Physiological and community responses of summer plankton to nutrient manipulation in the Gulf of Finland (Baltic Sea) with special reference to phosphorus

Abstract

Abstract Summer plankton communities from two locations in the central and western Gulf of Finland (Stns LL6 and JML, respectively), characterized by different nutrient dynamics, were manipulated with nitrogen and phosphorus additions in the laboratory. The aim was to study whether these populations would show indices of different phosphorus physiology or community structure. We were especially interested to determine whether the growth of diazotrophic filamentous cyanobacteria could be stimulated by phosphorus additions. No growth of these species was observed with daily addition of nutrients. The most evident population response was that of the small-size fraction organisms ( < 10 µm) in both experiments to N + P additions. In the size fraction > 10 µm the species favored by the nutrient additions were Dictyosphaerium subsolitarium van Goor in the central part and Snowella sp. in the western part of the Gulf. Phosphorus turnover rates measured by 32P-uptake were faster in the control and N-supplemented samples, as well in the exp-LL6 sample with N + P addition. The behaviour of the alkaline phosphatase activity was best explained by the ambient DIN : DIP ratio. The PN : PP ratios in exp-LL6 indicated more phosphorus deficiency than in exp-JML. Our conclusion is that there was difference in both population dynamics and biochemical indices between the central and western sea areas showing more signs of phosphorus deficiency in the central area of the Gulf.