Journal of Marine Research, 43, 211-236,1985
Blooms of the pelagic tunicate, Dolioletta gegenbauri: Are they associated with Gulf Stream frontal eddies?
I 2 by Don Deibel •
ABSTRACT Satellite-directed sampling was used to determine whether blooms of Do/ioletta gegenbauri are associated with warm filaments of Gulf Stream frontal eddies. Radio-transmitting drogues were used to mark the center of the bloom so that physical and biological covariables could be measured inside and outside of bloom waters. The bloom was not in the warm filament of a frontal eddy, but was 60 - 70 km northwest of the temperature front between outer-shelf water and the Gulf Stream-in upwelled water probably originating from the eddy's cold core. This cold-core remnant (CCR) water was stranded between 2 middle-shelf fronts. The doliolid bloom resulted from the asexual production of gonozooids by the oozooid stage. This occurred primarily in the nearshore temperature and salinity front and in or beneath the pycnocline between CCR and overriding outer-shelf surface water. Several of the doliolid populations were estimated to be capable of clearing 40-120% of their resident water volume each day-removing particles of less than 50 J.Lm equivalent spherical diameter. Their removal of small particles is thought to be one of the primary reasons for poor copepod recruitment and low net zooplankton concentrations in the midst of doliolid blooms. The phytoplankton community was co-dominated by dinoflagellates and diatoms. indicating the strong influence of the Gulf Stream in these mid-shelf waters. Dominant diatoms were Thalassiosira subti/is and Rhizosolenia sp., both typical of Gulf Stream upwelling in the Georgia Bight. The net zooplankton occurred in low concentrations. There were no species typical of coastal or Gulf Stream surface water. Those samples not dominated by D. gegenbauri were co-dominated by Oithona sp., Oncaea sp., Euterpina sp., and ostracods. Based on the frequency and duration of Gulf Stream frontal eddies, doliolid blooms may persist for 7-9 days. Blooms are most common in the winter and spring, due in part to the density regulated mixing characteristics of coastal and upwelled Gulf Stream water. Blooms of D. gegenbauri may form when upwelled. cold-core remnant water is advected onto the middle shelf and is stranded between 2 fronts. Doliolids are adapted to respond quickly to the "event" time scale of physical and phytoplankton dynamics.
1. Introduction Little quantitative information exists on the abundance of Dalia/etta gegenbauri in coastal waters, or on the physical and biological factors that accompany doliolid I. SkidawayInstituteofOceanography,P.O. Box13687,Savannah,Georgia,31406,U.S.A. 2. Presentaddress:MarineSciencesResearchLaboratoryand NewfoundlandInstitute for Cold-Ocean Science,MemorialUniversityofNewfoundland,St. John's,Newfoundland,Canada,AIC 5S7. 211 212 Journaf of Marine Research [43, 1 blooms. However, blooms of D. gegenbauri may cover thousands of square kilometers off the coast of Georgia, U.S.A. (Atkinson et af .• 1978; PaffenhOfer, pers. comm.) over the Aghulas Bank off southwestern South Africa (DeDecker, 1973), in the southern Bay of Bengal (Madhupratap et af .• 1980), and southwestern Japan Sea (Ogawa and Nakahara, 1979). D. gegenbauri has high rates of reproduction, growth, and feeding (Deibel, 1982a, b), suggesting that bloom populations are important mediators of energy flow through the pelagic food web of these various continental shelf communi- ties. To understand the role of D. gegenbauri in energy flow, one must first describe the physical and biological factors which appear to drive the formation of blooms. Blooms of D. gegenbauri are most frequent off Georgia, U.S.A., from February to May. At this time of year the thermal contrast between continental shelf and Gulf Stream water is greatest (Atkinson, 1977), and meanders and frontal eddies of the Gulf Stream are the dominant mechanisms of exchanging water between the shelf and deep ocean (Lee et af., 1981; Lee and Atkinson, 1983). Frontal eddies are mesoscale, cyclonic filaments of warm water with associated cold cores formed by the upwelling of cool, high-nutrient North Atlantic Central Water (Yoder et af .• 1983). Phytoplankton blooms form in cold cores in response to nutrient enrichment. Are doliolid blooms advected onshore in the warm filaments of frontal eddies, or do blooms form in association with the phytoplankton of the eddy cold cores? I found that D. gegenbauri forms blooms by reproducing asexually along middle- shelf temperature and salinity fronts-where phytoplankton have grown in response to the upwelling of inorganic nutrients, probably driven by frontal eddies of the Gulf Stream. A quick response to phytoplankton growth is important, because the physical and phytoplankton systems are driven on an "event" time scale, with a mean flushing time for the outer continental shelf of just 14 days (Lee et al .. 1981). Since doliolids can respond quickly to phytoplankton blooms by growing and reproducing, their populations may serve as major energy "traps" along productive continental-shelf fronts.
2. Methods Exploratory zooplankton tows were made across the shelf to intercept a frontal eddy which was tracked using daily satellite maps of sea-surface temperature (via telefax from Dr. S. Baig, NOAA/NESS, Miami). An area of high doliolid concentration that was 60 km offshore was marked with a pair of spar buoys each with a window-shade drogue sail (3 x 3 m), and radio transmitter. The sails were set at 6-m depth, where the bloom appeared to be most concentrated. Drogues were used so that stations could be made within and outside the bloom to compare doliolid concentrations and associated physical and biological variables. The station grid consisted of an initial (grid-I) and final (grid-2) group of 6 stations in a "cross" pattern, with the drogues at the intersections of the arms of the crosses (Fig. 1). The stations of grid-l were completed in 18 h, followed by 4 stations at the 1985] Deibel: Do/io/id blooms & Gulf Stream eddies 213
.·::.:·.:·······'9.. I : ' 8 \ ~X-_.-----~o , 4 •.-----~16 II .... 1.4,· ..• I . . ...: . 1.5 \ ..... 1.2 J I .... ·••• 1.3
6 . I 1.1
5
, KM o
Figure 1. The station grid. Stations 1.1, and 2-6 (.-.) were the 6 stations of grid-1, stations 1.6, and 7-11 (.----.) were the 6 stations of grid-2, and stations 1.1-1.7 (_ . _) were the 7 stations at the drogues. The station separation on the two grids ranged from 15-20 km. The 20 and 40-m isobaths are shown. Note that the figure is encloseri in a box which is 1N x 1W. drogues (stations 1.2-1.5) 8 h apart (24 h total). The stations of grid-2 were completed in 18 h. The final station was at the drogues (station 1.7). Both transects of grids I and 2 were oblique to the local isobaths, and the drogue stations were approximately parallel to the local isobaths. An expendable bathythermograph (XBT) was used to determine the vertical temperature structure. Duplicate water samples were taken at each of two depths-( I) the upper mixed layer (5-12 m), or near-surface if the water column was isothermal, and (2) the lower mixed layer (15-33 m), or near-bottom if the water column was isothermal. Subsamples were withdrawn from the Niskin bottles to determine salinity, the concentration of size fractioned particulate organic carbon (POC), chlorophyll a and phaeopigments, the particle-volume versus particle-size distribution using an electronic particle counter, and the phytoplankton concentration and species composi- tion. The size classes of POC were-( I) that fraction passing through 35-}lm nylon mesh, (2) that fraction passing 180-}lm mesh but retained on 35-}lm mesh, and (3) that fraction retained by 180-}lm mesh. To minimize the breakage of large particles the mesh was mounted in the neck of the filter funnels just above the glass-fiber filters, and the samples were drawn through with low vacuum «120 mm Hg). POC was determined using a Perkin-Elmer model 240 elemental analyzer. 214 Journal of Marine Research [43,1
CHARLESTON WIND
ET
EB ,~~\\\l ,d\\.•••.... \\\\\\I',III\\\."\\\\\\'11"~\\\\\\\\\I\IIII"""''' ~1!!1II\l1'
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FB ,ji/I///Ib •••\"\\\\\\\\\\\11111', I "",\\\11'" II II\\\\\\W"" - 150cms-'
1-3 7-3 13-3 19-3 25-3 31-3
Figure 2. Charleston coastal wind (NOAA weather station, Charleston, S.C., 32°54.0'N, 80082.0'W) and 40-hour low-pass filtered current meter records for 1-31 March, 1978. The records have been rotated 30° to align with the local isobaths. Scale arrows appear to the right ofthe records. Mooring E was located at 31°35.8'N, 79°40.2'W, at the 75-m isobath (see also Fig. 3). Current meter ET (hE Top") was at 17 m depth, and meter EB (hE Bottom") was at 72 m depth. Mooring F was located at 31°39.6'N, 79°50.9'W, at the 45-m isobath (see also Fig. 3). Current meter IT was at 17 m depth, and meter FB was at 42 m depth. The doliolid bloom was studied from 1620 h on 18 March, to 1320 h on 21 March (-).
The concentration of extracted chlorophyll a and phaeopigments in 2 size classes was determined fluorometrically (Strickland and Parsons, 1972). The size classes were-(1) that fraction passing 35-~m mesh, and (2) that fraction retained on 35-~m mesh. Particle-volume versus particle-size spectra were generated onboard ship using a Coulter Counter model TAIl. Particle volume was measured in particle sizes from 2.0 to 102 ~m equivalent spherical diameter (ESD). The coefficient of variation was 5% for the 100-~m orifice tube, and <13% for the 400-~m orifice tube. A series of depth-stratified zooplankton tows was made with an opening-closing, modified Tucker Trawl (Hopkins et al., 1973). The net was a 2.5-m long cylinder cone, made of 110-~m nylon mesh, with a 40 x 40-cm mouth. It had a mesh-opening to mouth-area ratio of about 10:1. A T.S.K. flowmeter was mounted above the mouth. At least 3 oblique tows were made at each station. If the water column was stratified, a tow was taken in the upper mixed layer, in the lower mixed layer, and in the thermocline. The bottle samples were taken at the mid-point of each of these strata. If the water column was isothermal, the tows were made to divide the water column into 3 strata of equal depth. The tows were slow (1.3-2.4 knots), and had a median sample 3 volume of 10.6 m (n = 50). Zooplankton samples were fixed in 5% buffered formaldehyde. After 6 weeks, all samples were transferred to propylene glycol-propylene phenoxetol. Concentrated samples were diluted using a Motoda Box plankton splitter, and the dominant taxa 1985] Deibel: Do/io/id blooms & Gulf Stream eddies 215 counted until at least 30 individuals in each taxon were recorded (95% confidence interval = ±33% of the count-Lund et al .• 1958). The life history stage of all doliolids was recorded. A minimum of 50 doliolids in each sample was selected at random for length measurement to the nearest 10 /lm. Shrinkage of doliolids due to preservation was not measured. The zooplankton sampling scheme was constrained by the need to make stations both inside and outside the bloom (10's of km) in as little time as possible (i.e., quasi-synoptically), taking samples in several depth strata at each station. Therefore, routine serial-replicate tows at each station within each depth stratum were not possible using the single-net Tucker Trawl. Four serial-replicate tows (i.e., two tows within 5 minutes of one another at the same station) were made at stations where doliolids were both concentrated and rare. Identical sampling schemes have been used in our laboratory routinely in these waters (Paffenhofer, 1980). The median coefficient of variation of the doliolid concentration calculated from the above-mentioned group of 4 serial-replicate tows was 38%. This was in the upper end of the range for the other macrozooplankton enumerated (18-41%). This means that the concentration of doliolids must differ by at least If3 or 3x between single samples to be statistically significant at the 5% level (from Model II single-factor ANOV AR and the Newman- Keuls test, Zar, 1974; Snedecor and Cochran, 1980).
3. Results a. Meteorology and drogue drift. Prevailing winds were from the south and southwest for at least 8 days prior to the study (Fig. 2). Wind from these directions is favorable to Gulf Stream upwelling off the Georgia coast. The drogues drifted northeast during the study, parallel to local isobaths and the axis of the Gulf Stream. Drift trajectory was affected strongly by the semidiurnal tides. The drogues were retrieved 34.8 km from where they were launched, resulting in a mean velocity (vector displacement/time), of
14.4 cm x s-1. The drogues remained in a mixed water mass with a temperature of about 16°C and a salinity of 36.35 °/00. b. Satellite temperature imagery. Satellite maps of sea-surface temperature show that the bloom was not associated with the western "wall" of the Gulf Stream or directly with a frontal eddy (Fig. 3). Rather it was in the midst of a complex middle-shelf temperature front between coastal and outer shelf water. The front was bounded by the 14.6 and 18.3°C isotherms on 20 March (Fig. 3). The large-scale map from which Figure 3 was taken shows a frontal eddy east-southeast of the study area. The temperature front of the warm filament of this eddy was delimited by the 23.0°C isotherm on 20 March (Fig. 3). The eddy front was located over the 200-m isobath, more than 70 km southeast of station 1.1. Warm filaments of frontal eddies are typically located over the outer shelf off Georgia, between the 45 and 200-m isobaths 216 Journal of Marine Research [43, 1
Figure 3. Sea-surface temperature map for 20 March 1978 (NOAA-S obit #7413).The map was traced directly from alphanumeric plots of sea-surface temperature. The exact location of the isotherms was determined by eye. The radiometer measurements were not corrected for atmospheric attenuation. The station symbols are the same as in Figure 1. The 20, 40 and 100-m isobaths are shown. Current meter moorings E and F are shown at the coordinates specified in Figure 2.
(Lee at al., 1981). The drogues were launched in a 25 x 20-km area of mixed water formed where the 17.1 and 18.3°C isotherms diverged just east of 32N-80W (Fig. 3).
c. Descriptive physical oceanography. Although the sa~pling grid covered only 48 km in the cross-shelf dimension, two fronts were detected (Fig. 4a-c). There was a temperature and salinity front near the 20-m isobath. Further offshore, near the 40-m isobath, there was a temperature front. For reference, I will designate the nearshore front the "salinity" front, and the offshore front the "temperature" front. There was a 20-1an band of mixed water between the fronts with a temperature of about 16.0°C and a salinity of 36.35 0/00. The high salinity of the water between the fronts indicated that it was undiluted Gulf Stream water (Mathews and Pashuk, 1977; Lee et al., 1981). In fact, all of the T-S observations from this study were within the Gulf Stream envelop defined for this area by Lee et al. (1981). Thus, the relatively low temperature (-16°C) of the water between the fronts could not have been caused by mixing with cold, low salinity coastal water. Associated studies in this area (Lee et al., 1981; Yoder et al., 1981, 1983; 1985] Deibel: Do/io/id blooms & Gulf Stream eddies 217
.=, ~ - ..•.~--.~ •••::> lim 8 a: Q Q
::>••• oc:l a: Q Q -~..
.~N
o 000 Q N ~ • (W1H.l.d30 218 Journal of Marine Research [43,1
Yoder, 1983), have shown that mid-shelf water with these T-S characteristics is upwelled Gulf Stream water, driven onto the shelf by either meanders or frontal eddies. The influence of a frontal eddy is shown by data from two current meters moored about 40 km south-southeast of the middle of the station grid in outer-shelf water (Fig. 2). Since flow in frontal eddies is cyclonic, water in the warm filament flows in opposition to the mean northeasterly flow of the Gulf Stream. In the rotated current meter records of Figure 2, mean Gulf Stream flow is north and frontal eddies show a southward current reversal. Southward flowwas of greatest magnitude and duration at mooring E (75-m isobath). Southward flow at mooring F (45-m isobath) began at the same time, but was <20 cm x s-1. The southward flowat mooring E lasted until 0000 h on 22 March, while the flow further inshore, at mooring F, lasted only until 0600 h on 20 March. These data suggest that the offshore stations of the grids, 3 and 10, may have been influenced by a frontal eddy from 17-20 March. Eight to ten current reversals were recorded at moorings E and F between January and May, 1978. This translates to a frontal eddy frequency of 7-9 days. Lee et al. (1981) have shown that the shoreward advection of frontal eddies can drive upwelled cold-core water onto the middle shelf-forming a salinity front 10's of km northwest of the instantaneous western wall of the warm filament of the eddy. This salinity front corresponds to the temperature and salinity front observed between stations 1.1,6, and 4 (Fig. 4a, b). Because of the above evidence, I will refer to the water mass between the fronts as cold-core remnant (CCR) water. Stations 4,6,8, and 9 were nearest shore, in the midst of the salinity front between coastal water and the central mass ofCCR water (Fig. 4b). They were the only stations where there was water with a salinity of less than 36.29%0. The drogue stations, 1.1-1.7, were in the midst of the CCR water, with temperatures between 15.8 and 17.0°C, and salinities of 36.33-36.36%0 (Fig. 4a, b). The most offshore group of stations included 3, 5, 7, 10 and 11. They all had warm, outer-shelf water at the surface, with a temperature of 17.2-19.1 °C, and salinity of 36.34-36.430/00(Fig. 4a, b). The outer-shelf surface water was overriding the CCR water below (Fig. 4a, c). The satellite maps showed that the temperature front between stations 2 and 3, and 10 and 11, was not the western wall of the Gulf Stream, which was at least 40 km to the southeast.
d. Distribution and abundance of D. gegenbauri. There were high concentrations of doliolids in the CCR water at both the salinity and temperature fronts, and in or just below the pycnocline between CCR water and the warm, overriding outer-shelf water (Fig. 5). Concentrations were lowest at the most offshore stations (3, 5, and 7) in the warmest, most saline water. Concentrations were also low at station 9-a result which did not fit the pattern observed during grid-I. During grid-I, doliolid concentration and biomass ranged from a low of 14 x m-3, 1985] Deibel: Doliolid blooms & Gulf Stream eddies 219
o
20.., .0
DROGUE _ 0 __ .-"'1.7
.!•• • ..• - I. ..i 1.4 I.' §: ,I.Z '-.. IT 1.1 ~. ~--tl--"i-~o I. •7
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3 Figure 5. The distribution of Dolio/etta gegenbauri in terms of concentration (x m- ) and biomass (llgC x 1-I), for grid-l, the drogue time series, and grid- 2. Concentration is shown to the right of each station by the solid histograms, and biomass is shown to the left by the open histograms. Details of the sections are explained in the legend for Figure 4.
3 and 0.1 J.LgC x 1-1, at station 5, mid-depth, to a maximum of 3216 x m- , and 69.5 J.LgC x I-I, at station 2, mid-depth (Fig. 5). The differences between concentra- tions in the surface samples at stations 6, 1.1, 2 and 5 were less than the between- replicates error (see Methods), as were differences between the surface and bottom sample at station 6, and the surface and mid-depth sample at station 2. The mean 3 doliolid concentration during grid-I was 728 ± 474 x m- (±2 SE, n = 16). Concentration and biomass in the drogue samples ranged from 26 x m-3 and 0.06 J.LgC x I-I at station 1.7, near-bottom, to 1041 x m-3, and 26.1 J.LgC x 1-1 at station I. I-surface (Fig. 5). Concentrations were lower than those of grid-I and were variable probably because the drogues were not launched in either front, but were in the large area of mixed CCR water between the fronts. It is clear that the drogues did not track a homogenous water mass precisely during the entire study (note the warm water at the surface during stations 1.1, 1.5 and 1.6-Fig. 4a). Differences between concentrations in the surface samples of stations 1.3, 1.4, and 1.5 were less than the between-replicates error, as was the difference between the concentrations in the surface samples of stations 1.6 and 1.7. 220 Journal of Marine Research [43, 1
The pattern of doliolid abundance of grid-2 was similar to that of grid-l (Fig. 5). The concentration ranged from 49.4 x m-3 in sample lO-bottom, to a maximum of 2169 x m-3 in sample II-surface. Biomass ranged from 0.09 J.LgC x 1-1 in sample lO-bottom, to 62.4 J.LgC x 1-1 in sample 8-bottom. The mean concentration of doliolids during grid-2 was lower than, but not different from, the mean of grid-l (498 ± 328 x 3 m- , n = 16). The differences in concentration between samples 8-bottom, and II-surface and mid-depth, and the remaining samples were greater tl).anthe between- replicates error. The differences in concentration between the surface samples of stations 7, 9, and 10 were less than the between-replicates error. The grand mean (±2 SE) concentration of D. gegenbauri for the entire study was 3 3 483 ± 207 x m- (n = 46). The 8 samples containing >1000 zooids x m- were all within water of < 16.8°C and a salinity of <36.30 °/00. Rank correlations between the concentration of doliolids and salinity were negative for both grids and for the entire
study (rs-sal = -0.43, P < 0.01, n = 34). This indicates that D. gegenbauri was most abundant in the CCR water and the near-shore salinity front between CCR and coastal water.
e. Life history stage and length-frequency analysis. If there are high concentrations of old nurses and gonozooids in a sample, and low concentrations of larvae and oozooids, then the doliolid population was reproducing asexually when collected. The converse indicates a population which was reproducing sexually. The distribution pattern of life history stages was identical during grids 1 and 2. Gonozooids were most abundant in samples 6, surface and bottom, 8-bottom, 1.1 and 1.6 at the surface, and 2 and 11, surface and mid-depth (Fig. 6). Six of these 9 samples had the highest absolute concentrations of old nurses (Fig. 6). The concentration of old nurses was 20-31 x m-3 in samples 6-surface and bottom, and 2 and 11, surface and mid-depth, and 0-11 x m-3 in the remaining samples. The relative dominance of old nurses and colonies in these samples is amplified by the distribution of trophozooids and phorozooids (data not shown). The concentration of phorozooids in these 6 samples 3 3 3 was 70-336 x m- , 190 x m- in sample 8-bottom, and 0-65 x m- in the remaining samples. The concentration oftrophozooids was 159-615 x m-3 in these 6 samples, and 0-156 x m -3 in the remaining samples. These 9 samples had the highest relative concentration of gonozooids (62-77% for grid-I, and 57-83% for grid-2- Table 1), and the lowest relative concentration of larvae and oozooids «1%). This indicates that asexual reproduction was dominant in the salinity front nearshore, in the CCR water in or just below the pycnocline, and on the cold side of the temperature front offshore. Length-frequency analyses supported this conclusion, showing that these samples were numerically dominated by small, immature gonozooids, <5 mm long. The biomass in these samples was dominated by mature gonozooids 6-9 mm long. Larvae were most abundant in samples 3-surface, 10 at all depths, 5-surface and bottom, and ll-surface and bottom (Fig. 6). These samples had among the highest 1000 ~ 1067
SURFACE ~ .&>•• 100 E ::J ~ 50 Z 0 li a: I- z 10 w u z 5 0u
GNOL GNaLIGNOL GNOL~GNOL GNOL GNaLdGNOL GNOL GNOLIGNOL GNOL GNOL GNOL GNOL GNOL GNOL 6 1.1 2 3 4 5 1.2 1.3 1.4 1.5 8 1.6 II 10 9 7 1.7 STATION 1000
500 MID-DEPTH iii 'E
j 100 E ::J ~ 50 z o li a: I- z 10 w u ~ 5 u
NOL GNOL 1.1GNOL GNOL GNOL GNOLIIGNaL GNOL GNOLJ GNC)L 6 1.1 2 3 4 5 1.2 1.3 1.4 1.5 8 1.7 STATION
1000 1632 11304 500 ;;-, NEAR BOTTOM E
iE 100 ::J Z 50 z o li a: I- z 10 w u oZ 5 u 1,1 G~G~L~lGN~GNQ~~G~~Q~~G~~GN~~O~OOO~GN~GOOLGNll~I~I~I~~ I~~ 6 1.1 2 3 4 5 1,2 1.3 1.4 1.5 8 1,6 II 10 9 7 1,7 STATION
Figure 6. The concentration (lOglO)of 4 life history stages of Dalia/etta gegenbauri (x m-3) at each station and depth. There were no mid-depth samples at stations 6, 4, 1.3, 8 and 9. G ~ gonozooids, N ~ old nurses, 0= oozooids, and L ~ larvae. 222 Journal of Marine Research [43,1 relative concentrations of larvae, ranging from 6--42% during grid-I, and 1-37% during grid-2 (Table 1). These samples also had low relative concentrations of gonozooids (25-58% during grid-I, and 18-58% during grid-2). Therefore, sexual reproduction was most common offshore in or near the warm water of the temperature front. Based on a high relative concentration of larvae, samples 1.I-bottom, 3-bottom, 5-mid-depth, and 7-bottom also were dominated by sexual reproduction (Table 1). Again, length-frequency analyses supported this conclusion. These samples were numerically dominated by larvae 0-1 mm long, and had biomass peaks made up of mature gonozooids 8-11 mm long. The ratio of the concentration of gonozooids to that of old nurses is a measure of the relative intensity of asexual reproduction (Fraser, 1949; Van Zyl, 1959; Braconnot, 3 1971). At total doliolid concentrations of <100 x m- , the ratio ranged from 5-10:1. 3 At concentrations of 100-500 x m- , the ratio ranged from 20-600:1. At concentra- 3 tions >1000 x m- , the ratio ranged from 49-1000:1. Asexual reproduction was dominant at the two fronts, at stations 6,8,4,9,7,2, and 11. Both sexual and asexual reproduction were occurring at the drogue stations in the middle of the CCR water mass, in or beneath the pycnocline. Sexual reproduction was predominant in the warm water of the offshore temperature front, at stations 3,5, and 10, and near-bottom in the CCR water, at stations 1.1, 1.6, and 11. f Seston analyses. Although there was a trend of decreasing scatter in the seston variables with increasing concentration of doliolids, few of the negative relationships between the abundance of doliolids and the concentration of their food (seston) were statistically significant. The concentration of doliolids was inversely correlated (rank correlation) with total particle volume (rs = -0.59, n = 13, P < 0.05), and the concentration of chlorophyll a in the small-size fraction (rs = -0.65, n = 13, P < 0.05), only during grid-I. There was no statistically significant association between the concentrations of D. gegenbauri and POC. Values of the seston variables were relatively high, and were indicative of enhanced frontal productivity (Table 2). In all samples, partjcle volume, from 2-102 I'm equivalent spherical diameter, ranged from 0.51-3.48 mm3 x 1-1, total POC from
130--400 JLgCx 1-I, and total chlorophyll a from 0.30-2.98 JLgx 1-I. The extreme values occurred primarily during grid-2. Most of the total seston passed through 35-JLm mesh. The mean (±SD) proportion of POC and chlorophyll a in this size fraction was 48 ± 2.1%, and 56 ± 2.7%, respectively. The carbon:chlorophyll a ratio was high (208 ± 88JLgC x JLgchlorophyll a-I, n = 35) and the Fo/ Fa ratio low (1.59-1.63) for seston from upwelled Gulf Stream water (Yoder et al., 1981). This indicates a high proportion of grazer-modified chlorophyll, senescent phytoplankton, or detrital carbon (perhaps doliolid fecal debris-Pomeroy and Deibel, 1980; Madin, 1982). Generally, there were high concentrations of particles, POC, and chlorophyll a in the near-bottom samples, suggesting that the CCR water contained higher concentra- 1985] Deibel: Doliolid blooms & Gulf Stream eddies 223
Table 1. The relative life-history stage composition (%)* of samples of Dolioletta gegenbauri from grids 1 and 2.
Sample Old Nurse Gonozooid Larva Oozooid Reproductive Status
Grid-l
6-5+ 2 62 As 6-B+ 1 66 <1 As 68 <1 5 ~t~+ 54 1 2 S(As) 1. l-B 44 10 20 5 2-5 2 65 As 2-M <1 77 <1 <1 As 2-B 2 68 <1 <1 As 3-5 4 41 19 11 S(As) 3-M 1 64 2 4 As(S) 3-B 3 37 17 23 S(As) 4-5 4 64 <1 4 As 4-B 2 66 As 5-5 <1 58 6 2 S(As) 5-M 42 11 5 5 5-B 25 42 6 5 Grid-2
8-5 2 53 5 As 8-B 83 As 1.6-5 <1 65 As 1.6-M <1 60 4 1 As(S) 1.6-B 3 69 5 3 As(S) 11-5 1 58 1 1 As(S) 11-M 1 57 1 As 11-B 2 35 15 7 5 10-5 43 8 17 5 10-M <1 41 8 23 5 10-B 18 37 27 5 9-5 2 54 <1 As 9-B 3 61 As 7-5 3 51 5 2 As(S) 7-M 1 51 'Z 6 As(S) 7-B 29 38 10 5
*Since the proportion of phorozooids, trophozooids, and probuds are not shown, the above rows will not sum to 100%. A hyphen (-) means that none of that life-history stage was found in the subsample. +upper-mixed-layer oblique tow. *oblique tow through the thermocline. flower-mixed-layer oblique tow. "As"is a primarily asexually reproducing population. "S"is a primarily sexually reproducing population. 224 Journal of Marine Research [43, 1
tions of particles than did the overriding outer-shelf water (Table 2). This is typical of cold-core upwelling (Yoder et al., 1981, 1983; Yoder, 1983). These observations are supported by rank correlations between temperature and total particle volume during grid-2 (rS-TPv = -0.73, n = 12, P < 0.01). Although chlorophyll a was generally most concentrated in near-bottom waters, between station variability and the aforemen- tioned negative correlation between the concentration of doliolids and that of chloro- phyll a in the small-size fraction made all negative correlations between chlorophyll a and temperature and salinity not statistically significant. During grid-l there was a positive correlation between the concentration of chlorophyll a and temperature (rs =
0.71, n = 13, P < 0.01), indicating moderately high concentrations of chlorophyll a in the warm water offshore, and intense grazing on phytoplankton in the colder CCR water and salinity front. The high chlorophyll a concentrations in CCR and cool near-bottom waters were due primarily to Guinardiafiaccida and Rhizosolenia sp., two large diatoms that are typical of productive, upwelled Gulf Stream water (Yoder et al., 1981, 1983; Marshall, I 1971). Dinoflagellates also were most concentrated in these waters (15-118 x ml- ). There was no evidence of an impact of doliolid grazing on these large diatoms and dinoflagellates, since there were no significant negative correlations between their abundance. Two smaller diatoms were dominant in the warm surface waters of the offshore temperature front-Leptocylindrus sp., and Chaetoceros sp., typical Gulf Stream surface-water taxa (Marshall, 1971). These species did not occur in CCR water or in the nearshore salinity front, suggesting again that the CCR water had upwelled from beneath the Gulf Stream.
g. Grazing by doliolid populations. The particle-volume versus particle-size spectra were of 2 types. The first type included samples with the highest particle concentra- tions and unimodal or biomodal spectra. Unimodal spectra, e.g., stations 1.1-bottom (Fig. 7b), and 2-bottom (Fig. 7c), had peaks of particle volume in sizes from 8-20 ~m E.S.D. The bimodal spectra, e.g., station 3, had peaks from 8-13 and 32-51 ~m E.S.D. (Fig. 7d). The second type of spectrum, typical of samples with low particle concentrations, was flat with equal particle volume concentrations of about 3 0.06 mm x 1-I in all size classes from 2 to at least 51 ~m E.S.D. (e.g., station 6-Fig. 7a; station 2-surface-Fig 7c). Grazing by D. gegenbauri seemed to have a role in producing and maintaining these flat spectra. There was strong coincidence of high biomass of D. gegenbauri and the flat, low-concentration particle-volume spectra. Samples at station 6 and station 2-surface are representative of all samples with high biomass of D. gegenbauri (Fig. 7a, c). Samples 1.1-surface and 2-bottom are typical of samples with intermediate concentrations of D. gegenbauri (Fig. 7b, c). Samples l.l-bottom and station 3 are samples with low biomass of D. gegenbauri, and high concentrations of particles with 1985] Deibel: Do/io/id blooms & Gulf Stream eddies 225
~ ~ ~ ~ E '" 0;-'" ;1 '" ~~ NCD •...'" CD'" '" ~ ""A ...... o~ ...•"I "'0 ~ .. :r ..'" "" .. ..'" u >- .•.. ..0 ~ ~ ~ E '" '" '" ;1 ": "''''' NCD ,..:~ cO",'" '" ""v ...... +1 .•• +1 .•• ~o '"0'".. "'''' '""''''..
e '; ": ~~ ~ ~ •...... '" 0 CD ...... +I'" A CDN 0 ••• """" "''''N"" """"
",!,:, .: ~ +'•• '"•. e '" c e u CD .: 0 0 C 0" "'''' "'''' U" •. 0 •...c.. CD'" "'N +' ... >. .... '", ...... +I +1 +1 .•• ;;: c 0 •. +' '" 0.30 biom r STA. 6-SURF. 'ggg 0.30 - -- ass r STA.2-5URF. I~ ;-0.24 J ;-0.24 "1., or~'Ulr 0 I 1.00 :: 0.18 .:: 0.18 r,.J I 1.00 O.~O ",- ~ 0.12 : 1 0..50. ~ 0.12 I , fiI °E ,...... L ~ - 0.06 0.10 0.06 I ~ •..•..•~~ 0..10 'e u" 'SO.oo 0.05 0.05 t':: .... r ~O.OO CO' z: A z III c ! z en ~ 0.30 JL-l STA.&-BOTT. 10.00 ; ~ 0.30 1000 en ~.OO ~ 5.00 ~ " 0.24 ~ l, / ,;; ...••• 0.24 ~ 0.18 r I ~. 1.00 ~ 0.18 1.00 ii 0.~0 o.!lO ~ 0.12 l,r,ni 50.12 I I > 006 ",WII 0, 0.10 0.06 • I 0.10 0.05 0.05 O. 0-12-3+51'" .",10-11••• 3_>. o DOUOLID LENGTH (rnml 2 ~ 5 Ii 13 zO 32 51 81 PARTICLE SIZE ("",ESD) 5TA.3-5URF. 10..00 0.30 ~:88 0.30 ~.OO ;-0.24 ;- 0.24 :: 0.18 1.00 :: 0.18 1.00 O.~O _ 0.50 _ ~ 0.1 ~ 0.\2 ..• '" -0.06 0.10 'e -: 0.06 010 'E 0.05 ~ 0.0s ~ ~OOO GO ~ 0.00 CO' z ?- B ! z o ! III en 10.30 10.00 ~ ~ 0.30 10.00 ~ u STA. 1.IBOTt 5TA.3-BCTT. 5.00 ~ " 0.24 5.00 ; •• 0.24 o 1.00 iii ~ 0.18 1.00 '"~ 0.18 \,\ i 0.60 '''-\./" O.!lO g 0..12 5 0..12 •/ ',,> r-"" J > 0.06 0.10. 0..06 .....•.•..~/ rJ-l ""', 0..10. J"JI I r'~I 0 0..0.5 0.05 0.00 0.00 0-1 2-3 +5 1-1 1-.1Ot1l:H3 •••••' > •• 0-' 2-3+56"7 8-9'0-11 1;!-1314-">16 DOUOUD LENGTH (rrwn) DOLICLIO LENGTH {mml 2 3 5 Ii 13 20 32 51 81 2 3 5 8 13 20 32 51 8'1 PARTICLE SIZE (~m ESD) PARTICLE SIZE (~m ESO) Figure 7. (a-d) Histograms of the biomass of Do/io/etta gegenbauri in I mm size classes and the particle-volume versus particle-size spectra from the Coulter Counter, for the east-west transect of grid-I. The Coulter Counter channel >811LmE.S.D. is a cumulative channel for all particles >811Lmcounted by the 400-lLm orifice tube. The discrete depth samples for particle counts came from the mid-point of the oblique zooplankton tows. Particle-volume versus particle-size spectra for all remaining discrete depth samples are available from the author. unimodal or biomodal particle-volume spectra (Fig. 7b, d). It seems that D. gegenbauri is capable of removing particles of not only 8-20 ILm E.S.D., but also up to 51 ILm E.S.D. (e.g. compare station 2-bottom to station 2-surface). A first-order estimate of the grazing impact of a population of D. gegenbauri can be made using laboratory-determined weight-specific grazing rates. Since bloom popula- tions were numerically dominated by gonozooids, I used the weight-specific grazing rates for that stage (Deibel, 1982b) to calculate a hypothetical volume-swept-clear by each doliolid population with a known zooid length composition. Since I found no statistically significant change in grazing rate with increasing gonozooid weight, I used a mean value of 17.0 ml x ILgC-1 x 24 h-1• The laboratory experiments were 1985] Deibel: Doliolid blooms & Gulf Stream eddies 227 conducted at a temperature (20°C) somewhat higher than that in the field (13.5- l8.9°C). Population grazing rates are expressed as a % of a unit water volume (in this 3 case, 1 m ), swept clear each day. A population grazing rate of 100% would mean the population was clearing 1 m3 of water each day. The pattern of population grazing rates was similar during grids 1 and 2, indicating that the drogues remained in a fixed position relative to the doliolid bloom (Table 3). Fourteen of the 51 samples had population grazing rates> 10% x 24 h -1. Four samples during grid-I, and the same 4 relative to the drogue position during grid-2, had grazing rates equivalent to sweeping clear their resident water volume in 2 days or less (underlined values in Table 3). The greatest grazing pressure was in the salinity front, in the CCR water below the overriding outer-shelf surface water, and at the edge of the offshore temperature front. The size classes responsible for the greatest grazing pressure were between 5 and 10 mm. The most frequently dominant size class was the 8-9 mm class (4 of the 8 samples). Although stations 1.1 and 1.6 show that the drogues were in an area of high concentration of doliolids at the beginning of each of the 2 grids, the reduced population grazing rates at stations 1.2-1.5 show that the drogues did not stay with high concentrations of doliolids at all times. This supports the evidence of changing vertical temperature structure at the drogues (Fig. 4a). I assume that this variability in doliolid concentration was due to the differential effects of wind and tide on the drogues and on the water column. The fact that stations 1.6 and 1.1 had similar concentrations of doliolids can only be viewed as fortuitous. h. The zooplankton community. Of the 46 zooplankton samples, 7 were dominated numerically by D. gegenbauri. Because of its relatively large size, D. gegenbauri may have dominated zooplankton biomass at another 7 stations. Oithona sp. was the dominant net zooplankter in 22 of the 46 samples. Oithona sp. is a coastal form, and is not typical of the Gulf Stream (Paffenhofer, 1980). Oncaea sp. was nearly equal in abundance to Oithona sp. It was dominant numerically in 1 of the 46 samples, but was always the most abundant cyclopoid when the doliolids were the dominant zooplankter. Oncaea sp. occurs in high concentrations in summer, bottom Gulf Stream intrusions, and is typical of mid-shelf waters (PaffenhOfer, 1980). During the present study Oncaea sp. occurred in great numbers along with high concentrations of D. gegenbauri and Oikopleura dioica. Oncaea sp. may feed on oikopleurid houses or doliolid tunics, or may use their surfaces for a substrate (Alldredge, 1972). After the cyclopoida, the harpacticoida were the most abundant net zooplankton. Euterpina acutifrons was the predominant harpacticoid. It was the most abundant net zooplankter in 9 of the 46 samples. E. acutifrons probably spends much of its life on large particles or aggregates, which may explain its positive correlation with O. dioica. The most abundant calanoid was Paracalanus sp., an indicator species of upwelling in the Georgia Bight (Paffenhofer, 1980). 228 Journal of Marine Research [43, 1 Table 3. Population grazing rate* of Dolioletta gegenbauri. Station Sample De~h SURF+ r~ID-DEPT BOTr' 6 36 NT 99 1.1 44 4.0 0.7 2 72 118 5.7 3 1.2,1.5 6.0 0.7 4 8.5 NT 12 5 23 0.2 1.2 1.2 0.9 4.0 1.6 1.3 4.7 NT 3.6 1.4 19,15 7.2,10 2.9,3.5 1.5 6.0 6.1 1.6 1.7 6.3 3.0 0.10 8 9.5 NT 106 1.6 61,23 6.2 3.6 11 52 42 3.2 10 5.5 0.2 2.3 9 4.6 NT 2.8 7 2.8 5.4 1.0 *~ of 1 m3 of water swept clear x 24 h-1 +upper-mixed-layer oblique tow *oblique tow through the thermocline 'lower-mixed-layer oblique tow NT = no tow taken at that depth. Two values for a sample are grazing rates calculated from 2 successive-replicate tows. The underlined values are population grazing rates > 40~ x 24 h-1. 1985] Deibel: Doliolid blooms & Gulf Stream eddies 229 Because of the small size of the chaetognaths in this study, and their low concentrations compared to summer, bottom intrusions in Onslow Bay, N.C. (Paffen- hOfer, 1980), their predation pressure on cope pods must have been small-and thus cannot account for the low concentrations of copepods found in doliolid bloom samples (see Discussion). Temora sp., Centropages sp., Undinula sp., and Eucalanus sp. were rare compo- nents of the net zooplankton community. Temora sp., Centropages sp., and Eucalanus sp. were found in higher concentrations in summer, bottom intrusions off northeastern Florida, and in Onslow Bay, North Carolina (Paffenhofer, 1980, 1983). Perhaps these large copepods do not have time to respond to winter /spring upwelling events, because these events are short lived (7-14 days) in comparison to the summer, bottom intrusions (1-6 weeks). The cold-core remnant water mass, isolated between 2 mid-shelf fronts, seems to have a unique, mid-shelf assemblage of net zooplankton. There must have been little mixing with coastal water, because no cladocerans were found in the doliolid bloom samples. Also, there must have been little mixing with pure, Gulf Stream surface water, because there was no evidence of Euchaeta sp., Mecynocera sp., or Lucicutia sp. 4. Discussion Thaliacean blooms have been associated with ocean-scale water masses and current systems (Fraser, 1949; Berner and Reid, 1961), with internal seiches in the Bay of Bengal (Sewell, 1926), with upwelling zones (Binet, 1976), and with ocean fronts (DeDecker, 1973; Atkinson et al.. 1978). Salp blooms often occur along with extremely low phytoplankton and zooplankton concentrations (Deevy, 1962; Fraser, 1949), and have been associated with low juvenile fish recruitment and poor commer- cial fisheries (Fraser, 1949). Fraser (1949) found that small sexual stage zooids made up salp blooms, and he noted that these zooids result from asexual reproduction. Binet (1976) hypothesized that salp blooms form in response to phytoplankton blooms resulting from the onset of spring water column stability. My initial hypothesis was that Dolioletta ge.genbauri is an open-ocean zooplankter, brought into continental shelf waters in the warm filament of Gulf Stream frontal eddies. Contrary to my hypothesis, I found a doliolid bloom about 60 km northwest of the Gulf Stream -temperature front, among a complex of mid-shelf fronts. In comparison to Gulf Stream and surrounding shelf waters, there were high concentra- tions of particles, POC, and chlorophyll a at these fronts. The bloom occurred between two fronts-a temperature and salinity front at the 20-m isobath, and a temperature front at the 40-m isobath. The fronts were about 20 km apart. The mixed water mass between the fronts was cool, but had a high salinity equal to that of the Gulf Stream-so its low temperature could not have resulted from the admixture of warm, 230 Journal of Marine Research [43,1 high salinity Gulf Stream surface water with cold, low salinity coastal (i.e., inner- shelf) water. a. Gulf Stream shelf-water interactions. There are two possible origins of Gulf Stream water of 16°C on the middle continental shelf off Georgia (T. Lee, pers. comm.). It could be surface Gulf Stream water that has been atmospherically cooled, or it could be upwelled water from the cold-core of Gulf Stream frontal eddies. The physical and biological evidence supports the second alternative. The concentrations of particles, POC and chlorophyll a (Table 2) were all many times higher than is typical of Gulf Stream surface water-and were similar to concentrations measured in other studies of Gulf Stream upwelling (PaffenhOfer et al.. 1980; Yoder et al.. 1981, 1983). Secondly, the zooplankton community contained no representatives of surface Gulf Stream waters, but was similar in composition to communities observed in other studies of Gulf Stream upwelling (Paffenhofer, 1980, 1983). Finally, the CCR water during this study had the same T-S characteristics (16°C, 36.300;00)as the North Atlantic Central Water which forms the upwelled cold-cores of Gulf Stream frontal eddies (Mathews and Pashuk, 1977; Lee et al.. 1981). The evidence suggests that the doliolid bloom formed in remnant, upwelled cold-core water that was stranded on the middle continental shelf between 2 fronts (Fig. 3,4). These mid-shelffronts are similar to those which occur off S. Africa, where Thalia democratica and Doliolum denticulatum blooms form in an along-shore band, about 90 km offshore, between an eddy of the Agulhas Current and the front of the incipient Benguela Current (DeDecker, 1973; Lazurus and Dowler, 1979). Upwelled, cold-core water can reach the middle shelf (Lee et al .• 1981) and high nutrient concentrations in cold-core water result in aperiodic phytoplankton blooms (Yoder et al .• 1981, 1983; Yoder, 1983). The physical and phytoplankton dynamics of cold-core water are well described in the literature, and are beyond the scope of this report (Lee et al.. 1981; Yoder et al.. 1981, 1983; Yoder, 1983). Please see the preceding references for contemporary reviews of frontal eddy dynamics. Thaliacean tunicates are perfectly adapted to respond to short-term phytoplankton blooms with high grazing, asexual reproduction, and population growth rates (Heron, 1972a,b; Diebel, 1982a,b). b. Origin of bloom populations. "Seed" stocks of D. gegenbauri may live in the Gulf Stream, or in shelf waters. Previously, investigators have asserted that thaliaceans are advected into coastal waters from oceanic sources (Fraser, 1949; Grice and Hart, 1962). However, D. gegenbauri is present throughout the year in continental shelf waters off Georgia (Deibel, unpub1.; PaffenhOfer, pers. comm.). The extremely high fecundity of D. gegenbauri means that only a few old nurses are needed to start a bloom. Probably there are sufficient D. gegenbauri in middle and outer-shelf waters to initiate a bloom when CCR water is confined between two mid-shelf fronts. If doliolid blooms were caused only by physical mechanisms, e.g., advection from the 1985] Deibel: Doliolid blooms & Gulf Stream eddies 231 Gulf Stream and concentration at a mid-shelf convergence front, they would be composed of an even mixture of adult and juvenile stage, and all 5 life history stages would be present. This was not the case, since the bloom was dominated by small gonozooids, which are produced asexually by the old nurses (Fig. 6). Non-bloom populations had gonozooid to old nurse ratios of about 10:1, while bloom populations had ratios greater than 1000:1. This is a biological response to physical driving forces, and not simply a case of concentration by physical mechanisms alone. If doliolid blooms were advected into shelf waters from the Gulf Stream, then the net zooplankton community should include many Gulf 5!tream representatives. This was not the case either, since some of the most common Gulf Stream copepods, e.g., Euchaeta sp., Mecynocera sp., and Lucicutia sp., were not present in any of the samples. c. The seasonality and time scale of doliolid blooms. This bloom of D. gegenbauri showed no sign of dispersing over the 3 day study period (Fig. 5). Thaliacean blCIOms have been tracked for days to months (Fraser, 1962; Braconnot, 1971; Atkinson et al., 1978). The current meter records from January to April, 1978, showed the leading edge of frontal eddies passing through outer-shelf waters every 9 days. Since it appears that doliolid blooms depend on eddy-associated upwelling and water column stability, the mean duration of blooms may be from 9-14 days. This is ample time for the old nurses to asexually produce gonozooids (Deibel, 1982a). Bloom duration may be shortened by across-front mixing caused by strong winds associated with storms that pass through the area in the winter and spring every 7-9 days (Lee et al., 1981). The seasonal occurrence of doliolid blooms may be due to seasonal changes in the way frontal eddies interact with shelf water. A band of upwelled cold-core water on the middle shelf, stranded between two fronts, may be necessary for a doliolid bloom to occur. Seasonal plots of the density of shelf and upwelled Gulf Stream water suggest that this physical pattern is most likely from December to March (Atkinson, 1977). This may be partial explanation for why doliolid blooms are most common in the winter and spring. D. gegenbauri. and the salp, Thalia democratica, may be the only zooplankton herbivores in the Georgia Bight capable of rapid population response to short-lived phytoplankton blooms. Copepods in these waters take at least 3 weeks to respond to upwelling events (PaffenhOfer, 1980). Without thaliaceans these large phytoplankton blooms may be advected from the shelf about every 14 days. Doliolid blooms may have an impact on energy flowbeyond their own short existence, by trapping phytoplankton biomass and converting it into fecal material and tunicate biomass, and thus "smoothing" the energy pulses of aperiodic phytoplankton blooms. d. Seston and the grazing impact of doliolids. D. gegenbauri seemed to have a marked impact on the concentration and size distribution of seston particles. During grid-I, the concentration of D. gegenbauri was inversely correlated with total particle volume, and 232 Journal of Marine Research [43, 1 with the concentration of chlorophyll a. As is typical of Gulf Stream bottom intrusions at the shelf break (PaffenhOfer et al., 1980), the particle-volume vs. particle-size spectra in the present study were usually bimodal, with peaks in particle sizes from 8-20 and 32-51 #lm E.S.D. However, when doliolids were abundant, particles were removed from both peaks. These particle sizes are those most important for juvenile (8-20 #lm) and adult copepods (32-51 #lm). In 8 of the 46 samples of this study, D. gegenbauri was clearing its resident unit 1 water volume in <48 h (40-120% x 24 h- ). There is no other estimate of the population clearance rate of thaliaceans. Primary production rates in the cold-cores of Gulf Stream frontal eddies are high, ranging from 2-6 gC x m-2 x 24 h~l (Yoder et al., 1981, 1983). Phytoplankton doubling times have been estimated to be as short as 1-2 days. Even these high rates of phytoplankton growth are probably not sufficient to maintain populations in those areas of greatest doliolid concentration. Samples with high concentrations of D. gegenbauri were removing POC at 1-2 x its replacement rate (based on the estimated maximum and mean rate of primary production). This removal rate also was evident from the flat, low concentration particle-volume vs. particle-size spectra in samples with high concentrations of doliolids (Fig. 7). e. The exclusion of zooplankton by D. gegenbauri. It has been asserted that net zooplankton are rare in thaliacean blooms (Fraser, 1949, 1962; Van Zyl, 1959; Madhupratap et al., 1980). In this study, the doliolids were inversely associated with most of the net zooplankton, but there were few statistically significant inverse correlations. Atkinson et al. (1978), and Ogawa and Nakahara (1979) also failed to find significant correlations between thaliaceans and other net zooplankton. Sameoto (1978) reported significant positive correlations between salps, and pteropods and euphausiids. However, the mean concentrations of the various net zooplankton were 1/2 to 1/10 of those reported for summer, bottom Gulf Stream intrusions off North Carolina and Florida (Paffenhofer, 1980, 1983). The exclusion of -net zooplankton from doliolid blooms could be caused by anyone of 4 mechanisms. (1) High concentrations of doliolids may have clogged the zooplankton net, causing artifactual exclusion by reducing the volume of water actually strained, and thus reducing the catch of zooplankton. (2) Large, oceanic salps can ingest copepod eggs, nauplii, and adults (Van Zyl, 1959; Kashkina, 1978) and thus may reduce directly the concentrations of associated net zooplankton. (3) The high asexual reproduction rates of D. gegenbauri may allow it to increase in concentration quickly in response to phytoplankton blooms-more quickly than can other net zooplankton with longer generation times and slower growth rates (Heron, 1972b; Deibel, 1982a). (4) The ability of D. gegenbauri to consume small particles at high rates may reduce the concentration of food available to juvenile copepods, resulting in low copepod recruitment rates. The first mechanism.can be disposed of easily. Fraser (1949) and Grice and Hart 1985] Deibel: Doliolid blooms & Gulf Stream eddies 233 (1962) found some samples with high concentrations of thaliaceans and copepods. In this study, one of the samples with high concentrations of D. gegenbauri also had one of the highest concentrations of net zooplankton (station 11 mid-depth). Because of this, and because of the high mesh-to-mouth-area ratio of the cylinder-cone zooplankton net, fouling can be eliminated as a mechanism causing artifactual exclusion of net zooplankton. Carnivory by thaliaceans is harder to discard. D. gegenbauri is 10-100 x smaller by weight than the open-ocean salps studied by Van Zyl (1959) and Kashkina (1978). Accordingly, D. gegenbauri's individual grazing rates are orders of magnitude less than are those of oceanic salps. Since the bloom was dominated by gonozooids which were 7-9 mm long, the maximum size of particle which can be ingested is about 80 ~m. Also, I have never seen animal remains in the stomach or fecal material of D. gegenbauri (Deibel, unpubl.; Pomeroy and Deibel, 1980). I doubt that D. gegenbauri consumes copepod nauplii or copepodids. They may ingest copepod eggs, but it has not been shown experimentally. The exclusion of net zooplankton from doliolid blooms is undoubtedly due to a combination of mechanisms 3 and 4. The high asexual fecundity and population growth rates ofthaliaceans are well known (Heron, 1972a,b; Deibel, 1982a). Each old nurse of D. gegenbauri can produce asexually thousands of gonozooids in a matter of days, in comparison to generation times of the other net zooplankton which range from weeks to a month. It seems that frontal eddies flush the outer Georgia shelf frequently in the winter and spring (every 14 days), and so maintain a temporally young ecosystem, often dominated by the fast-responding doliolids. Early juveniles of small copepods require particles of < 10 ~m diameter to feed and grow (PaffenhOfer, 1980; PaffenhOfer et al., 1980). D. gegenbauri seems to reduce the concentration of all particles of <50 ~m E.S.D. to very low levels (Fig. 7). This means that they compete directly with juvenile copepods for food particles in the size range of <10 ~m E.S.D., and may reduce copepod recruitment significantly. Doliolids utilize rapid and fecund asexual reproduction to colonize middle-shelf fronts off Georgia, U.S.A. Although not directly associated with the warm filament of Gulf Stream frontal eddies, doliolid blooms may depend on the physical and biological effects of the cold cores of eddies on middle-shelf phytoplankton production. Much remains to be done. We need to know more about the spatial and temporal dimensions (from weeks to seasons) of thali ace an tunicate blooms, and about the fate of thali ace an body and fecal biomass. The effect of nitrogenous excretory products of thaliaceans in blooms on phytoplankton metabolism is unknown and may be substantial. Population grazing models should be more complex and realistic, taking into account the rates of phytoplankton and tunicate growth. As we become increasingly aware of the biological dynamics of coastal fronts, some of the above questions hopefully will be addressed. Acknowledgments. I thank the crew of the R/V Blue Fin. who contributed fundamentally to the success of this work. I thank Dr. G.-A. PaffenhOfer for his guidance, support, and field 234 Journal of Marine Research [43, 1 assistance. Steve Bishop made the chlorophyll measurements. I am indebted to Dr. Tom Lee for the current meter records, to Dr. S. Baig for the daily satellite maps, and to Dr. R. Legeckis for the satellite-derived sea-surface temperature photographs. I am grateful to Drs. G.-A. Paffen- hOfer, L.R. Pomeroy, L.P. Atkinson, T. Lee and 3 anonymous reviewers for many helpful comments on a long earlier draft. Thanks to Maureen James for operating the word processor. This manuscript was adapted from a dissertation submitted in partial fulfillment of the requirements for the Ph.D. degree, Department of Zoology, University of Georgia, Athens. It was supported by Department of Energy contract DE-AS-09- 76EV009 36 to Dr. G.-A.P. Marine Sciences Research Laboratory Contribution Number 535; Newfoundland Institute for Cold- Ocean Sciences Number 40. REFERENCES Alldredge, A. L. 1972. Abandoned larvacean houses: unique food source in the pelagic environment. Science, 177, 885-887. Atkinson, L. P. 1977. Modes of Gulf Stream intrusion into the South Atlantic Bight. Geophys. Res. Lett. 4, 583-586. Atkinson, L. P., G. -A. Paffenhofer and W. M. Dunstan. 1978. The chemical and biological effects of a Gulf Stream intrusion off St. Augustine, Florida. BulL Mar. Sci., 28. 667-679. Berner, L. D. and J. L. Reid, Jr. 1961. 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