ O~tSt~R[
I'
Jl.~y 24 191~1
,oF\
51)
,
i
I
IO
L._
,
,
I
,
I
,
]0 SO 70 90 DISIAN£E {~-FSb~)RE (l~M)
l
J~lly ,';' IItU I
lib
1
-~ _
,
llO
--L
I0
5O
I0
a
,
T
,
T
( I1)l,l/m 3 )
i
T
,
;
i
7
T
T
i
;
,
~
I
7___7 .
.
.
.
.
.
.
,
(lllniJ/i)
iiiiipj/i)
i
[llltl)/l)
.
.
and (b) c h l o r o p h y l l
DI$1ANCE OFFq~IOR£ (KM)
Im,iJ/m :1) Jl~y 22 ~:J 1981
CllL
i
(fl~tlll j )
V e r t i c a l d i s t r i b u t i o n of (a) temperature and n i t r a t e , and p a r t i c l e volume from J u l y 22-24, 1981.
90 O',M)
.
I
24 lull1
J~l~ ,'2 / a P,tI~ I
NO 3
i\.
r
,~
T .
.
,
.
.
i ~
.
.
.
_ . [ _
~ _ L _ _ L _
~'~
* . . . .
a -
OlS1ANC[ OtFSHOI~ (KN)
~~-~_
Y_~_7__i~ ~ - , f - -
1
.
-io
.t.
x~ ~2
u
ILl
,
I
to
I
:tJ
l
,
-
'
,
,
,
1
+
I
I
-
,
I
,
I
l
i
FIG. 16.
9U tlxM+
,
,
_'2.\_
I
i
flu
I
t+oj
10
,~+IA+
"It,
DI',IAN~i O~f';+~tJRi
JO
~O
M~MMM
./tll+ +++J I~)+~ 1
II+M'
t,~O I
(pMt
~]~t)
oo4~
~':
.
I lU
,
,
h
.
~
~
....
.+
h
'
,
~
,I+
(~M)
q
\
l
',~
cc
,
OISIAPdLE UtlbtlOlq
,; ~h''~b
, .q.,
I
(n,,),m ~I
u,~ u
+
L i l t ,+ ,l,q +,+fl ~.++, '+, q J d l .
~,
,
,;0
I
L__
~,
t ~IU
I
,
/I
]LJ
I
(ltlSlltJRl
r
''
,
,
?'
~w)
_ +J"
~'7
UIS/,~NI[
JU
t
~ . , i r l . h' ' W . I , W , . '
-.
Vertica! distribution of temperature, nitrate, chlorophy|l a and particle volume from July 27-29, 1981.
.
-.c. 'i-I
"k'% ~
I
+t~ ',~; 'tJ t,l+,l+.'lll [,+t*,+It)lil
I
I
ltl Xl, '+, '1 I
I
..............
~uU~
'"" ~ ;
,,,I
I
-
u+
,
t'2
1
!'I
m m
Z
-t
m
m
7
o
I0
I
....
~0
FIG. 17.
~
<',L'
7°
IlU tO
~J ~3 70 OI$1ANtl L~lS~il~
~
IJO ~P,M)
I I0
o • ~
~.~
'\ \
~lSIA~k ~ F ~
~
V e r t i c a l d i s t r i b u t i o n of temperature, n i t r a t e , volume from August 6-8, 1981.
~1o
~
~-
.
S
J._~
~
.
l
~,,,L~,.:~:~ i., ~ -,;,,
I
c hlo r o p h y ll a and p a r t i c l e
¢KWI
t
E
SO
tO
I0
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iO
5O
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I0
,
1
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I
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I
,
,
A~jusi
,
,
,
.-L
t
t
t
,
,
I
t
I
I
FIG. 18.
I ~3 (KW)
,
,
~-~1~
,~ ~ ~
~__1
,
I _~ _J , t 3U ~3 70 DIMANC[ LWFS)~[
I
ll) i u ~ i
• | O, lll~)l.th, u ~,llj)
__~
t
I UlIg)Ul allll e
t
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,
,
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,
I
,'f
,
%.f.
t
I
,'.f
,
,
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,
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,~
30 50 70 DISIANC[ O~FSHORE(KW)
,'f
t
AuguSl 11 19{}1
NOj
I
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I
,
,
110
__
i _
I
5~
50
ID
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,
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AuguSl II 19~I
1(3
.
,
.
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.
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.
e
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.
i
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IT:T.
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I
(.I,~I0
Vertical distribution of (a) temperature and nitrate, and (b) chlorophyll a and particle volume from August 8-12, 1981.
i I I0
i
I
I
A uijust I~,i9~ t
I0
L ~ l
I
,--~-_
r" tn m
p~
0
m
7
0
lit
5o
~)
IO
5O
I0
f~UrdhWe
,
I
,
,
,
I
10
,
,
,
t
I
',,'
,
,
I
1__,
,
I ....
i
50
?0
gO
FIG.19.
NO 3
,
I ~
,
.o
.o ,
o~
l ,
' I
DISIAN~E OFFf~IOR£ (WW)
Augu~l 13.198 1
\o,
Aul)usl 14 1981
NO,I
AUC~U51 15 19BI
NO3
I
Au~,51 16,1981 ,
I
5O
IU
I0
50
I0
a
*
I
,
,
I
.
~
I
,
l
I-
I
,
,
,
\
I
I
I
,
,
,
I
L
I
30 50 OlSI^NEE OFFSH(]~[ (KM)
CHL a (mg/,n~) Au~st i~ 19~i
I
(.w~/l,(~} Al;~llc.lFl I')III
I
A~(Ju~I 15 19Ul
OL (.~.#)
l
(.~vm3)
O1
Z
,
,
--L_
L
/
I0
(f1|n3/LJ
and p a r t i c l e
volume from August 13-16,
1981.
\
\
#J b~l 70 ~tJ [}ISI^N~[ UI}~K~E (~H}
PJI, ~ V,~J,w, (o.iJ.l}
/'
P~ t~b: wa.i~ (lla~i]/1)
P~V~,~
Vertical distribution of (a) temperature and nitrate, and (b) chlorophyll
I10
L._>' 1
I
I
DISI^NCE L~FS~edRE (KM)
30
,' ~'~,'
IoC) _AuQU;I t4,1901
I
lempe~at~e {%) 15.Bill
I
[oc) ~ J S l ~6,1981
I0
110
F¢
3
414
514
23 July 1981
29 July 1981 0.17 0.30 12.00
T I
9,05
I U
0.19 0.31
U
5,23
!
T
0.23
T
4.10
0,19
0.12
0.32
0.01
0.07
0,99
0.13
0.08
lJg I - I
ug 1- I
0.19
Phaeo.a
Chl.a
U
Depth Range
*ESD = Equivalent Spherical Diameter
I = Intrusion
T : Thermocline
U = Upper Mixed Layer
313
Station
5.42
0.55
0.42
5.55
0,50
0.28
4.61
0.66
0.35
Total Particles mm3 1- I
0.43
0,09
0.08
0.43
0.13
0.07
0.43
0.14
0.07
2-8
3.26
0.27
0.21
3.06
0.23
0.14
2.26
0.25
0.17
8-32 mm3 1- I
1.73
0.19
0.13
2.06
0.14
0.07
1.92
0.27
0.II
32-128
Concentrations in size ranges (~m ESD*)
8
16
19
8
25
26
9
21
21
2-8
60
49
51
55
46
48
49
38
47
8-32
32
35
30
31
28
27
42
41
32
32-128
% t o t a l volume w i t h i n s i z e ranges
Patch 2: Concentrations of chlorophyll a, phaeopigments a, and p a r t i c u l a t e matter at patch center; and concentrations and percentage distributTon of p a r t i c u l a t e matter within size ranges.
15 July 1981
Date
TABLE I :
rn m
p-
C~
rn l
0
r
3.13
I
3.03
I
674
*ESD = Equivalent Spherical Diameter
I
T 2.69
0.27
1.59
T
640
665,666, U
0.40
633,634, U
U = Upper Mixed Layer T : Thermocline I : Intrusion
15 August 1981
I I August 1981
0.66
0.32
1.60
I U
0.37
T
T
0.12
U
602
-I
600,601
~g 1
Chl.a
6 August 1981
Depth Range
501,502
Station -I
0.59
0.13
1.64
0.57
0.16
1.15
0.38
0.16
0.95
0.09
0.06
~g 1
Phaeo.a
6.68
0.53
4.49
1.24
1.02
2.55
1.38
I.II
1.36
0.74
0.33
Total Particles 3 -I mm 1
0.27
0.12
0.50
0,28
0.22
0.31
0.26
0.20
0.27
0.24
0.II
2-8
0.79
0.29
1.32
0,54
0.61
I.I0
0.72
0.64
0.58
0.33
0.15
mm3 1-1
8-32
5.62
0.13
2.67
0.42
0.18
1.14
0.40
0.27
0.51
0.17
0.07
32-128
Concentrations in size ranges (~m ESD*)
4
22
11
22
22
12
19
18
20
32
33
2-8
12
54
30
43
60
43
52
58
43
45
45
8-32
84
24
59
34
18
45
29
24
37
23
21
32-128
% t o t a l volume w i t h i n size ranges
Patch 3: Concentrations of chlorophyll a, phaeopigmentsa, and particulate matter; and concentration and percentage distribution of particulate matter within size ranges
27-28 July 1981
Date
TABLE 2:
396
G-A. P~FFE%H,~FEK and T. N LEE
August 71, except that p a r t i c l e volume began to decrease in the UML and t~e~mocline after
August 6.
By August 15 a l l
intruded water.
variables except p a r t i c l e volume had decrease~ in the ~ L
and
Division of p a r t i c l e volume into 3 size groups showed that "arge parzicles
(32 to 128um ESD) increased in concentration in the i n t r u s i o n , throughout the study period, whereas small
and medium sized p a r t i c l e s decreased between August 11-15.
were the diatoms G. f ~ a c ~ a ,
Rhtzoso~enia
4.
Spp. and Stephanopyx~s
_~rge p a r t i c l e s
Sp.
DISCUSSION
Subsurface intrusions of Gulf Stream water reach the middle shelf o f f
Flori~
and Georgia
during summer when winds are approximately northward during the passage of a Gulf Stream frontal
eddy (LEE, ATKINSON, LEGECKIS 1981) at times when the Gulf Stream is in an onshore
position (LEE and PIETRAFESA, 1988). While f r o n t a l eddies occur at an average frequency of one per week, major intrusions occurred at the rate of one per month during the summer of 1981.
We repeatedly observed that i n t r u -
sions of cold, n i t r a t e - r i c h Gulf Stream water onto the southeastern shelf led to the development of patches of phytoplankton. cesses.
This implies that patches were induced by physical pro-
Here we compare the t r a j e c t o r i e s ,
longevity and dimensions of intrusions and discuss
changes in chlorophyll a and p a r t i c u l a t e matter over time. 4.1
Trajectories
Patch 1 moved onto the shelf near 29°N between June 10-27 (Fig.3, LEE and P!~TRAFESA, 1988) and was displaced northward as a tongue at mid-shelf
along isobaths.
A current reversal
resulted in i t s offshore displacement between June 28 and July 7 (Fig.3) near the same geographic location.
Patch 2 was displaced onshore at 30°25'N (July 13-22) and then alongshore.
Patch 3(aspart of intrusion 2) which i n i t i a l l y
had the same t r a j e c t o r y as patch I, was displaced
northward from July 20-30 to about 30°30'N about 40km southward and offshore (Fig.3).
(Fig.7,
nitrate),
and from July 31 to August 4
Thus, part of the intrusion which was on mid-
shelf near 30°N (Figs 3 and 10, the l a t t e r showing abundant
Stephanop~xis c e i l s of approxim-
at e ly lOOum width) remained on the shelf forming a well-defined patch (patch 3, Figs. 9 and I0).
Stephanopgxis
used to trace patch 3. it
was abundant within the 2mm31-I
p a r t i c l e volume isoba~h and could be
This patch was displaced towards the 20m isobath a f t e r August 4 where
remained throughout the rest of the study period.
and PIETRAFESA (1985, t h e i r F i g .t 1 ,
our Fig.20),
YODER, ATKINSON, BISHOP, BLANTON, LEE
however, postulated that the phytoplankton
maximum, observed nearshore near 30°N by August 10-13 (patch 3) originated from a maximum located nearshore at 290N on July 27-30 at 29°N and was displaced northward. ing 7 do not support t h e i r findings. followed the same route.
Data from Moor-
Our results show that intrusions I and 2 i n i t i a l l y
P~rtIculatc m3~t~r
81
80
< ."
7g
.~9"
81
w. /
80
i
<{"
¼)
'
~ \ C . L \ \ ~t "
81
',; i
........ " • i. . • ',-'~ \\ ~Ar>--"
,, :~ ,<,.-,,,o r .~,,_= .,<,<>. \"
7g
81
~-~
,; 3
81
FIG.20.
80
2~
7g
7g
,,'t
..A
• ......:,
t;. 'i
;" •
/
C~
:;
' " " 't
a
&~7~. . `
~.;
,
1/
;
I \~._J,-;t
80 •
?
2g~kk,~?.~-'~.',,,
-"-"-'" ' 3 0
eOTTOM
80
< 2
)
t )< 2/
•' ~
,<
7g
Patc
I
V#tV-.k~,~.
~
-~
~ ",
"~L
81
eOTTO,,, /
80
~g
7g
Near bottom d i s t r i b u t i o n of c h l o r o p h y l l a and phaeopigments combined from July 20-23 to A-ugust 10-13, 1981 (modified from YODER et ~ . 1985). Patch 2 could not be i d e n t i f i e d by August 10-13.
395
G.-A. P~FFE~HOFERand T N LEE
The l i f e t i m e of patch I on the shelf was 27 days from a r r i v a l to departure over the middle shelf.
Patch 2 was observed fo r at least 21 days (Fig.5 was the i n i t i a l
observation) and patch 3 f o r 27 days.
and Fig.20 the f i n a l
Because patches 2 and 3 were s t i l l
on the shelf when
we terminated observations, the average longevity of a summer upwelling on the middle shelf probably exceeds 25 days.
This time is s u f f i c i e n t f o r a phytoplankton bloom to develop and
for zooplankton populations to respond to enrichment (YODER e t a~. 1983, 1985; PAFFENHOFER, SHERMAN and LEE 1988). enhanced because (a) no d i r e c t
Persistence of phytoplankton patches on the southeastern shelf is
weak winds induced l i t t l e
vertical
mixing,
(b)
there was limit e d or
influence of the Gulf Stream on the middle and inner shelf (LEE and PIETRAFESA,
1988: HAMILTON, 1988), and (c) large diatoms formed a large part of patches 2 and 3 and were not grazed by zooplankton.
4.2 It
E o r ~ z o m t a l D~mens~om~
was d i f f i c u l t
to accurately determine the horizontal dimensions of patches because of
the spacing of transects and because we could not cover each patch completely. When 1.0mm3l-I of p a r t i c l e volume was used to define the edge of patch I, the maximum width was 63km. shelf,
the length exceeded 140km, and
Patch widths are r e s t r i c t e d by the width of the continental
and thus could not exceed 75km.
Lengths can exceed 180km i f
onshore flow persists
fo r more than 2 weeks, as we observed using bottom temperature d i s t r i b u t i o n to define i n t r u sion 1 (Fig.4).
In this case, the inner, middle and part of the outer shelf were covered
with upwelled water of
less than 20°C over an area exceeding lO,O00km2,
Shorter periods
of upwelling resulted in patches of smaller dimensions.
On July 22-24 patch 2 was about -I 47km wide and exceeded lOOkm in length, using lug chlorophyll a 1 to define i t s l i m i t s (Fig.6).
Later i t exceeded 110km in length and 50km in width, as observed (YODERet aZ. 1985,
Fig.20) on July 27-30 . (Figs. 7,20).
Patches 2 and 3 could not be c l e a r l y separated during July 27-30
The northern part of patch 3 could have contained part of patch 2 (Figs. I I ,
12, Fig.20: July 27 to August 13).
Due to lower layer flow towards shore the width of patch
3 decreased from 52km on August 6 to about 35km by August 13-16 (Figs. 9,12). exceeded 155km on August 13-16.
Its
length
The development of patch 3 was also d e t a i l e d by YODERet
a~. (1985, Fig.20, July 27 to August 13). According to OKUBO (1978)
"...
patch size depends on the balance between the aggregative
process of organism growth and reproduction and the anti-aggregative process of d i f f u s i o n " . Minimum c r i t i c a l sion d-I
patch r a d i i range from 2km at 1 c e l l d i v i s i o n d-I to 40km at 0.1 c e l l d i v i -
(OKUBO, 1978).
On the southeastern shelf the rate of c e l l d i v i s i o n rates should
have exceeded 0 . I d-I f o r a period of >10 days in patches I and 3 (YODER e~ a~. 1983).
This
would allow the patches to maintain themselves as t h e i r minimum dimensions were about 50km.
Particulate m a t t e r
3~9
MACKAS, DENMAN and ABBOTT (1985) note that n u t r i e n t enrich~e~ is o~e source of b i o l o g i c a l variances in the ocean and described 3 factors common to e~richmen~: v e r t i c a l movement of water, strong horizontal heterogeneity of the physical and c~emical environment, and advecr i v e f i e l d s coupled to regional bathymetry. rence of patches at the same location. shelf between 29° and 31°N.
The l a t : e r results in the persistence or recur-
All 3 elements are c h v a c t e r i s t i c of the southeastern
Intrusions 1 and 2 upwelled ont~ the shelf near 29°N and were
displaced northward, mainly directed by bathymetry at mid-shelf, and had strong horizontal heterogeneity.
Concerning the persistence of phytoplankton patches we know only of the study
of SAVAGE and WIMPENNY (1936) who followed a patch of the diatom Rh~zosolenta stg~ifo~nisin the North Sea from June to September 1934. from 8 to 334 1- I . MACKAS et al.
During that period c e l l concentrations increased
Maximum patch dimensions were about 190km length and 90km width.
(1985) found that plankton community patches are usually elongated p a r a l l e l
to local currents
and isobaths, as we also observed on the middle shelf in summer. They
found a 3:1 r a t i o f o r average alongshore:cross-shelf patch dimensions. to species occurrence, the Stephanopyx~s (Fig.10).
Referring s t r i c t l y
patch (patch 3) on August 8-12 had a r a t i o of 4:1
Comparing lengths and widths on the basis of chlorophyll i ,
phaeopigments a and
p a r t i c l e volume, patch dimensions r a t i o s ranged from about 3:1 in patch 2 (YODERet ai. 1985, Fig.20,
July 27-30)
to 6:1 f o r
the nearshore maximum on August 10-13 (Fig.20).
The r a t i o
of 2.5:1 which we obtained f o r patch I is an underestimate because we could not define the northern and southern l i m i t s of the patch (Fig.4). 4.4
Changes in Chlorophyll a and Particulate Ma~ter over Time
Near the center of patch 2 we observed substantial increases in chlorophyll ~ and l i t t l e change in particle volume from week to week (Table I). variable in the UML and thermocline.
There was l i t t l e change in either
We relate the pronounced increase in phaeopigments
in the intruded water to grazing a c t i v i t y since,
in the presence of abundant zooplankton
(PAFFENHOFER et ai. 1988), neither particle volume U or chlorophyll a decreased. This indicates rapid phytoplankton growth rates and not phaeopigment formation due to lack of nutrients. In the UML and intruded waters of patch 3 we observed increases, and then decreases, chlorophyll a, phaeopigments a and particle volume (Table 2).
of
We attribute these decreases
to grazing effects, particularly since the decreases occurred in small and medium-sized particles.
The probability is low that these decreases were due to sinking because the concentr-
ation of large particles, which usually sink faster than small ones, increased nearly 50% during that period.
Total particle volume in the intrusion increased because the large part-
icles were too wide (mostly adult Eucalanus pileatus KNOWLES, 1978).
> 60um) to be ingested by zooplankton other than copepodid and and late copepodid and adult
Temora s t y l i f e r a
(PAFFENHOFER and
While chlorophyll a did not increase between August 6-11, the concentration
of large p a r t i c l e s did.
The small amount of phaeopigments on August 15 indicates that (a)
zooplankton had grazed on d e t r i t a l occurred due to lack of n u t r i e n t s .
and fecal
material
and (b)
little
phytoplankton death
'l)d)
G -A. P-kFFE%HOFER and T N LEE
To determine the development and persistence of a plankton bloom i t
is essenzial
to zrack
the bloom and the water mass in which i t occurs. This was achieved by combining 3-dimensional hydrographic surveys with simultaneous current meter observations.
Our results show that
complete coverage of an intrusion event requires that the study exceeded 4 weeks in time. Because of the lack of s u f f i c i e n t time the fates of patches 2 and 3 were not determined. Earlier observations
showed that mid-shelf water (entire water column) at 30° and 31°N is
slowly displaced during summer in an alongshore direction (ATKINSON, LEE, BLANTON
and
CHANDLER, 1983). Thus, the longevity of patches 2 and 3 should have been considerably longer than the 21 and 27 days, respectively, that were studied.
Our observations lead us to con-
clude that most of the particulate matter in patch I was eventually advected from the shelf. Most of that in patches 2 and 3 remained on the shelf and was eaten by zooplankton, benthos,
or microorganisms.
ACKNOWLEDGEMENTS This research was supported in part with the Department of Energy by contracts 76EV00936 and DE-ASO5-76EV05163, and National 0CE85-00917.
DE-ASO9-
Science Foundation Grants 0CE81-17761
and
The captains and crews of R/V B~ue Fin and R/V Cape Florida cooperated profess-
i o n a l l y and contributed to our success.
Without the assistance of Byron Sherman, J u l i a
C l i f t o n , Carolyn Benson and Thomas Brandt we could not have accomplished our goals. Hagin provided Figure 3.
Dannah McCauley typed the manuscript.
Boyette prepared the figures.
Bill
Suzanne Mclntosh and Anna
Chandler provided base map software.
comments of two anonymous reviewers improved our manuscript.
Michael
The constructive
We would l i k e
to thank a l l
those who supported our e f f o r t s .
REFERENCES ATKINSON, L.P., T.N. LEE, J.O. BLANTON and W.S. CHANDLER (1983) Climatology of the southeastern United States continental shelf waters. Jo~rna~ o f Geophysical Research, 88, 4705-4718. ATKINSON, L.P., T.N. LEE, J.O. BLANTON and G-A. PAFFENHOFER (1988) Summer upwelling on the southeastern continental shelf of the USA during 1981: Hydrographic observations. Progress in Oceanography, 19, 231-266. ATKINSON, L.P., P.G. O'MALLEY, J.A. YODER and G-A. PAFFENHOFER (1984) The e f f e c t of summertime shelfbreak upwelling on n u t r i e n t f l u x in southeastern United States continental shelf water. Journa~ o f Marine Research, 42, 969-993 BLANTON, J.O., L.P. ATKINSON, L.J. PIETRAFESA and T.N. LEE (1981) The intrusion of Gulf Stream Water across the continental shelf due to topographically-induced upwelling. Deep-Sea Research, 28A, 339-405. HAMILTON, P. (1988) Summer upwelling on the southeastern continental shelf of the USA during 1981: The structure of shelf and Gulf Stream motions in the Georgia Bight. Progress in Oceanography, 19, 329-351. HAURY, L.R., J.A. McGOWAN and P.H. WIEBE (1978) Patterns and processes in the time-space scales of plankton d i s t r i b u t i o n s . In Spatia~ Patterns in Plankton Communities, J.H. STEELE, e d i t o r , Plenum Press, New York and London, 277-327. LEE, T.N., L.P. ATKINSON and R. LEGECKIS (1981) Observations of a Gulf Stream f r o n t a l eddy on the Georgia continental shelf, April 1977. Deep-Sea Research, 28A, 347-378. LEE, T.N. and L.J. PIETRAFESA (1988) Summer upwelling on the southeastern continental shelf of the USA during 1981: C i r c u l a t i o n Progress in Oceanography, 19, 267-312. MACKAS, D.L., K.L. DENMANand M.R. ABBOTT (1985) Plankton patchiness: Biology in the physical vernacular. Bulletin of Marine Science, 37, 652-674.
Particulate m a t t e r
40[
OKUBO, A. (1978) Horizontal dispersion and c r i t i c a l scales for phytoplankton patches. In; Spatial pac~ern~ ix ?~ank~o~ communities, J . H . STEELE, editor, Plenum Press, New York and London, 21-42. PAFFENHOFER, G-A. (1983) Vertical zooplankton distribution on the northeastern Florida shelf and i t s relation to temperature and food abundance. Journal of Plankton Research, 5, 15-33. PAFFENHOFER, G-A. and S.C. KNOWLES (1978) Feeding of marine planktonic copepods on mixed phytoplankton. Narine Biology, 48, 143-152. PAFFENHOFER, G-A., B.K. SHERMANand T.N. LEE (1988) Summer upwelling on the southeastern continental shelf of the USA during 1981: Abundance, distribution and patch formation of zooplankton. Progress in Oceanography, 19, 403-436, SAVAGE, R.E. and R.S. WIMPENNY (1936) Phytoplankton and the herring. Part I I . 1933 and 1934. Fisheries Investigations Series II, 15, 1-88. WEBER, A.H. and J.O. BLANTON (1980) Monthly mean wind f i e l d for the South Atlantic Bight. Journal of Physical Oceanography, 10, 1256-1263. YENTSCH, C. and O. MENZEL (1963) A method for the determination of phytoplankton chlorophyll and phaeophytin by fluorescence. Deep-Sea Research, I0, 221-231. YODER, J.A., L.P. ATKINSON, S.S. BISHOP, E.E. HOFMANNand T.N. LEE (1983) Effect of upwelling on phytoplankton productivity on the outer southeastern US continental shelf. Conti~enta~ She~f Research, I, 385-404. YODER, J.A., L.P. ATKINSON, S.S. BISHOP, J.O. BLANTON, T.N. LEE and L.J. PIETRAFESA (1985) Phytoplankton dynamics within Gulf Stream intrusions on the southeastern US continental shelf during summer, 1981. Continental Shelf Research, 4, 611-635.