Phytoplankton growth, microzooplankton herbivory and community structure in the southeast Bering Sea: insight into the formation and temporal persistence of an Emiliania huxleyi bloom

Using the seawater dilution technique, we measured phytoplankton growth and microzooplankton grazing rates within and outside of the 1999 Bering Sea coccolithophorid bloom. We found that reduced microzooplankton grazing mortality is a key component in the formation and temporal persistence of the Emiliania huxleyi bloom that continues to proliferate in the southeast Bering Sea. Total chlorophyll a (Chl a) at the study sites ranged from 0.40 to 4.45mgC l . Highest phytoplankton biomass was found within the bloom, which was a mixed assemblage of diatoms and E. huxleyi. Here, 75% of the Chl a came from cells >10 mm and was attributed primarily to the high abundance of the diatom Nitzschia spp. Nutrient-enhanced total phytoplankton growth rates averaged 0.53 d 1 across all experimental stations. Average growth rates for >10 mm and o10mm cells were nearly equal, while microzooplankton grazing varied among stations and size fractions. Grazing on phytoplankton cells >10mm ranged from 0.19 to 1.14 d . Grazing on cells o10mm ranged from 0.02 to 1.07 d , and was significantly higher at non-bloom (avg. 0.71 d ) than at bloom (avg. 0.14 d ) stations. Averaged across all stations, grazing by microzooplankton accounted for 110% and 81% of phytoplankton growth for >10 and o10mm cells, respectively. These findings contradict the paradigm that microzooplankton are constrained to diets of nanophytoplankton and strongly suggests that their grazing capability extends beyond boundaries assumed by size-based models. Dinoflagellates and oligotrich ciliates dominated the microzooplankton community. Estimates of abundance and biomass for microzooplankton >10mm were higher than previously reported for the region, ranging from 22,000 to 227,430 cells l 1 and 18 to 164 mgC l . Highest abundance and biomass occurred in the bloom and corresponded with increased abundance of the large ciliate Laboea, and the heterotrophic dinoflagellates Protoperidinium and Gyrodinium spp. Despite low grazing rates on phytoplankton o10mm within the bloom, the abundance and biomass of small microzooplankton (o20mm) capable of grazing E. huxleyi was relatively high at bloom stations. This body of evidence, coupled with observed high grazing rates on large phytoplankton cells, suggests the phytoplankton community composition was strongly regulated by herbivorous activity of microzooplankton. Because grazing behavior deviated from size-based model predictions and was not proportional to microzooplankton biomass, alternate mechanisms that dictate levels of grazing activity were in effect in the southeastern Bering Sea. We hypothesize that these mechanisms included morphological or chemical signaling between phytoplankton and micrograzers, which led to selective grazing pressure. r 2002 Elsevier Science Ltd. All rights reserved. *Corresponding author. E-mail addresses: [email protected] (M.B. Olson). 0967-0645/02/$ see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 0 6 4 5 ( 0 2 ) 0 0 3 2 9 6