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Trophic role and top-down control of a subarctic protozooplankton community

Riisgaard, Karen; Swalethorp, Rasmus; Kjellerup, Sanne; Juul-Pedersen, Thomas; Nielsen, Torkel Gissel

Plankton succession was investigated in the subarctic Godthåbsfjord, Western Greenland, from March to August 2010. The trophic role of protozooplankton (ciliates and heterotrophic dinoflagellates) was evaluated with emphasis on their seasonal succession and as prey for the copepod community. The integrated protozooplankton biomass ranged between 0.1 and 4.0 g C m-2, and was dominated by ciliates. Over the 6 mo study period, maximum potential ingestion rates of the protozooplankton ranged from 0.02 to 1.2 g C m-2 d-1, corresponding to 30 to 194% of primary production d-1 or 0.5 to 37% of phytoplankton biomass d-1. The highest copepod biomass (24 g C m-2) occurred in spring, with Metridia longa alone contributing up to 92% of the biomass. A grazing experiment with M. longa feeding on a natural plankton assemblage confirmed that this species cleared cells in the size range 10 to 60 µm with an average clearance rate of 2.4 ml µg C-1 d-1. The copepod community, dominated by the genera Calanus, Metridia, Pseudocalanus, Oithona, Microsetella and Triconia/Oncaea, accounted for 72 to 93% of the copepod biomass in the spring. After the large calanoid copepod species left the surface layer, the protozooplankton increased numerically and were the most important grazers for some weeks until a late summer copepod community, dominated by cyclopoids Oithona spp., controlled the protist community. Our study indicated that protozooplankton succession is regulated by copepod grazing during most of the season, and that these protists provide an essential source of nutrition for the copepod populations.

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