Phytoplankton growth, microzooplankton herbivory and community structure in the southeast Bering Sea: insight into the formation and temporal persistence of an Emiliania huxleyi bloom
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- 作者：Olson ; M. Brady ; Strom ; Suzanne L.
- 刊名：Deep Sea Research Part II: Topical Studies in Oceanography
- 出版年：2002
- 出版时间：December, 2002
- 年：2002
- 卷：49
- 期：26
- 页码：5969-5990
- 全文大小：271 K
文摘
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 4.45μgCl−1. 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 >10μm and was attributed primarily to the high abundance of the diatom Nitzschia spp. Nutrient-enhanced total phytoplankton growth rates averaged 0.53d−1 across all experimental stations. Average growth rates for >10μm and <10μm cells were nearly equal, while microzooplankton grazing varied among stations and size fractions. Grazing on phytoplankton cells >10μm ranged from 0.19 to 1.14d−1. Grazing on cells <10μm ranged from 0.02 to 1.07d−1, and was significantly higher at non-bloom (avg. 0.71d−1) than at bloom (avg. 0.14d−1) stations. Averaged across all stations, grazing by microzooplankton accounted for 110 % and 81 % of phytoplankton growth for >10 and <10μm 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 >10μm were higher than previously reported for the region, ranging from 22,000 to 227,430cellsl−1 and 18 to 164μgCl−1. 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 <10μm within the bloom, the abundance and biomass of small microzooplankton (<20μm) 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.