Summary and conclusions

Prog Oceanog ',ol IV. pp J.37--l...&l. ta~Printed m Grea~ Br,tam

Oqru-,'~;i x" $ ¢ } 1 ~ . 50 Ce~".r,ght ~ 1~',,'~ P~-~amon Pre~ pie.

~.',I r,ghts reser~.eJ

SUMMER UP~,VELLING ON THE SOUTHEASTERN CONTINENTAL SHELF OF THE U.S.A. DURING 1981

Summary and Conclusions GLST-',V-ADOLF P-XFFE'4FIOFER', LARRY P. ATKINSON'', J.-',CKSO\ O. BLANTO'~'. THOMAS*'. LEE ~, LAWRENCE R. POMEROY-~ and J.~,~tES A. YODER" "Sk~da.'ay Institute o]" Oceanography. P. O. Box 13687. Savannah. G.4 3141h. (_" S..4. "'Departmem of Oceanography. Old Dormmon Umverstty. Norfolk. VA 23508--4L~00. U.S ,4. ~Rosensttel School of Marine attd Amtospheric Sctence. Untverst~y of Miamt. 4000 Rwkenbacker Causeway. Mtami. FL 3314Q, U.S.A. ZDeparmwm of Zoology av,d Institute ~( Ecology, Univer~'tU of Georgia. Athens. GA 30602. U.S.A.

The goal of our i n t e r d i s c i p l i n a r y study whose results we presented in the preceeding papers was to determine the causes of cold, subsurface summer intrusions of Gulfstream water into the middle and inner shelf, t h e i r frequency of occurrence and size, and processes associated with their development, persistence and fate.

During summer 1981 cold water masses from

greater depths of the Gulf Stream intruded onto the middle and inner shelf when the following conditions simultaneously occurred: northward wind stress, 1988). that all

and (c)

(a) passage of a Gulf Stream frontal eddy, (b) sustained

onshore position of the Gulf Stream (LEE and PIETRAFESA,

Since these observations were made during only one summer season we cannot be certain three above mentioned conditions are necessary for upwelled water to reach the

inner shelf.

We know that frontal eddies are a necessary condition since only those intru-

sions reached the inner shelf which were i n i t i a t e d by frontal eddies (LEE and PIETRAFESA, 1988).

In addition, northward winds or onshore position of the Gulf Stream must be sustained

over at least 4 to 8 days for an eddy-induced upwelling to reach the middle shelf.

A modell-

ing study describing responses of shelf waters to northward wind or an onshore Gulf Stream position showed that either of these two conditions w i l l lead to upwelling near Cape Canaveral (LORENZZETTI, WANG and LEE, 1988).

However, each condition requires frontal eddy upwelling

to start an intrusion across the outer shelf.

Frequency-domain Empirical Orthogonal Funct-

ions (EOF) were determined from current and temperature data from shelf and Gulf Stream current meter arrays, and from buoy-measured winds to obtain coherent current and temperature fluctuations in the 2- to 14-day period band (HAMILTON, 1988). which the f i r s t

He found three modes of

two were related to Gulf Stream frontal eddy and intrusion motions, and

the third to wind forcing.

To i n i t i a t e an intrusion of cold water on the outer shelf, either

a strong intrusion signal

(mode 2) by i t s e l f

or a combination of modes I

(frontal eddy)

and 2 was needed. However, sustained upwelling-favorable winds are needed (mode 3) to transport upwelled water to the inner shelf. I t appears that only additional extended observations during the summer w i l l

c l a r i f y whether all

three of the previously mentioned conditions

need to co-occur for an intrusion to reach the inner shelf or i f two of the three can suffice.

437

J.3~

G.-A. P~FFE~HOFERCt ~/

We observed two large intrusion events between June and August 1931: The f i r s t one occurred between June I0 and 27 when water as cold as 12°C was advected onto the shelf south of 30°N and eventually stranded on the continental shelf, ie. i t was isolated from its cold source by warmer w a t e r .

This intrusion extended over parts of the outer,

middle and inner shelf reaching the shore near Daytona Beach, Florida. I80km and its maximum width was 63km.

Its length exceeded

Nitrate concentrations decreased from about 15 to

I~M (see also ATKINSON, O'MALLEY, YODER and PAFFENHOFER, 1984) in about 2 weeks. 28 currents

By June

at mid-shelf shifted southward and advected the intruded water off the shelf

by July 7 (ATKINSON, LEE, BLANTON and PAFFENHOFER, 1988). A second large intrusion, which also originated south of 30°N, was advected onto the shelf between July 20 and 30.

Within this intrusion, which reached the 20m isobath, a patch of

chlorophyll a/particulate matter

of 155km length and 35km width had developed by mid-August

Most nitrate was exhausted after 2 weeks (PAFFENHOFER and LEE, 1988). l i v i n g bacteria had reached concentrations concentrations

of

> 106ml-I

At that time free

at all depths, exceeding normal

by one order of magnitude (POMEROY, PAFFENHOFER and YODER, 1988).

While

the upper mixed layer overlying the intrusion was dominated by microheterotrophs (bacteria and protozoa), phytoplankton were most abundant in the intruding waters. There the concentration of small- (2 to 8um ESD) and medium sized particles (8 to 32pm ESD) decreased after an i n i t i a l

increase while large particles (32 to 128um ESD), consisting of large diatoms

of up to lOOum width, increased (PAFFENHOFER and LEE, 1988). Grazing control of the mediumsized particles in the intrusion was attributed to small calanoid copepods. Grazing control also seemed operative at mid-shelf during August when chlorophyll a and particulate matter did not increase despite sufficient nitrate (>IuM).

Here the development and decay of a

patch of doliolids in and below the thermocline was observed (PAFFENHOFER, SHERMANand LEE, 1988).

Inverse correlations between the abundance of bacteria and doliolids suggested that

these mucous-net feeders, when abundant, exerted some grazing control on bacteria (POMEROY, PAFFENHOFER and YODER, 1988). Both intrusions had the following characteristics in common: I.

They were advected into middle and inner shelf waters when a cold cyclonic frontal eddy occurred together with northward upwelling-favorable wind stress at a time when the Gulf Stream was in an onshore position.

2.

They intruded onto the shelf south of 30°N, later were advected alongshore, eventua l l y covering most of the middle and part of the inner shelf between 29° and 31°N (8,000 to 10,O00km2) and remained on the shelf for more than 3 weeks.

3.

Shelf waters remained strongly s t r a t i f i e d when intrusions were present.

4.

Nitrate was exhausted within about 2 weeks after the intrusions moved onto the shelf.

Summar 7 and conclusions

5.

J,39

Isotherms in bottom waters followed the isobaths on the middle and inner shelf as did isopleths of chlorophyll a and particulate matter.

6.

These distributions (5) resulted in phytoplankton patches with maximum dimensions in the alongshore direction.

In the second intrusion copepod patches formed with

horizontal dimensions similar to those of the particulate matter. Processes i n i t i a t e d within the intrusions dominated the biology of shelf waters (YODER, ATKINSON, BLANTON, DEIBEL, MENZEL and PAFFENHOFER, 1981). Primary production on the middle shelf over the period of June-August 1981 was estimated at I50gC m-2 (YODER, ATKINSON, BISHOP, BLANTON, LEE and PIETRAFESA, 1985). T h i s estimate exceeds e a r l i e r estimates of annual production.

The fate of this primary production depends on three conditions:

First, durations

and velocities of flow reversals following the upwelling: Intrusion I was entirely removed from the shelf because of a strong and long-lasting flow reversal 28 to July 7.

to the South from June

During intrusion 2 the flow reversed to the south for only 4 days followed

by a pronounced onshore flow, advecting most of these intruded waters to near the 20m isobath. There the probability of rapid removal from the shelf was l o w .

Second, the fate of the

primary production depends on the species and size of phytoplankton:

widths between about

8 and 30um ESD enhanced their removal by zooplankton because they sink slowly and can be eaten by most juvenile and adult zooplankton and possibly protozooplankton.

Third,

the

species or group of zooplankton: asexually reproducing particle-feeders like salps and doliolids can respond to enrichment within several days by increasing their concentration daily by up to 50% (PAFFENHOFER and LEE, in press)and exerting almost immediate grazing pressure on phytoplankton.

Calanoid copepods, however, increase in concentration slowly and require

about 2 weeks before their concentration abundant phytoplankton.

is s u f f i c i e n t l y high to significantly graze the

A calanoid patch began to decay after the concentration of particu-

late matter between 8 and 30~m ESD had started to decrease (PAFFENHOFER e~ a~. 1988). The upwelling process t r i g g e r s a chain of events. plankton by introducing n u t r i e n t - r i c h

water into

I t physically induces the growth of phytothe euphotic zone and onto the shelf.

There the formation of phytoplankton patches occurs depending on the size and shape of the intrusion.

The intruded water remains on the shelf as an i d e n t i f i a b l e cold water mass u n t i l

advected o f f the shelf in a flow reversal ( i n t r u s i o n I ) . The phytoplankton patch b i o l o g i c a l l y induces zooplankton patch formation because strong reproduction occurs only where phytoplankton are abundant.

Once the food source is reduced, the decay of zooplankton patches begins.

This sequence of events is water, with

its

interrupted when a strong flow reversal displaces the upwelled

phyto- and zooplankton, back into the Gulf Stream.

Since they are then

r a p i d l y displaced downstream there may be s u f f i c i e n t survival of organisms to serve as seed populations in other regions of upwelling such as downstream of the Charleston Bump or in Onslow B a y .

Offshore

and downstream displacement of p a r t i a l l y

developed intrusions may

occur more frequently in colder seasons when upwellings usually do not penetrate inside the 30m isobath (ATKINSON and LEGECKIS, 1981; DEIBEL, 1985; PAFFENHOFER, unpublished results) and winds are stronger (WEBER and BLANTON, 1980).

~)

G . - A . P-~FFE',,H,',FER el a/

Our observations show that during summer strong primary (YODER e: a;. (PAFFENHOFER e; ~L.

19B5) and secondary

1 9 8 8 ) production occurs over the e n t i r e shelf in water masses which

o r i g i n a t e from Gulf Stream upwellings.

I t was assumed that during cooler seasons, particu-

l a r l y in winter, most of the production would be r e s t r i c t e d to the shelf-break because the high density of the shelf water would prevent upwelled water from intruding beyond the outer shelf.

However, recent observations (DEIBEL, 1985; PAFFENHOFER, unpublished observations)

showed phyto- and zooplankton-rich water at mid-shelf at 31° to 32°N. This water originates from the Gulf Stream (DEIBEL, 1985).

To understand and quantify these cold season processes,

i n t e r d i s c i p l i n a r y studies l i k e the one described in these papers are necessary. Our findings to date and future observations on Gulf Stream shelf i n t e r a c t i o n s could possibly be applied to other wide shelves bordered by strong boundary currents l i k e the East China Shelf and the Agulhas Bank. water onto the l a t t e r

Cyclonic f r o n t a l eddies contribute to upwelling of cold bottom

(SWART and LARGIER, in press),

Stream upwelling i n i t i a t i o n .

s i m i l a r to our observations f o r Gulf

Residence times of upwelled water on the Agulhas Bank seems

to occur over prolonged periods since a general westward flow was observed over the Agulhas Bank (CARTER, SWART and LARGIER, in press).

We w i l l

need additional time-series studies

on the southeastern and other broad shelves bordered by boundary currents to a r r i v e at a general understanding of shelf-boundary current i n t e r a c t i o n s .

ACKNOWLEDGEMENTS This research was supported by the f o l l o w i n g contracts from the Department of Energy: DEAsog-76EVO0936, DE-ASO9-76EVO0889, DE-ASO9-80EV10331, DE-ASO5-76EV05163 and DE-ASO9-76EVO0639. We would l i k e to thank three anonymous reviewers f o r t h e i r constructive c r i t i c i s m of e a r l i e r versions of our work.

REFERENCES 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 continenta shelf waters. Journa~ o f Marine Research, 42, 969-993. ATKINSON, L.P., T.N. LEE, d.O. BLANTON and G-A. PAFFENHOFER (1988) Summer upwelling on the southeastern continental shelf of the USA during 1981: Hydrographic Observatiors Progress in Oceanography, 19, 231-266. CARTER, R.A., V.P. SWART and J.L. LARGIER (1988) Thermocline c h a r a c t e r i s t i c s , advective processes and phytoplankton dynamics in Agulhas Bank waters. South African Jour~al of Marine Science, (in press). DEIBEL, D. (1985) Blooms of the pelagic tunicate, Do~io~etCa gegenbauri:Are they associated with Gulf Stream f r o n t a l eddies? Journal o f Marine Research, 43, 211-236. HAMILTON, P. (]988) Summerupwelling 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. 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 Oceanographg, 19, 267-312.

Summary' and concluslons

J-;i

LORENZZETTI, J., J.D. WANG and T.N. LEE (1988) Summer upwelling on the southeastern cont i n e n t a l shelf of the USA during 1981: C i r c u l a t i o n modelling. Progress ~ Oceanography, 19, 313-327. PAFFENHOFER, G-A. and T.N. LEE (in press) Development and persistence of patches of Thaliacea So~th African Journa~ of Marine Science.

PAFFENHOFER, G-A. and T.N. Lee (1988) Summer upwelling on the southeastern continental shelf of the USA during 1981: 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. Progress ~n Oceanographw, 19 , 373-401. PAFFENHOFER, G-A., B.K. SHERMAN and T.N. LEE (1988) Summer upwelling on the southeastern continental shelf of the USA during 1981: Abundance, d i s t r i b u t i o n and patch formation of zooplankton. Progress in Oceanography, 19, 403-436. POMEROY, L.R., G-A. PAFFENHOFER and J.A. YODER (1988) Summer upwelling on the southeastern continental shelf of the USA during 1981: Interactions of phytoplankton, zooplankton and microorganisms. Progress 6n Oceanography 19,353-372. SWART, V.P. and J.L. LARGIER (in press) On the thermal structure of Agulhas Bank waters. South African Journa~ of Marine Sciences.

WEBER, A.H. and d.O. BLANTON (1981) Monthly mean wind f i e l d for the South A t l a n t i c Bight. J o u ~ a l o f Ph~sioa~ Oceanography, 10, 1258-1263. YODER, J.A., L.P. ATKINSON, J.O. BLANTON, D.R. DEIBEL, D.W. MENZEL and G-A. PAFFENHOFER (1981) Plankton p r o d u c t i v i t y and the d i s t r i b u t i o n of fishes on the southeastern US continental shelf. Science, N.Y. 214, 352-353. YODER, J.A., L.P. ATKINSON, S.S. BISHOP, J.O. BLANTON, T.N. LEE and L.J. PIETRAFESA (1985) Phytoplankton dynamics w i t h i n Gulf Stream intrusions on the southeastern US cont i n e n t a l shelf during summer 1981. Continenta~ SheLf Research, 4, 611-635.