Distribution and composition of biogenic particulate matter in a Gulf Stream warm-core ring

Deep-SeaResearch. Vol.32. No. I1. pp. 1347to 1369. 1985. Printed in Great Britain.

11198~1149/S5$3.(ll + (I.(ll O I~X5PergamonPressl,td.

Distribution and composition of biogenic particulate matter in a G u l f Stream warm-core ring DAVID M. NELSON,* HUGII W . DUCKLOW,'I" GARY L. HITCIICOCK,:I:'H" MARK A . BRZEZINSKI,* TIMOTtIY J. COWLES,§~:~ CtIRISTOPIIER GARSIDE,]] RICHARD W . GOULD, JR.,¶ TERRENCE M . JOYCE,§CttRISTOPHER LANGDON,~ JAMES J. MCCARTIIY** a n d CIIARI.ES S. YENTSCIIll

(Received 16 November 1984; in revisedform 6 May 1985: accepted 20 May 1985) Abstract--We have characterized the biogcnic particle lield in Gulf Stream warm-core ring 82-B in June of 1982. Our observations include chlorophyll a and phacopigments, ATP, particulate organic carbon and nitrogen, biogcnic silica, total particle w,lume and sizc distribution, bactcrial abundance and picoplankton biomass, and the abundances of diatoms, dinoflagellatcs and coceolithophorids in the upper 7111 m along two transects of the ring. A distinct maximum in phytoplankton biomass occurrcd within the thcrmocline (211 to 411 m) at thc ring's ccntcr of rotation. This maximum had not been present in late April, and apparently developed within 3 to 4 weeks after the ring stratified in mid May. It exhibited a high degree of axial symmetry about the center of the ring, with biomass decreasing outward from ring center. A second biomass maximum associated with shelf surface water was being entrained into the anticyclonic flow field of the ring 60 to 711 km from its center. Maximum chlorophyll a and ATP concentrations observed in the two biomass maxima were similar, but the ring-center maximum was 2 to 10 times richer in particulate carbon, biogenic silica, particles >5 lam in diameter, dinoflagellatcs, diatoms and estimated organic detritus, while the entrained shelf water had 2 to 5 times greater abundances of unicellular monads. Heterotrophic bacterial abundance and biomass, and the abundance of coccoid cyanobacteria were maximal in the region of highest rotational velocity 40 to 50 km from ring center. In this region the abundances of bacteria and cyanobacteria were 2 to 5 times as great as at the center of the ring. Two possible mechanisms can explain the development of an axially symmetrical maximum in biogenic particulate matter in the center of a warm-core ring: concentration by the flow field and in situ growth. Our data on the distribution and composition of biogenic material in ring 82-B indicate a greater likelihood that this particular ring-center maximum devcloped in situ.

INTRODUCTION

OVER TIlE past 10 years detailed satellite imagery of sea surface temperature has shown

that eddies propagated from western boundary currents are common, persistent and * College of Oceanography, Oregon State University, Corvallis, OR 97331. U.S.A. t Lamont-Doherty Geological Observatory, Columbia University, Palisades, NY 10964, U.S.A. Present address: Horn Point Laboratory, University of Maryland, Cambridge, MD 21613, U.S.A. Graduate School of Oceanography, University of Rhode Island, Kingston, RI (12882, U.S,A. § Woods Hole Oceanographic Institution, Woods Holc, MA 02543, U.S.A. [I Bigclow Laboratory for Ocean Sciences, Wcst Boothbay Harbor, ME 114575, U.S.A. ¶ Department of Oceanography, Texas A & M University, Collcgc Station, TX 77843, U.S,A. ** Museum of Comparative Zoology, Harvard University, 26 Oxford Street, Cambridgc, MA 02138, U.S.A. l't Present_address: Department of Oceanography, Nova Univcrsity, 80(XI Occan Drive, Dania, FL 331XJ4, U.S.A. :H: Present address: College of Oceanography, Oregon State Univcrsity. Corvallis, OR 97331, U.S.A. 1347

1348

I). M. NI!I.NON('I al.

energetic features off the east coasts of major land masses (ROBINSON, 1983). The larger meanders in these current systems can become unstable, separate from the main current and persist for weeks to months as cyclonic eddies, seaward or anticyclonic eddies, shoreward of the current. Eddies of this type, designated as rings (Rl('] IARI)S()N, 1983), are known to be generated by the Gulf Stream (SAUNDERS, 1971; GOrTIIARDT, 1973), the Agulhas (DUNCAN, 1968), Kuroshio (ToMOSADA, 1978) and East Australian currents (NH SSONet al., 19771. Gulf Stream warm-core rings are anticyclonic eddies injected into the Northwest Atlantic Slope Water between the cold wall of the Gulf Stream and the continental shelf of the northeastern United States (SAUNDERS, 19711. They arc typically Ill0 to 200 km in diameter and 700 to 1000 m deep at the time of formation, and consist of a rotating central core of Sargasso Sea water surrounded by a more rapidly rotating (up to I(111cm s ~) annulus of Gulf Stream water. They can bc accurately located and tracked from satellite infrared images of sea surface temperature (HOOKERand OLSON, 1985; Brown et al., 1983) and from images in the visible spectrum showing phytophmkton pigments (GORDONet al., 1982). A considerable body of data has been accumulated concerning the formatkm, movement and fate of these rings. The following generalizations can be made: rings typically form to the south of New England or Nova Scotia and move in a southwesterly direction at 3 to 5 km day -I (LAJ and RICJlARDSON, 19771. They frequently interact with the Gulf Stream, Slope Water and even with shelf water, usually by entraining surface waters into the anticyclonic flow (JoYcE and STALCUP, 19851. Strong, episodic interactions with the Gulf Stream can result in overwashing or even resorption of the ring at any point in its southwesterly trajectory (JOYCEel al., 1984). Rings seldom, if ever, penetrate south of Cape Hatteras where the western edge of the Gulf Stream is typically'within 80 km of the shelf break. The life span of a ring that crosses west of the New England seamount chain and reaches Cape Hatteras is usually about 6 months (JoycE and WlEBE, 1983). Warm-core rings isolate mesoscale portions of the northern Sargasso Sea plankton ecosystem when they separate from the Gulf Stream. Newly formed warm-core rings have many chemical and biological properties similar to those of their Gulf Stream and Sargasso Sea source waters. They initially contain low to moderate phytoplankton biomass in water that is warm, saline and depleted in nutrients relative to the surrounding Slope Water (GorDon el al., 1982). Warm-core rings generally have been thought to be less productive than surrounding water masses (e.g. TRANTI'~Rel al., 19811). Wc initially expected that Gulf Stream warm-core rings would gradually become enriched with nutrients as they underwent exchange with the Slope Water. Furthermore, we hypothesized that, in response to nutrient enrichment, the productivity levels, community structure and plankton dynamics would evolve smoothly from properties characteristic of the oligotrophic Sargasso Sea toward those of the more eutrophic Slope Water. Such a process would be analogous to that by which cyclonic cold-core Gulf Stream rings with Slope Water origins evolve gradually toward Sargasso Sea characteristics (WIEBE"et al., 19761. It is clear now that warm-core rings often undergo biological transformations that are both greater in magnitude and more rapid than those observed in cold-core rings; initially low in phytoplankton biomass and primary productivity, they frequently become local maxima in both properties (TRANTERet al., 1980; BRADFORDet al., 1982; GORDONet al., 1982; JoY(E et al., 1984; HITCIICOCK et al., 1985; SMITII and BAKER, in press; FRYXELLet al., in press). The processes responsible for this phenomenon are not well understood.

1349

Biogcnic particulate m a n e r in a warm-core ring

In the spring and summer of 1982 we conducted an extensive time-series study of the physical structure and dynamics, biology and chemistry of a single ring (ring 82-B). This study included three multi-ship cruises to ring 82-B beginning in April and extending into August (JoYCE and WIEBE, 1983). In this article we present data from ring 82-B in the spring and early summer of 1982, showing that this ring: (a) was initially a local minimum in near-surface plankton biomass, (b) developed a localized maximum in biogenic particulate matter at its center within a period of two months or less, concurrent with the onset of seasonal stratification, and (c) developed a phytoplankton and microbial assemblage that was qualitatively very different from that in the surrounding surface waters. Our main purpose in this paper is to describe the mesoscale distribution of biological properties in the surface layer (0 to 100 m) of a Gulf Stream warm-core ring in relation to its physical structure. Our compositional data provide evidence of the relative importances of in situ growth and lateral entrainment processes in determining these distributions. METHODS

We conducted two transects of ring 82-B during June 1982; the first (TI) consisted of six stations occupied during 16 and 17 June, while the second (T2) included eight stations taken over the 23 to 25 June time period (Fig. 1). During each transect ring-center position was estimated based upon drifter locations and satellite imagery. The best estimate of truc ring-center position was subsequently calculated as a continuous function of time (HooK~:R and OLSON, 1985), permitting station locations to be plotted in polar coordinates (r,O) relative to a moving ring center (see Fig. 1 inset). While our intention was for each transect to pass through the center of the ring, it can be seen from Fig. 1 that T2 passed much closer 75*

70*W

65*

60*

40*N

55*

Fig. 1. A p p r o x i m a t e location of ring 82-B in March, May and June. Inset: locations of stations along two transects of the ring occupied by R.V. Knorr in June. Station Iocati~ms arc plotted in polar (r,0) coordinates relative to the moving center of the ring. ( o - - - o ) Transect 1, 16 and 17 June. I,o - e) Transcct 2, 23 to 25 Junc.

135(I

I). M. Ni,.l.soneta/.

to the revised ring-center positions than did TI. There was a major entrainment of shelf surface water in the northeast quadrant of tile ring lroln approximately 21 to 25 June (see Results) and T2 passed through this entrained water mass. At each station we collected seawater samples at 15 to 18 depths in the upper 700 using 3(11 Niskin "Go-FIo" samplers inountcd on a roscttc with a Ncil Brown CTD. This collection was accomplished in two casts; the first sampled 6 depths in the upper 110 m and the second 9 to 12 depths between 110 and 700 m. On the shallower cast the watcr volumc required for the various analyses exceeded 30 I, so 2 bottles were closed at each of the 6 depths. These paired samples were then combined on deck in 90 I rectangular polypropylene vats that were fitted with polypropylene covers to exclude airborne particulate matter, and spigots about 2 cm above the bottom for drawing samples. These vats were sheltered from direct sunlight. All measurements for each station and depth were performed upon the same combined seawater sample. Fewer analyses were performed on samples from the deeper cast, so only one sampling bottle was closed at each depth. To minimize the effects of particle sinking after the samplers were closed and to keep the handling of samples consistent with that on the shallow cast, samples from the deep cast were also drained in their entirety into the 90 1 polypropylene vats and then subsampled for individual analyses. The data set collected on each water sample in the upper 110 m included salinity, nutrients (NO~, NO~, NH~-, PO~- and Si(OH)4), particulate carbon and nitrogen, biogenic particulate silica, chlorophyll a, phaeopigments, ATP, particle volume and size spcctrum, bacterial abundance and cell volume, and phytoplankton abundancc and species composition. Samples collected from 110 to 700 m were analyzed for salinity, nutrients, particulate carbon and nitrogen and biogenic particulate silica. In addition to these discrete samples, continuous data records were obtained from the CTD on both casts (temperature, conductivity, oxygen). The methods used in obtaining these measurements are summarized in Table 1. Several of these measurements can provide estimates of the carbon biomass of various components of the suspended particle assemblage if certain assumptions are accepted. These estimates, the analytical values from which they were derived and the conversion factors of algorithms used are listed in Table 2. Cross-sectional distributions of selected properties were contoured in ring-coordinate space for T2, where thc differences in distances from ring center closely approximated actual distances between stations (Fig. 11. However for T1, which did not pass near the center, the contour plots were drawn by considering the center of the transect to be the closest point in the transect to ring center (Fig. 3). The location of each station on T1 is shown by its distance, in polar coordinates, from that midpoint. All data were contoured by linearly interpolating between data points. Temperature, salinity and density were contoured from the continuous data, and the other properties from the discrete data sets. Cluster analysis of selected data from the shallow bottle casts taken along T2 was performed using statistical packages available through Bigelow Laboratory for Ocean Sciences and the University of Maine-Orono IBM 4341 computer. For the analyses reported here, Canberra-Metric similarity values (S'rEeIIENSONet al., 1972) were computed from an 8 station × 6 depth x 12 parameter [chlorophyll a, phaeopigment, ATP, biogenic silica, particulate organic carbon and nitrogen, Coulter Counter-estimated biomass, picoplankton (<1 I.tm) biomass, and the numerical abundances of diatoms, dinoflagellates, coccolithophorids and very small algae designated as 'other'] correlation matrix. To

<0.1 <0.1

McCarthy Nelson Cowles Ducklow Yentsch Gould

Particulate C,N

Biogenic SiO2

Particle volume, size spectrum

Bacterial abundance, size

Cyanobacterial abundance

Phytoplankton abundance, major taxa

0.8 0.7* 0.6 t

Millipore A A Whatman GF/F Nuclepore -

-

Nuclepore

0.2

0.2

0.7*

Whatman GF/F

Nuclepore

Pore size (lam)

Filter type

Method

Settling chamber; inverted microscopc

Direct counts

Direct counts

Coulter counter

NaOH digestion: spectrophotometry

Pyrolysis; gas chromatography (CHN analyzer)

Luciferin/luciferase

Acetone extraction; fluorometry

Relerences

HASLE (1978a,b)

MURPHYand HAUGEN (1985)

DUCKI.OW(1982)

PAASCHE(1973) KRAUSSEet al. (1983)

KARL and HOLMHANSEN (1978)

HOLM-HANSEN et al. (1965)

* Nominally rated pore sizes. Recent information (LAws et al., 1985) indicates that 0.4 lam is a more realistic estimate of the effective lower size limit for the particles collected. t Aperture size 281) lam, resulting in minimum detectable particle diameter of ca. 5 ~tm.

1.0

2.8

2.8

0.1

Langdon

ATP

1.0

Sample size (1)

Hitchcock

Performed by

Summary o f analytical methods used in characterizing biogenic particulate matter in Gulf Stream warm-core ring 82-B

Chlorophyll, phaeophytin

Analysis

Table 1.

HOLM-HANSEN(1970) DucKLow (1982). WATSON

Strathmann equation C:ATP mass ratio. 250 C:Volume = l.l x l0 ,3 g pm 3 (cyanobacteria assumed to be 0.5 lam -3) C,t~,rit~,~ = Ct,,t~l- (ATP x 250)

Particle volume per size class ATP Cell abundance (heterotrophic and cyanobacterial), size Total particulate C. ATP

Carbon in living cells

Picoplankton carbon

Detrital carbon

et al. (1977)

STRATHMANN(1967)

None

Direct analysis

References

>5 ~m particulate carbon

Conversion algorithm

Total (>0.4 lam) particulate carbon

Derived from

Table 2. Summary of methods used in deriving estimates of carbon-based biomass from analyses of suspended particulate matter

Biomass estimate

1353

Biogcnic particulatc mattcr in a warm-core ring

compare objectively parameters with large differences in dynamic range, parameter values (P) were normalized to the observed range in that parameter: p, _

(P-

Pmin)

× 100.

(P~,,x - P.,i,,) RESULTS

A brief history of ring 82-B Ring 82-B formed in early to mid February 1982. It was the second warm-core ring generated by the Gulf Stream in that year (hence the designation 82-B). A detailed account of the physical evolution of ring 82-B will be presented elsewhere (JovcE and KENNELLY, in press; ScItMlq'r and OLSON, in press; OLSON et al., in press), but this overview of major events is presented as background to our description of biological properties and transformations within the ring between April and June. When sampled in April ring 82-B was elliptical, with major and minor axes of about 100 and 80 km. It had an isothermal, isohaline core of 15 to 16°C and 36.0 to 36.3%o extending from the surface to about 350 m in the center of the ring. By April the ring had undergone deep convective mixing, and had cooled by 2 to 3°C since its formation (ScHMFvr and OLSON, in press). The greatest depth of the 10°C isotherm was about 550 m. The 26.80o C H L O R O P H Y L L-O (p.g/kg)

o-, 25 5

260

26 5

BIOGENIC SILICA (n mol/kg)

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3

600

800

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50

J

JUNE

i IOO

.f

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ioo

75

(e)

,oo

Fig. 2. Representative vertical profiles (0 to 100 m) of density, chlorophyll a and biogenic silica in the slope water and at the center of warm-core ring 82-B in April and June. (a-c) 21 to 33 April, (d-f) 13 to 15 June. ( ) slope water; (- - - ) ring ccntcr.

(f)

1354

D . M . NELSON et al.

isopycnal intersected the sea surface, isolating the ring thermostad (SEITZ, 1967) from the surrounding Slope Water. Nutrient concentrations were vertically uniform throughout the upper 350 m ([NOel ~ 5.8 txM [PO~-] ~0.25 laM, [Si (OH)4] ~ 2,8 ~tM; Foxetal., 1984a), reflecting recent convective overturn and the absence of vertical stratification. Surface chlorophyll a concentrations in the ring were about 0,3 lag kg-~, which contrasted sharply with the 2 to 4 lag kg-~ concentrations in the Slope Water (Fig. 2). The April Slope Water K,LO.ETERS FRO. c E . r E . •

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"(f) intervQI • 2 5 nmol / kg"

Fig. 3. Sections of temperature, salinity and biogenic silica in the upper 7(X) m along the two transects of ring 82-B shown in Fig. 1. (a-c) Transect 1, which passed ca. 50 km northeast of the center of the ring, (d-f) transect 2, which passed within < 10 km of ring center.

1355

Biogenic particulate matter in a warm-core ring

station was occupied during the slope/shelf spring bloom (16 April to May) as revealed by satellite imagery (BRowN et al., 1985). The spring bloom in the ring was delayed relative to the surrounding waters beginning on 2 May and persisting until at least 10 May when satellite coverage of April to May surface pigments ends (BROWN et al., 1985). Throughout the April sampling period the vertical distributions of biogenic properties within the ring were relatively uniform in the upper 100 m (Fig. 2). By mid June the ring had drifted about 200 km to the southwest since our last observations in early May (Fig. 1). A seasonal thermocline had formed by mid May and was well developed in early June (Figs 3 and 4). The concentration of nitrate in the surface layer had been depleted to 0.01 to 0.1 pM (GARSn~E, 1985). Phosphate and silicic acid were undetectable by conventional analyses (Fox et al., 1984b). Surface salinities at

KILOMETERS FROM RING CENTER SW

I

I

116 117

1

/ STATION

118

I

I

2~

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210

( (I

I00

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SALI • mtervol = O.1%o IIII

0

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I

114

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(b) ~ _ ~ ~ _ E

50

l

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TEMPERATURE

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0

Ioo.

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/

I

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I

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1

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/

O- e interv

Fig. 4. Scctions of salinity, temperature and density in the upper 125 m along transect 2.

1356

NH.SON e/ al.

D.M.

ring center ranged from 35.3 to 35.75?'0o in June, as compared with 36.2%o in April. This is strong evidence of surface exchange between the ring and the slope or shelf water during the time interval between early May and mid June. An advective intrusion (a 'streamer'; EVANSet al., 1984) of shelf or slope surface water spiralled into the ring during the 14 to 18 May and 11 to 16 June periods, as revealed by satellite imagery. Then from 21 to 25 June an intense cold streamer of shelf water was entrained into the anticyclonic flow in the northeast quadrant of the ring, and transported out to the cold wall of the Gulf Stream (EVANSet al., 1984). In contrast to the April condition, the ring no longer contained a local minimum in surface chlorophyll a in June. Both the center of the ring and the shelf-water streamcr contained elevated chlorophyll a concentrations relative to the ring periphery and surrounding SlopeWater (Fig. 5a). When we returned to ring 82-B in August, it was greatly diminished in size, and interacting frequently with the Gulf Stream. Salinities in the upper 50 m ranged from <35 to >36.3%0, indicating both the presence of Gulf Stream water and also our difficulty in tracking ring-center positions at that time. A drifter placed in the center of the ring on our

KILOMETERS FROM RING CENTER

O ....

KILOMETERS FROM RING CENTER

60 4,0 zo ? zp 4p ~ 8,ONE swS,O ~ ~ 2,0 ? zp ~ 60

sw

116 ~

STATION IJB 2

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/

i

i

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hs

I zt

ze

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Transect 2

Transect 2 "

CHLOROPHYL

2

COULTER POC (:.5,~Lm)

interval = O.~ )J.g/kg"

l

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9

I ~ NE

•

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interval : 2~.1moi/kg

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(d) I i

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interval • 50 ng/kg"

(b)

50

tO0

POC ( > 0.7/am )

i n t e r v a l • 2).t r n o l / k g "

.

/~-

z --.~

(c)

Fig. 5. Sections of chlorophyll a, A T P , particulate organic c a r b o n , carbon in particles > 5 lam in d i a m e t e r (as e s t i m a t e d by a C o u l t e r c o u n t e r ) and biogenic silica in the u p p e r 110 m along transect 2.

,

j

Biogctlic particulate matter in a warm-core ring

1357

last visit was released into the Gulf Stream on 19 September 1982, suggesting almost complete reabsorption by that time. Gross structure and symmetry

The two transects of ring 82-B were separated in time by 8 days, and offset from one another spatially by 40 to 50 km with respect to the center of the ring (Fig. 1). According to estimates of ring-center position based upon detailed analysis of satellite imagery, T2 penetrated to within 10 km of the surface manifestation of ring center, while TI passed about 50 km north of ring center. The same spatial offset is apparent in the temperature and salinity sections (Fig. 3a, b, d, e); the T2 sections (Fig. 3d, e) show a thermostad of >15°C, >36%o water extending to >350 m depth in ring center and having an east-west diameter of >80 km at 100 m depth. In contrast, T1 encountered no > 15°C water beneath the seasonal thermocline and probably passed through only the northernmost edge of the >36%o core of the ring. Thus, the subsurface temperature and salinity fields are consistent with the surface thermal images of the ring obtained by satellite; both suggest that T2 approximates a diametric section of the ring while TI represents a chord located 40 to 50 km off center. The various plankton biomass data sets show three conspicuous distributional patterns, all of which correspond to the physical structure of the ring or its environment. These are (1) a generalized axial symmetry about ring center with maxima or minima near the center; (2) subsurface maxima extending across part or all of the transect, and (3) a strong, distinctive distribution of properties associated with the shelf-water streamer in the eastern portion of the ring. These features are discussed below. On a 0 to 700 m scale the clearest evidence of spatial coherence between the physical structure of the ring and the biogenic particle field is the distribution of biogenic silica (Fig. 3c,f). Biogenic silica increased with depth.in the permanent thermocline, making the outer (r > 40 km) and lower (z > 350 m) thermostad a local minimum with biogenic silica concentrations generally <50 nmol kg-t. Within 30 km of ring center biogenic silica concentrations exceeding 50 nmol kg-~ extended to about 400 m. This feature appears only in T2, which passed through the center of the ring, and splits the distribution of the very lowest biogenic silica concentrations (<35 nmol kg-~) into two distinct subsurface fields, each offset from the ring center by 20 to 30 km. In T1, which did not pass within 40 km of ring center, the <35 lamol kg-~ field appears continuous. These sections suggest an axially symmetrical distribution extending at least 50 km outward from ring center, with a particle-impoverished toroid surrounding a central vertical shaft of slightly more particle-rich water. Closer examination of the biogenic silica sections shows that contour depths at the center of T1 correspond to those offset from the center of T2 by 40 to 50 km in either direction in almost every detail: concentrations <50 nmol kg-~ extend from about 80 to 400 m and there is a continuous layer of concentrations <35 nmol kg-~ about 200 m thick within this depth interval. Also, the sharp maximum at about 30 m is present in both sections, but the concentrations at this maximum are approximately 3 times higher in T2 than in T1. Offsetting T2 by 40 to 50 km in either direction preserves the maximum at 30 m, but decreases its magnitude to approximately that observed in T1. Similarly, the distributions of dissolved oxygen, nitrate and silicate, and of particulate carbon, nitrogen, chlorophyll a, ATP, bacterial biomass and phytoplankton abundance observed in T I (not shown) can be matched with those in T2 by considering T1 to be an off-center pass through

1358

D.M. NELSONet al.

an axially symmetrical field whose center is included in T2. This is strong evidence for a substantial degree of axial symmetry in nutrient and biomass distributions within the ring. Also, because the two transects were separated in time by 8 days, their spatial matchup with respect to the parameters listed above argues against major temporal changes in these distributions below about 20 m on time scales of one week. Examination of the satellite-collected infrared images for the period 10 to 30 June suggests that temporal variability of sea-surface properties was greater than shipboard measurements of the ring-center subsurface biomass maximum indicate. Both warm and cold streamers penetrated the ring on time scales of 2 to 5 days (EvANs et al., 1984). With the exception of the entrained shelf-water streamer we sampled on 23 to 25 June, we have few data on the depth penetration of the streamers revealed by remote imagery. The relative constancy of the subsurface biomass maximum indicated by comparison of our two transects suggests that the biological influence of these streamers was generally confined to the upper 50 m of the water column at the periphery of the ring. The T2 temperature and salinity sections (Figs 3d, e and 4) show a core of cold, low salinity water in the upper 100 to 150 m, centered approximately 60 km east northeast of ring center at Sta. 14. This is a shelf-water streamer, entrained by the ring from the northeast. The time series of satellite images indicates that it was present from about 21 to 25 June (EvANs et al., 1984). This streamer also exhibited high chlorophyll a concentrations (Fig. 5a). Furthermore, as we will show below, it contained an assemblage of biogenic particles qualitatively distinct from that at ring center. Biomass in the upper 100 m

In April, ring 82-B was isothermal to >100 m, and represented a pronounced local minimum in surface chlorophyll a (Fig. 2). By 23 June it had developed a strong seasonal thermocline and pycnocline at approximately 20 to 30 m (Fig. 4b,c). Subsurface maxima in chlorophyll a, biogenic silica, ATP, Coulter-estimated carbon, particulate organic carbon, detrital carbon, bacterial abundance, and total picoplankton (< 1 lam) carbon all occurred within the seasonal pycnocline (Figs 5 and 6).

R

2.8

,,

¢ ',,s,.z, z 0 m ,',-

i / ,'

2.0 t

g ,,,=L

s

4.5

Fig. 6.

, b ,t i

", 9 18 36 72 PARTICLE DIAMETER (~rn)

Size spectra o f particles w i t h i n the t w o high-biomass cores observed in ring 82-B in June.

( o - - - o ) ring centcr, (e e) the shelf-water streamer in the northeast quadrant of the ring. Both size distributions arc fr()m the depth of the chlorophyll a maximum.

Biogcnic purticulatc maUcr i,i a w;irm-corc ring

1359

Several of the biogenic particle fields had two separate corcs of high concentration: one at ring center and the other associated with the shelf-water streamer 60 to 80 km east of ring ccntcr. In the chlorophyll a section (Fig. 5a) these two maxima show approximately equal concentrations (1 to 1.5 tag kg-l). The ATP distribution (Fig. 5b) also shows these two high biomass cores, and indicates that the maximal levels of living biomass achieved in the two cores were about equal (approximately 0.6 lag ATP kg-J). However, these two cores were of very unequal intensity with respect to several other biomass parameters, indicating sharp differences in their composition and probably reflecting different origins. For instance, both the ring center and the shelf streamer contained maxima in particulate carbon (Fig. 5c), but the particulate carbon concentrations within the corc of entr~incd shelf water wcrc less than half thosc observcd in the ring-center maximum. In thc section of suspended particle volume (Fig. 5d), as determined using a Coulter counter whose aperture allowed detection of particles >5 lam in diameter, the streamer no longer appears as a distinct maximum, but contours as a tongue of somewhat elevated particle concentrations extending eastward from the ring-center maximum. The biogenic silica section (Fig. 5c) shows no trace of clewdcd co,~ccntrations in the cnlr~fincd shelf writer. Direct microscopic examination of the phytoplankton reveals that the ring-center biomass maximum of T2 was due to high numbers of diatoms (>5(],[)[](J cells I ~), and relatively high numbers of coccolithophorids and dinoflagellates (up to 18,000 and 27,0(10 cells I-~ , respectively), while the entrainment feature maximum was dominated by very small unicellular monads (1 to 3 lam in diameter), with some coccolithophorids (43.11(1(J and 11,000 cells 1-I respectively). Diatom abundance did not exceed 650 cells 1-I in any of thc samples examined from the entrainment feature. The major species present, as well as the major groups and cell numbers, were different at the two sites (GOULD, in preparation). The size distribution of particles >5 lam in diameter was also very different in the streamer from that at ring center (Fig. 6). Particle mass at ring center was dominated by a broad band of particles with equivalent spherical diameters of 8 to 30 lam. Particles in this size range were virtually absent in the shelf-water streamer, where most of the detected biomass was < 10 [am in diameter. To summarize, the subsurface particle maximum in the shelf-water streamer contained chlorophyll a and ATP at concentrations similar to those in the ring-center maximum, but less particulate carbon, much less material in the >5 lam size fraction and very little siliceous material. The streamer also contained higher lcvels of picoplankton carbon than were present in ring center, but very low estimated levels of detrital carbon (nonliving particulate carbon) (Fig. 7c). In contrast, the ring-center biomass maximum was characterized by relatively larger particles, a greater abundance of diatoms, and larger amounts of detritus (>50'70 detrital carbon). The picoplanktonic biomass, consisting of heterotrophic bacterioplankton and autotrophic coccoid cyanobacteria, was not distributed within the ring 82-B system like the other biomass components. This biomass pool had maxima centered in thchigh-velocity region (Stas 15 and 17; Figs 7a and b). Like other properties, picoplankton biomass was maximal in the seasonal pycnocline, but differed from other biomass parameters in that it was not maximal either at ring center or in the streamer. The abundances of heterotrophic bacterioplankton (DUCKLOW, 1984) and of cyanobacteria (C. YENTSCIt, unpublished data) were maximal in the high-velocity region, and the mean size of bacterial cells was larger there than at ring center (DuC'KLOWand HILL, 1985). An objective analysis of these distributions obtained from a cluster analysis which preserves orthogonality with equal weighting of all data sets, shows that the data can be

D . M . NI~I.,',;oN('t al.

1360

grouped into 6 distinct clusters of >65% similarity (Fig. 8 and Table 3), Most significantly, these clusters correspond to distinct physical features of the ring. For instance, cluster 111 represents the high-biomass shelf-water intrusion, and cluster V the high-biomass core at ring center (Table 3),Cluster VI is centered at Stas 15 and 17 about 40 km to either side of ring center, close to the azimuthal velocity maximum for this period (JovcE and KENNELLY, in press). Clusters I and II show greater variability, and represent the low-biomass waters below the pycnocline, in general, this analysis confirms the visual impression left by Figs 4 to 7 that the distribution of qualitatively and quantitatively distinctive biogenic particle assemblages was controlled by the dynamic physical structure of the ring system.

KILOMETERS FROM RING CENTER

SW 80

60

~

20

?

20

4pi

6,0 80 NE

50

•

•

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,,\ BACTERIA

. X~" ~'"'

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EST~M,TEOOETRITAt."CARBO~ p_too!/kg Fig. 7.

(

Sections of bacterial abundance, picoplankton biomass and estimated dctrital carbon in the upper 1 I0 m along transect 2.

SP D SWS SLW RC HVC

Region*

0.724 0.656 0.842 0.894 0.866 0.81 l

Similarity levelt 0.27 0.04 0.82 0.21 0.57 0.39

Chlorophyll a (lag kg-I) 100 58 72 49 715 237

Biogenic silica (nmol kg-t)

Mean values in cluster

120 43 398 250 427 283

ATP (ng kg t)

5.9 2.8 10.3 8.3 17.0 8.6

Particulate organic carbon (lamol kg -I)

1.2 0.5 4.3 2.6 11.5 4.8

>5 lam carbon (lamol kg -t)

1.0 0.6 1.5 1.0 0.8 1.3

Picoplankton carbon (lamol kg -l)

Cluster analysis of biogenic particulate properties within the upper 110 m along transect 2 of Gulf Stream warm-core ring 82-B+ 23 to 25 June 1982. Refer to Fig. 8for cluster locations along transect

* SP = sub-pycnocline; D = deep; SWS = shelf-water streamer; SLW = Slope Water; RC = ring center; HVC = high velocity core+ t Canberra-metric percent similarity level at which sample properties form a cluster.

I II Ill IV V VI

Cluster

Table 3.

1362

D.M. NELSONet al. KILOMETERS SW 80

60

I

20

0

I

i

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Fig. 8. Spatial distributions of the six biogenic particle assemblages showing Canberra-metric similarity levels >0.65 by cluster analysis of 11 parameters. Locations of these clusters appear to be controlled predominantly by the physical structure of the ring. See Table 3 for major properties of the particle assemblage in each cluster.

DISCUSSION

Between 23 April and 15 June 1982 the center of ring 82-B was transformed from a region of locally minimum (Fig. 2) to one of locally maximum biomass (Fig. 5). To some extent this change reflects a decrease in Slope Water biomass (Fig. 2b, e) but it is also clear that biomass increased within the ring as evidenced by the subsurface biomass maximum embedded in the seasonal thermocline (Figs 2e, f, 4 and 5). Development of a local biomass maximum at the center of the ring is consistent with observations that other anticyclonic eddies exhibit higher planktonic biomass than surrounding waters (TRANTER etal., 1980, 1983; JEFFREYand HALLEGRAEFF, 1980; Sco'rT, 1981). Comparison of the April and June conditions shows that this transformation can be much more rapid than the biological transitions observed in cold-core rings (WIEBE et al., 1976). The two high-biomass cores apparent in our June transect data coincided spatially with distinct hydrographic features. One was contained within an entrained plume of cold, lowsalinity shelf water. The other was most intense at the ring's center of rotation and diminished with distance from ring center, showing a high degree of axial symmetry. This symmetry suggests that it developed in response to some unique property of the ring environment. The mechanisms that could produce a biomass maximum at the center of a warm-core ring must fall logically into two broad categories: either the living material at ring center grew outside the ring (or in the ring periphery) and became concentrated in the center by the flow field, or it grew and accumulated in situ. These processes need not be mutually exclusive, and it is entirely possible that the distributions of different kinds of biogenic particles are governed by different mechanisms. The difficult problem of differentiating between local and far-field origins of plankton biomass and chemical properties inside eddies has been approached by other investigators. TRANTERet al. (1980) observed the development of a local maximum in surface chlorophyll a inside East Australian Current eddy F in the spring of 1978. On the basis of continuous tracking of the eddy by drifters and comparison of temperature and salinity profiles, they concluded that the phytoplankton maximum was produced in situ. They attributed the enhanced production to increased nutrient concentrations resulting from

Biogenic particulate matter in a warm-core ring

1363

deep convective mixing in the ring interior. JEFFREYand HALLEGRAEFF(1980) provided further evidence for an in situ origin of the phytoplankton maximum in eddy F. They observed, as we did, higher diatom abundance at the ring center than near the edge, and found that nanoplankton accounted for about 50% of the chlorophyll a at the center, but >70% near the edge. They noted that the ring-center maximum could have been generated either in situ by deep convective overturn or by some unspecified concentrating mechanism "due to hydrodynamic forces at the center of the rotating eddy". FLIERL(1977) showed from theoretical analyses of ring motions that there is an area of varying size at the core of a translating ring in which non-swimming particles should bc trapped. This arca is largest near the surface. In the absence of lateral mixing such a trapping mechanism would ensure that biomass generated inside the ring must either remain there or sink. The role of horizontal advective processes in transporting materials and properties toward the interior of eddies has been discussed by ANGELand FASHAM(1983), primarily with respect to cyclonic (cold-core) rings. Assuming radial advection and diffusion of rapidly growing species, they concluded that the time scale associated with the transport of particle tracers across a 20 km wide frontal zone by these processes is of the order of 50 to 500 days. These time scales are consistent with the observations of WIEBE et al. (1976) in cold-core rings, but are considerably longer than those that appear to characterize biomass changes in warm-core rings. Recently SIMPSONet al. (1984) have described a quasi-permanent semi-stationary offshore eddy in the California Current system, in which various advective processes appear to exert primary control over biological and chemical distributions in a threelayered eddy system. In that system, a subsurface warm-core eddy below 200 m entrained both cold coastal water and warm North Pacific Central Water into the surface layers, forming a warm surface layer and a cold-core eddy above the warm-core eddy. SZMPSONet al. (1984) and HAURY (1984) observed enhanced biomass inside the eddy, but also observed significant asymmetries in the distributions of phytoplankton pigments and zooplankton biomass and species abundance. SIMPSONet al. (1984) concluded that deep convective overturn, and thus in situ generation, could not explain the distributions they observed, and that the enhanced biomass levels in this eddy system did not have a local origin. YENTSCit and PIIINNEY (1985) proposed a model whereby mixing along isopycnal surfaces results in elevated rates of vertical nutrient transport in the periphery of a warmcore ring. This model predicts that if phytoplankton biomass and productivity in both the ring and the surrounding surface waters are limited by nutrient availability, the edge of the ring should display a localized maximum in both properties. Phytoplankton biomass distributions of this type have been observed in warm-core eddies off Australia and New Zealand (TRANTERet al., 1983) and we observed high microbial abundance and biomass in the periphery of ring 82-B (Fig. 7a,b). However, several measures of phytoplankton biomass in ring 82-B show the opposite distribution, with biomass decreasing outward from ring center (Fig. 5). As indicated in the Introduction, we initially expected Gulf Stream warm-core rings to be semi-isolated systems of Sargasso Sea properties that evolve slowly toward Slope Water characteristics. However, data from satellite images of sea surface temperature and shipboard observations of temperature, salinity, chemical and biological properties all indicate a significant import of slope and shelf surface water into the interior of the ring between the time of seasonal stratification in mid May and that of our June transects. The

1364

D . M . Ntt.soN et al.

ring-center biomass maximum we observed in June could have resulted from lateral transport of material generated in the periphery or imported by streamer events (EvANs et al., 1984) followed by smoothing of the distributions by the rotational flow to produce the observed axial symmetry. It is also possible that the high biomass levels in June were generated in situ because some property of the ring interior enhanced phytoplankton growth, retarded its removal, or both. Evidence for lateral exchange

Numerous satellite images of sea surface temperature show that plumes of Gulf Stream, slope or shelf water entrained into the anticyclonic flow field of a warm-core ring can at times spiral inward toward the center of the ring (EvANs et al., 1984). This process is not well understood dynamically. If these spirals penetrate well into the interior of a ring (as some satellite images indicate that they do) the ring center may serve as a collecting point for any particulate matter entrained from the adjacent water masses. There are several conceivable fates for this particulate matter, including sinking, in situ consumption and decomposition, and dissipation by lateral mixing, but the rates of these processes are not known. However, during any time when the transport of particulate matter to ring center exceeds the sum of these removal processes, particulate matter must accumulate at or around the center of the ring. The accumulation of passive particles in ring center is consistent with the distribution of detritus shown in Fig. 7c. Detritus, by definition, is not growing, and yet its apparent distribution in ring 82-B in June showed a single, pronounced maximum exhibiting a high degree of symmetry around the ring's axis of rotation. Ft, JzRt,'s (1977) theoretical analysis of water movement in a translating ring predicts that passive particles near the surface at ring center should remain there, regardless of their origin or mode of formation. Had therc been no detrital maximum at ring center, it would have implied that any trapping mechanism must be weak and that particles must be growing actively to form a maximum at ring center. The presence of a detritai maximum in the center of ring 82-B tends to support Fiierl's model, but does not rule out either advective transport or in situ production as the source of this material. We noted earlier that the surface salinity in the center of 82-B decreased from about 36,2 to 35.5%0 between April and June. If this reduction is a result of lateral exchange between ring-center water and shelf-water streamers, a crude two-point mixing model with ring center (S = 36.2%0) and shelf water (S = 34.0%o) as the endpoints can be used to calculate the fractional exchange of mass required to account for the reduced ring-center salinity of 35.5%o. This fractional exchange is given by (36.2 - 35.5)/(36.2 - 34.0) = 0.32. Thus, the surface salinity change in the ring center suggests that shelf-water streamers could have replaced approximately one third of the ring-center surface water in two months, with a complete replacement time of 6 months. This is a conservative estimate of exchange; entrainment of surface waters with >34%o salinity (such as Gulf Stream or Slope Water) would have to be greater to produce the observed salinity change. Thus, there was significant surface exchange between the ring and adjacent water masses between April and June. It is clearly possible that this exchange transports biogenic particulate mattcr into the ring. A similar analysis for the isotopic composition of particulate organic nitrogen in ring 82-B indicates that between early May and mid June the contribution of shelf water or Slope Water-derived particulate nitrogen to the core region of the ring could have been as high as 50% for the euphotic zone (ALTABETand McCARTllV, 1985).

Biogcnic particulate matter in a warm-core ring

1365

Evidence for accumulation t~[ biomass by in situ growth Several mechanisms have been proposed whereby warm-core rings can develop localized maxima in phytoplankton biomass by enhanced in situ growth. Within eddies of the East Australian Current system (TRANTERet al., 1983) maximum phytoplankton biomass occurs at the ring periphery. Enhanced phytoplankton biomass at the periphery of the East Australian eddy was associated with a frontal zone between the ring center and subantarctic waters (JEFFREYand HALLEGRAEFF,1980). Those authors proposed that horizontal and vertical shear at the frontal boundary led to nutrient enrichment and a shoaling of the seasonal thermocline into the cuphotic zone. Alternatively, TRANTER et al. (1983) attributed the enhanced phytoplankton biomass to upwelling, and YENTSCnand PmNNEV (1985) suggested the possible importance of mixing along sloping isopycnal surfaces. Since warm-core eddies often have increased pigment concentrations at their edges, the question is: why did 82-B have maxima in many biomass properties at its center? Two observations suggest that the particle maximum in ring center had bccn developed primarily in situ, rather than advected into the center of the ring. First, the biomass maximum observed in ring center had a composition distinct from that of the shelf-water streamer, suggesting that the ring-center maximum could not have arisen from recent intrusions of shelf-water into the ring. It is possible, however, that other streamers entering the ring between cruises introduced seed populations quite different from those present in the entrainment feature sampled in June. A maximum estimate of the magnitude of these introductions was calculated using the largest available estimate of volume exchange between the ring and Slope Water (50%; ALTABET and MCCARTIIY, 1985) and an upper limit to the biogenic silica levels present in the slope and shelf waters during May. Satellite imagery revealed that the April slope and shelf water spring phytoplankton blooms were over by the time we left the ring in early May (BROWN et al., 1985). Thus, the biogenic silica concentrations measured in the Slope Water in April (Fig. 2c) should provide a maximum estimate of the biogenic silica content of streamers entering the ring during May and early June. A l: 1 mixture of Slope Water containing 350 nmol kg -1 of biogenic silica and ring-center water containing 100 nmol kg-t (Fig. 2c) would contain an average particulate biogenic silica concentration of 225 nmol kg-t . The volume-weighted mean biogenic silica concentration in the upper 40 na of the ring in June was 400 nmol kg-1 with values ranging to over 900 nmol kg-I (Fig. 20, indicating that considerable in situ production had to occur to produce the observed June biomass distribution. A second observation supporting in situ development of the ring-center biomass maximum is the distribution of bacteria and small photosynthetic monads within the ring (Fig. 6a,b). Bacterial abundance and total picoplankton biomass distributions show that within the high-velocity region and entrainment feature of the ring these properties were 2 to 5 times greater than in ring center. If the ring-center particle maximum had arisen from the accumulation of entrained material, then all entrained particles should have been maximal at the center of the ring unless they were being selectively removed (e.g. by protozoan grazers). The distinct ring-center minima in bacteria and picoplankton thus can be taken as evidence against the passive aggregation of these particles. We do not know why picoplankton biomass was maximal in the frontal regions of the ring, where neither phytoplankton nor zooplankton biomass was elevated. However, this observation is Consistent with several others that bacterial biomass is often enhanced near frontal systems (FLOODGATEet al., 1981; POMEROYet al., 1983; HOLLIGANet al., 1984).

1366

1). M. Nl~l,sonet al.

PEELEet al. (in press) found that bacterial abundance and production, but not total ATP or microbial RNA synthesis (which measure both autotrophic and heterotrophic pico and nanoplankton attributes; KARL,1980), were maximal in the frontal regions of warm-core ring 81-F in March 1982. Those data and ours suggest that warm-core rings arc areas of enhanced bacterial biomass and productivity, as they are of phytoplankton biomass, but that the mechanisms generating these different maxima are different and still unclear. Data on the distribution of macrozooplankton (WIE~ et al., in press) and microzooplankton biomass (ROMANet al., 1985) across ring 82-B in June, together with direct estimates of grazing rates by both size classes of zooplankton (T. J. Cowt.l~s and M. R. ROMAN,personal communication), do not indicate a pronounced change in the rate of consumption of small-particle biomass with distance from ring center. Therefore, it seems clear that the observed ring-center biomass maximum was produced and sustained by processes other than differential removal by grazers. If the ring-center biomass maximum developed by in situ growth, this growth and accumulation almost certainly bcgan when the seasonal thermoclinc dcvcloped in thc ring. It was in that sense a rather typical spring bloom, responding to the improving light and mixing regimes in accordance with SVERDRUI"S(1953) critical depth concept. Seasonal stratification occurred later within the ring than in the surrounding slope water (Fig. 2), and it is possible that the ring-center biomass maximum we observed in June was simply the remnants of the spring bloom. On the other hand, the biomass field within the ring apparently did not change significantly in the 8 days that separated our two transects. If the ring-center biomass maximum was a quasi-steady-state feature of the phytoplankton distribution it had to be sustained by some set of processes that continued through June and were unique to the ring center. FRANKS(1984) recently proposed that the relaxation of the thermocline in a warmcore ring that results from the ring's loss of energy and rotational velocity with time may result in upweiling throughout the thermostad of the ring. JovcE et al. (1984) have estimated mean upward vertical velocities of 0.5 to 1.5 m day -I at the center of ring 82-B during the April to June period, based upon the observed vertical displacement of isopycnal surfaces with time. Vertical velocities of this magnitude would result in enhanced upward transport of nutrients in the ring, and may play a significant role in supporting the elevated levels of phytoplankton biomass observed there (NECsONet al., 1984). CONCLUSIONS

The following five points emerge from our study of the biogenic particle field in ring 82-B in June. 1. Many biomass parameters displayed a high degree of spatial congruence with the physical structure of the ring. In the upper 100 m there was a pronounced biomass maximum within the seasonal thermocline that was most intense at the ring's center of rotation and diminished axially outward. 2. The axially symmetrical biomass maximum in the upper water column was established within 30 days after stratification of the surface layer in mid May. The symmetry of the biomass distribution about ring center was exemplified by the biogenic silica field, which reflected ring structure throughout the upper 600 m. 3. The biomass distribution was influenced strongly by the entrainment of shelf water into the anticyclonic flow field of the ring in the upper 100 m. The entrainment feature

Biogenic particulate matter in a warm-core ring

1367

sampled in June comprised localized chlorophyll a and ATP maxima 60 to 80 km from the center of the ring that were equal in magnitude to the ring-center maxima. 4. The ring-center and shelf-streamer biomass maxima contained particle assemblages of very different compositions. The two maxima exhibited approximately equal chlorophyll a and ATP concentrations, but the ring-center maximum was approximately twice as high in particulate carbon and particles >5 p.m in diameter, five times as high in organic detritus and 10 times as high in biogenic silica. The streamer had four times more picoplankton (cells <1 lam in diameter) including bacteria. The phytoplankton at ring center was dominated by diatoms and dinoflagellates, while that in the streamer w~ts predominantly composed of coccolithophorids and small, unicellular monads. 5. Our best evidence suggests that the biomass maximum in the center of the ring grew and developed in situ. Though lateral exchange between the ring center and surrounding water occurred, our estimates of the maximum increase in ring center biomass that could result from these processes indicate that they were insufficient to account for the magnitude of the biogenic particle maximum at the center of ring 82-B in June. Acknowledgements--We wish to thank Julic A. Ahcrn, Nancy Coplcy,Suzannc Hill, John Ncvins and David A. Phinney for their assistance in the various analyses and Greta A. Fryxcll for her hclp in phytoplankton identilications. Dana R. Kcster and Mary Frances Fox performed nutricnt analyscs and allowcd us to scc thcir unpublished data. Richard H. Backus, James K. B. Bishop, Patricia A. Blackwelder, Robert Evans, Alfred Hanson, Theodore J. Smayda and Peter H. Wiebe provided comments on earlier versions of this paper, for which we are very grateful. An anonymous reviewer provided us with an exceptionally detailed set of comments, which helped us greatly in presenting and discussing the data. This work was supported by thc Division of Occan Sciences, National Science Foundation, through grants to the Bigelow Laboratory for Occan Scicnccs, Harvard University, Lamont-Doherty Geological Observatory of Columbia University, Oregon State University. the Univcrsity of Maryland, thc University of Rhodc lshmd, Texas A & M Univcrsity and the Woods lh)lc Oceanographic Institution. This is Contribution No. 2123 from thc College of Occanography, Orcgon State University and No. 5865 from the Woods Hole Oceanographic Institution.

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