Seasonal variation in zooplankton and suspended faecal pellets in the subarctic Norwegian Balsfjorden, in 1996

Zooplankton and the vertical distribution of suspended faecal pellets at a central station in Balstjorden were investigated during 25 cruises between March and October 1996. Calanus finmarchicus followed by Metridia sp. dominated the biomass of large (> 500 mum) zooplankton. as is typical for subarctic fjords in northern Norway. However, small zooplankton (< 500 m), in particular nauplii, Oithona spp.. Microsetella sp., and protozooplankton, also contributed significantly to zooplankton biomass. The biomass of small zooplankton varied similarly to that of large zooplankton during the study period ranging from about 0.2 to 3.5 g C m(2), except For increased biomass of large zooplankton during an advective episode in May/June. The grazing impact of the small forms must be at least equal to that of the large zooplankton. Among the faecal pellets, cylindrical pellets of copepod origin prevailed. Filiform pellets were most probably underestimated due to the sampling procedure. Small pellets less than 80 mum in length and probably of nauplii and protozoan origin, contributed considerably to the total suspended faecal pellet biomass. They accumulated in parallel with their potential producers in the surface layers, in particular, during summer, and contributed to the typical summertime retention of nutrients and particulate biogenic matter. The fate of phytoplankton-derived matter in north Norwegian coastal waters is influenced by grazing, resulting in either rapidly or slowly sinking Faecal matter, depending on the size spectrum and the prevailing feeding modes of the grazer community. The importance of an adequate sampling of the entire zooplankton community to determine the impact of grazing on the fate of phytoplankton-derived biomass is stressed.