Effects of N:P:Si ratios and zooplankton grazing on phytoplankton communities in the northern Adriatic Sea. I. Nutrients, phytoplankton biomass, and polysaccharide production

Abstract

The northern Adriatic Sea has been historically subjected to phosphorus and nitrogen loading. Recent signs of increasing eutrophication include oxygen def~ciency in the bottom waters and large-scale formation of gelatinous macroaggregates. The reason for the formation of these macroaggregates is unclear, but excess production of phytoplankton polysacchandes is suspected. In order to study the effect of different nutrient (nitrogen~phosphorus:silicon) ratios on phytoplankton production, biomass, polysacchandes, and species succession, 4 land-based enclosure experiments were performed with northern Adriatic seawater. During 2 of these experiments the importance of zooplankton grazlng as a phytoplankton loss factor was also investigated. Primary productivity in the northern Adriatic Sea is thought to be phosphorus limited, and our experiments confirmed that even low daily phosphorus additions Increased phytoplankton biomass. However, this only occurred when nitrogen additions were high. Alternatively, when nitrogen was added in low concentrations, u l t h simultaneous high phosphorus a d d ~t ~o n s , phytoplankton biomass declined Nitrogen deficiency induced the highest production of polysacchandes per unit of cell carbon, while nutrient-sufficient and phosphorus-deficient treatments caused a higher production of polysaccharides in total. In order to decrease the frequency of algal blooms and high polysaccharide production in the northern Adnatic, it appears necessary to reduce the amounts of incoming nutrients. Since phosphorus has a high turnover rate in low P : high N waters of the northern Adnatic, and slnce our experiments show that a shortage of nitrogen can produce reduced levels of phytoplankton biomass and total polysaccharides, a reduction of the nitrogen discharge would probably be the best countermeasure for eutrophicat~on In the northern Adnatic Sea.