Microzooplankton grazing impact in the Bering Sea during spring sea ice conditions

Abstract

Microzooplankton grazing impact on phytoplankton in the Bering Sea during spring was assessed in 2008, 2009 and 2010 using two-point dilution assays. Forty-nine experiments were completed in a region encompassing shelf to slope waters, including the 70m line along the edge of the shelf. A variety of conditions were encountered, with a concomitant range of trophic states, from pre-bloom low chlorophyll-a (Chl-a)<3μgl−1 during heavy ice cover to late spring open water diatom blooms with Chl-a up to 40μgl−1. Microzooplankton biomass was dominated by large heterotrophic dinoflagellates and ciliates. Both athecate and thecate dinoflagellates, as well as some species of ciliates, fed on diatom cells and chains. Other types of protists, notably thecate amoebae and parasitoid flagellates, were also observed preying on diatoms. Total microzooplankton biomass ranged from 0.1 to 109μgCl−1 and was positively related to Chl-a concentration. Significant rates of microzooplankton herbivory were found in 55% of dilution experiments. Maximum grazing rate was 0.49d−1, and average grazing rate, including experiments with no significant grazing, was 0.09±0.10d−1. Phytoplankton intrinsic growth rates varied from slightly negative growth to >0.4d−1. Microzooplankton grazing was significant in both non-bloom and bloom conditions, averaging 46±75% of phytoplankton daily growth. Based on the amount of phytoplankton carbon consumed, we estimated potential microzooplankton community growth rates of up to 1.3d−1. Our results confirm the importance of protist grazers in planktonic food webs of high latitude ecosystems. We also conclude that our finding of significant grazing by microzooplankton on spring blooms in the Bering Sea does not support theories about phytoplankton bloom formation based on escape from grazing, due either to predation resistance or to slow growth of herbivorous protists at cold temperature.