Interaction effects of N : P ratios and frequency of nutrient supply on the plankton community in the northern Baltic Sea

An experiment was carried out in brackish seawater mesocosms to investigate the impacts of different N:P ratios and frequency of nutrient supply on phytoplankton and zooplankton communities in the Archipelago Sea, part of the northern Baltic Sea, The experiment used a 2 x 2 factorial design in which nitrogen (N) and phosphorus (P) were added in either an N-deficient or Redfield ratio either daily or at 1 wk intervals. Both phytoplankton total biomass and the biomass of most phytoplankton groups increased most in the enrichments with a Redfield ratio. The effect of nutrient addition frequencies varied with time and between the 2 nutrient ratios. The biomass of heterocystous N-2-fixing cyanobacteria was highest in the daily nutrient enrichment, while chlorophytes increased most in the weekly Redfield treatment. The concentration of cyanobacterian hepatotoxins increased during the experiment, but was not affected by the nutrient enrichments. The biomass of calanoid copepods increased more in the weekly than in the daily enrichments when the nutrients were supplied in a Redfield ratio. Small rotifers, on the other hand, were favored by the Redfield nutrient enrichment, especially when the nutrients were added daily. Based on the results of this study it seems that a daily nutrient supply favors cyanobacterial growth, while an intermittent but heavier nutrient discharge may result in a more advantageous plankton community, with fewer cyanobacteria and a zooplankton community dominated by larger species.