Impact of microzooplankton on the progression and fate of the spring bloom in fjords of northern Norway

The objective of the study was to determine the role of microzooplankton in the fate of primary production and progression of the spring phytoplankton bloom in high latitude fjords.The 3 fjords, Balsfjord, Malangen and Ullsfjord (Norway), varied in oceanic influence and in the rate of development of the spring bloom.The abundance of Phaeocystis pouchetii was relatively low in the spring of 1997 compared to previous years, and diatoms dominated the biomass of the phytoplankton assemblage in all 3 fjords.The mean biomass of microzooplankton in the top 20 m averaged 72, 66 and 80 mg C m -3 and values integrated to 170 m averaged 4560, 4450 and 6820 mg C m -2 in Balsfjord, Malangen and Ullsfjord, respectively.The composition of the microzooplankton was consistent among the fjords and over time, with the proportion of biomass split evenly between nanoflagellates, dinoflagellates and ciliates.Grazing rates of the microzooplankton community were measured with the dilution technique.The impact of microzooplankton grazing was similar among the fjords, accounting for on average 68, 63 and 55% of the production of the < 200 µm phytoplankton in Balsfjord, Malangen and Ullsfjord, respectively.When integrated to 20 m, based on a microzooplankton biomass-specific ingestion rate, microzooplankton grazing accounted for an estimated 12 to 26% of the gross primary production.However, this does not account for the carbon requirements of a substantial proportion of the microzooplankton that occurred below 20 m.Clearance rates by ciliates of nanophytoplankton cells of a similar size to the single cells of P. pouchetii were determined from the uptake rates of fluorescently labelled algae (FLA).Generally, the taxa of ciliates that were found to ingest FLA accounted for ≥ 50% of the abundance of the ciliate population in each fjord.Taxon-specific ciliate clearance rates of FLA in surface waters varied with ciliate size from 5.6 to 1.3 µl cell -1 h -1 .The FLA-consuming ciliate population cleared a total of 27 × 10 3 to 141 × 10 3 µl l -1 d -1 in surface waters.The ingestion rates of the FLA-consuming ciliates were equivalent to between 11 and 29% of the total microzooplankton consumption.It is possible that the grazing pressure exerted by the microzooplankton on single cells and small colonies was high enough to decrease the overall competitiveness of the P. pouchetii populations and contribute to their low abundance.