Seasonal patterns in grazing mortality of bacterioplankton in polar oceans: a bipolar comparison

Despite the relevance of high latitude oceans to models and budgets of biogenic carbon, and the central role of heterotrophic microbes in global biogeochemical cycles, the patterns of energy flow through the lower food web in polar regions are poorly understood. To assess bacteriabased food webs in polar regions, the distribution, growth, and respiration and grazing losses of bacteria must be characterized. We report on the results of a seasonal (late winter through late summer) study of protist grazing in both Resolute Bay, Northwest Territories, Canadian Arctic and McMurdo Sound, Antarctica, and summarize the literature on the relations between the growth and grazing mortality of polar bacterioplankton. Bacterial abundance varied 5-fold in the Arctic and 25-fold in the Antarctic. Average bacterial growth rates ranged from 0.1 to 1.1 d super(-1). During comparable seasons, bacterial abundance was 2- to 3-fold higher and growth rates were 2- to 3-fold lower in the Antarctic than the Arctic. When grazing occurred, microzooplankton consumed nearly all of the local bacterial production. Grazing losses of bacteria were negligible immediately before and after the phytoplankton bloom. We propose that at these times, bacterioplankton were nutrient limited and protists were predominantly herbivorous. Protozoan grazers appear to alternate between bacterivorous and herbivorous nutritional modes as prey fields change in response to the seasonal progression in submarine irradiance and concentration of dissolved nutrients. The timing and magnitude of the phytoplankton bloom and the duration of the post-bloom period exert a significant influence on the flux of bacterioplankton carbon through microzooplankton and ultimately the coupling of the microbial and metazoan food webs.