Spring-bloom dynamics in Kosterfjorden, western Sweden: Variation in phytoplankton production and macrozooplankton characteristics

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Abstract
An investigation evaluating whether the dynamical processes of primary producers influence the macrozooplankton community was carried out over a 57-day period covering the build-up, culmination, and retardation of a phytoplankton bloom. The bloom was initiated in the beginning of March, when solar radiation had reached c. 150 ly per day and culminated in mid-March with an exceptionally high net production of 9.3 g C/m2 per day, and with a dense biomass (chlorophyll a around 20 mg/m3) down to 20 m. An exchange of the upper water layer caused a retardation of the bloom after less than 2 weeks. The primary net production amounted to 74 g C/m2 for the period between 7 March and 26 April. Laboratory bioassays showed that light determined the vertical decline of the assimilation number, while nutrient deficiency caused the decreasing assimilation number with time. The zooplankton abundance and biomass was low during the phytoplankton bloom and began to increase 3 to 4 weeks thereafter. The succession in population structure of Calanus finmarchicus Gunnerus indicated that spawning occurred concomitantly with the bloom. High weight-specific respiration rate, as measured by the ETS activity, and high trypsin activity of the zooplankton fraction containing mainly copepods occurred when phytoplankton was abundant, but the accumulation of lipids did not start until the phytoplankton bloom was over. It is suggested that early spring blooms, although utilized by the small populations of zooplankton, mainly give a nutritional input to the benthic community. Early spring blooms may also serve as a trigger for zooplankton reproduction.

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