AME 29:267-278 (2002)  -  doi:10.3354/ame029267

ABSTRACT: Thalassiosira spp., large-chain forming centric diatoms, typically dominate the biomass during phytoplankton blooms in the North Water Polynya (76 to 79°N, centred on ca. longitude 75°W), which is the largest recurring polynya in the Canadian Arctic. We used an experimental method based on semi-continuous cultures to investigate mechanisms responsible for bloom maintenance and associated changes in microbial food web constituents. We compared 2 treatments: (1) a new nutrient system in which the cultures were partially enriched every 2 d with nutrient-rich seawater from depth to simulate horizontal or vertical advection, and (2) a recycled nutrient system in which large particles (>2.0 μm) were partially removed every 2 d to simulate grazing and sinking losses without nutrient replacement. The experiment lasted 8 d. In the new nutrient treatment, large diatoms, particularly Thalassiosira spp. and to a lesser extent Chaetoceros spp., consumed the added nutrients and continued to dominate production and biomass of the protist community. The total eukaryotic community production in the Œrecycled¹ community shifted to one dominated by dinoflagellates and ciliates in the absence of diatom growth. These 2 end points corresponded to 2 types of communities observed in the North Water Polynya in June 1998. Net production rates for viruses and bacteria were not significantly different between treatments. These results demonstrate the importance of advective processes in maintaining a prolonged diatom bloom. An underlying microbial food web dominated by large (>20 μm) ciliates and dinoflagellates was able to maintain similar rates of net production respective of new versus recycled nutrient supply. Dominance of the protist communities by large cells under both conditions is likely to favour the sustained high productivity of zooplankton and megafauna that characterize the North Water Polynya.

KEY WORDS: Diatom blooms · Ciliates · Dinoflagellates · Microbial food webs · New and recycled nutrients