Dynamics of Chaetoceros socialis blooms in the North Water

The North Water, a large polynya in northern Baffin Bay, has an extensive period of continuous primary production lasting up to 6 months. The well-known richness of this polynya in higher trophic levels, including numerous marine mammals and birds, must be based on the magnitude and duration of that production. As part of the International North Water Polynya Study, we observed that two groups of phytoplankton were responsible for most of the primary production in 1997-1999: the large centric diatoms, Thalassiosira spp., and the smaller, colonial centric diatom, Chaetoceros socialis Lauder. We studied the phytoplankton community assemblage in the North Water in August 1997, April-July 1998, and August-September 1999 using epifluorescence microscopy on samples fixed and filtered at sea, with unpreserved cells observed in July 1998. Blooms of C socialis occurred at southern stations in June and throughout the polynya from July to September. Maximum concentrations reached 30,100 cells ml(-1) (496 mug Cl-1) in the western area in early July. Blooms of C socialis directly followed those of Thalassiosira spp., which were more intense but also more transient. C socialis appeared to maintain its population in the euphotic layer for up to 3 months by surviving at low nutrient levels, sinking to depths where dynamic physical regimes brought nutrient replenishment, and producing resting spores (up to 233 mug Cl-1). The resting spores may survive low-nutrient periods and be introduced, some from fecal pellets, to waters of more favorable growth conditions; in July, 86% of fecal pellets and 76% of aggregates observed in shallow, floating sediment traps contained resting spores of C socialis. Colonies of C. socialis were a potential mid-water food source and major contributor to the benthos: C socialis contributed up to 91% of total phytoplankton cells (49% phytoplankton carbon) identified in moored sediment traps. Mats of polymer gels (mucous), likely formed from the observed C socialis colonies, some containing frustule and setae fragments as well as resting spores, contributed massively to the material in traps in the southern and western polynya. C socialis appears to be a very important primary producer throughout the productive season of the North Water. (C) 2002 Elsevier Science Ltd. All rights reserved.