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' FT \\\\\\\' 'fl" 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.