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Ingestion of Phytoplankton and Bacterioplankton by Polar and Temperate Echinoderm Larvae

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Ingestion of Phytoplankton and Bacterioplankton by Polar and Temperate Echinoderm Larvae Differences in respiration rates among samples (not meas- References ured) could account partly for the unexplained variance, al- though rates at 0 °C are expected to be lower than the 12 percent Cullen, J. 1990. On models of growth and photosynthesis in phyto- assumed for 20 °C (Sakshaug, Kiefer, Andresen 1989). plankton. Deep-Sea Research, 37, 667-683. In conclusion, phytoplankton growth rates at the mixed layer Eppley, R.W. 1972. Temperature and phytoplankton growth in the sea. were on the average 53+22 percent of the maximal rates ex- Fishery Bulletin, 70, 1063-1085. Holm-Hansen, 0., and M. Vernet. pected (0.58 per day) for the ambient temperature (Eppley 1972; 1990. RACER: Phytoplankton dis- tribution and rates of primary production during the austral spring Spies 1987). Maximum growth rates were observed in a non- bloom. Antarctic Journal of the U.S., 25(5), 141-144. bloom assemblage, and lowest growth rates were associated Kocmur, S., M. Vernet, and 0. Holm-Hansen. 1990. RACER: Nutrient with low nitrate concentrations at the surface. Growth rates depletion by phytoplankton during the 1989 austral spring bloom. can be modeled as a function of irradiance, but at saturated Antarctic Journal of the U.S., 25(5), 138-141. irradiance, they are mainly dependent on the chlorophyll-to- Laws, E.A., and IT. Bannister. 1980. Nutrient- and light-limited carbon ratios. growth of Thalassiosira fluviatilis in continuous culture, with implica- We would like to thank the captain and crew of the RIV Polar tions for phytoplankton growth in the ocean. Linnology and Ocean- Duke for their help, C. Fair for technical assistance, and E. ography, 25, 457-473. Brody for graphics. This project was funded by National Sci- Redalje, D., and E. Laws. 1981. A new method for estimating phyto- plankton growth rates and carbon biomass. Marine Biology, 62, 73- ence Foundation grants DPP 88-17635 to 0. Holm-Hansen and 79. M. Vernet and DPP 88-18899 to D. Karl. Sakshaug, E., D. Kiefer, and K. Andresen. 1989. A steady state descrip- tion of growth and light absorption in the marine planktonic diatom Skeletonema costatum. Limnology and Oceanography, 34, 198-205. Sommer, U. 1989. Maximal growth rates of Antarctic phytoplankton: Only weak dependence on cell size. Limnology and Oceanography, 34, 1109-1112. Spies, A. 1987 Growth rates of Antarctic marine phytoplankton in the Weddell Sea. Marine Ecology Progress Series, 41, 267-274. Ingestion of phytoplankton here the rates of particle ingestion for representative field and and bacterioplankton by laboratory experiments with morphologically similar echino- derm larvae from polar (Odontaster validus) and temperate (As- polar and temperate terina miniata) environments. echinoderm larvae Natural microbial populations collected at the ice edge in McMurdo Sound, Antarctica, and approximately 2 kilometers offshore of Santa Cruz, California, in Monterey Bay were seri- RICHARD B. RIvKIN, M. ROBIN ANDERSON, ally size fractionated through 64-micrometer and 10-micrometer and DANIEL E. Gu5TAF50N, JR. Nitex mesh and a 1.0-micrometer Nuclepore filters (designated the <64-micrometer, <10-micrometer, and <1.0-micrometer Horn Point Environmental Laboratory size fractions, respectively). We are assuming that only algae University of Maryland assimilated the carbon-14 sodium bicarbonate in the <64-mi- Cambridge, Maryland 21613 crometer and <10-micrometer size fractions and that primarily bacteria incorporated methyl, tritiated thymidine in the <1.0- micrometer size fraction. The <64-micrometer and <10-mi- Echinoderm larvae are widely distributed in the plankton of crometer fractions were incubated with carbon-14 sodium bi- polar and temperate oceans (Mileikovsky 1971). Although phy- carbonate (1-2 microcuries per milliliter final activity) for 6 to toplankton are considered to be their primary food source, 36 hours and the <1.0-micrometer fraction was incubated with recent studies suggest that echinoderm larvae may be nutri- methyl, tritiated thymidine (approximately 7-10 nanomolar tionally quite opportunistic. They may assimilate a variety of TdR per liter) for 6 to 12 hours. Laboratory cultures of the dissolved substrates and ingest both autotrophic and hetero- chlorophyte Dunaliella tertiolecta were labeled with carbon-14 trophic microbiota (Manahan, Davis, and Stephens 1983; Riv- sodium bicarbonate (1-2 microcuries per milliliter final activity) kin et al. 1986; Strathmann 1987; Manahan et al. 1990). The for at least 12 hours. Mid- to late-stage bipinnaria larvae were seawater concentration of both dissolved and particulate ma- added to the radiolabelled prey, and after incubating replicate terial is spatially and temporally variable, hence the nutritional bottles (n = 3 or 4) for 2 to 6 hours at ambient temperatures in modes may differ for larvae in distinct geographic regions or the dark, the larvae were gently collected onto 73-micrometer for larvae from the same region during different times of the Nitex screening, rinsed several times with ambient tempera- year. As part of a collaborative study to evaluate the nutritional ture seawater to removing adhering particles and backwashed importance of dissolved and particulate resources, we report into isolation dishes. Using micromanipulation, 8 to 10 larvae were isolated into replicate (n = 5) scintillation vials, and their radioactivity was counted using liquid scintillation spectrome- try (Rivkin, Anderson, and Gustafson in preparation). All sam- present address: Ocean Sciences Centre, Memorial University of Newfound- ples were corrected for quench by the external standards land, St. Johns, Newfoundland, A1C 5S7 Canada. method and for background radiation. The data were tested for 156 ANTARCTIC JOURNAL significance (among replicate bottles within a treatment and 15 among treatments) using nested and two-way analysis of vari- ance. 12 The ingestion of radiolabeled prey has been widely used to study the dynamics of grazing by crustacean and protozoan .c zooplankton; however, it had not been used to measure grazing in echinoderm larvae. The rates of clearance and ingestion of a D. tertiolecta, common food source, by 0. validus and A. miniata . 6 were compared (figure 1). There were no significant differences -j among replicate bottles within a treatment however clearance 3 and ingestion rates were significantly (p<O.00l) faster by the temperate than polar larva. Figure 2 shows the clearance and ingestion rates of naturally 0 occurring particulate prey by 0. validus and A. miniata. There (10 jm (64 Am (1.0 ,tsm were usually no significant differences among replicate bottles 60 within a treatment. The clearance rates of 0. va/idus for <64- micrometer and <10-micrometer algae were not significantly (p=O.l05) different (figure 2A). In contrast, the clearance rates of A. miniata on <10-micrometer algae were significantly 40 greater (p<O.00l) than for <64-micrometer algae (figure 2A). a validus a The rates of clearance of the <64-micrometer algae by 0. -J and A. miniata were not significantly different (p=0.477) 0C .0 whereas A. miniata cleared <10-micrometer algae significantly a 20 (p<0.001) faster than 0. validus. 0 The rates of algal ingestion by 0. validus, calculated as the 0. product of clearance rates and prey carbon per microliter, was significantly greater (p<O.00l) for <64-micrometer than the <10-micrometer algal size fraction. In contrast, the rate of (10 Am (64/hm <1.0 um ingestion of <64-micrometer and <10-micrometer algae by A. miniata was not significantly (p = 0.101) different. Phytoplankton Bacteria Odontaster validus readily ingested <1.0-micrometer bacteria and the clearance rates were significantly (p<0.05) greater than Figure 2. Rates of (A) clearance (in microliters per larva per hour, for the <64-micrometer and <10-micrometer algae (figure 2A). 1iL larva- 1 h) and (B) ingestion (in picograms of carbon per larva At this time of year, the biomass of phytoplankton is greater per hour, pg Carbon larva- 1 11- 1 ) of the <64-micrometer, <10-mi- than bacteria in McMurdo Sound (however see Rivkin 1991); crometer, and <1.0 size fractions of natural planktonic populations hence, the ingestion rate of bacterial carbon was significantly by Odontaster validus in McMurdo Sound, Antarctic, (darkened (p<0.05) lower than that for phytoplankton carbon. In contrast, bars) and Asterina miniata in Monterey Bay, California (cross- did not appear to ingest bacteria. hatched bars). The experiments were carried out in mid-August A. miniata 1990 (Monterey Bay) and late December (McMurdo Sound). On the The rates of ingestion of prey carbon, measured during the dates of these experiments, the ambient chlorophyll a concentra- field experiments, were compared with the rates of metabolism tions (in micrograms per liter) were: in Monterey Bay, <64-microm- eter = 2.03 and <10-micrometer = 0.33 and in McMurdo Sound, <64-micrometer 1.88 and = 10-micrometer = 0.47. Bacterial abundances in McMurdo Sound were 2.8 x 108 cells per liter. The error bars are one standard deviation. 15.0 5.0 12.0 4.0 - -c (Manahan et al. 1990) and the metabolic carbon demand (table). 9.0 3.0 Grazing on the natural microbial populations could satisfy 100 a percent of the carbon demand of 0. validus but <1 percent of it C . 6.0 0 the metabolic requirements of A. miniata. The metabolic carbon a demands were satisfied when the algal biomass was higher 0 3.0 1.0 g such as in the experiments where A. miniata ingested D. Tertio- lecta (compare the table and figure 1). These results suggest that temperate larvae may have a much higher particulate food 0.0 0.0 Clearance Ingestion requirement and threshold for clearance than polar larvae. Rate Rate This project was supported by National Science Foundation grants DPP 88-18354 and 88-20132 to J.S. Pearse and R.B. Riv- Figure 1. Rates of clearance (in microliters per larva per hour, jiL kin, respectively.
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