Abstract
A three-year time series of hydrography, nutrients, chlorophyll, a, Secchi depth and phytoplankton composition taken at the Ocean Weather Station Mike (OWSM)(66°N, 2°E) was investigated. The pattern in the seasonal development of phytoplankton biomass was very similar in all three years, despite some differences in the magnitude of the spring bloom. From January to March typical winter conditions prevailed with low water mass stability, high and homogeneously depth distributed nutrient concentrations, and phytoplankton biomass below 0.05 mg m−3 chlorophyll a. During April a pre-bloom situation still prevailed, characterised by still low, but now steadily increasing biomass. The spring bloom started in May concurrent with incipient thermal stratification. Maximum phytoplankton biomass was observed between mid May and mid June, but the Chl a concentrations never exceeded 2.8 mg m−3. By July the biomass had decreased to pre-bloom levels. Generally nutrients were not depleted at this time. The decrease in phytoplankton biomass was followed by a smaller second bloom of short duration in early autumn, with Chl a concentrations around 1 mg m−3. After the autumn bloom the biomass decreased to winter levels. The winter and pre-bloom period were dominated by small nanoplankton (< 5 μm), and coccolithophorids, where Emiliania huxleyi accounted for a considerable proportion. The spring bloom was dominated by small diatoms, either Fragilariopsis pseudonana (1990, 1992) or Thalassiosira bioculata var. raripora (1991). The absence of large diatoms seemed to be a common feature of the winter and spring all three years. After the bloom, small nanoplankton (<5 μm) again became increasingly important. Dinoflagellates were also occasionally an abundant component of the phytoplankton community but the succession did not seem to reach a “dinoflagellate phase” at Stn M during summer. The autumn samples did not reveal any major changes in the composition of the phytoplankton community compared to summer.
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