AME 18:37-54 (1999)  -  doi:10.3354/ame018037

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

Edna Granéli1,*, Per Carlsson1, Jefferson T. Turner2, Patricia A. Tester3, Christian Béchemin4, Rodger Dawson5, Enzo Funari6

1University of Kalmar, Department of Marine Sciences, POB 905, S-391 29 Kalmar, Sweden
2Biology Department, University of Massachusetts Dartmouth, North Dartmouth, Massachusetts 02747, USA
3National Marine Fisheries Service, NOAA, Southeast Fisheries Science Center, Beaufort Laboratory, Beaufort, North Carolina 28516, USA
4CREMA-L'Houmeau (CNRS-IFREMER), BP5, F-17137 L'Houmeau, France
5Chesapeake Biological Laboratory, University of Maryland, Solomons, Maryland 20688, USA
6Laboratorio di Igiene Ambientale, Istituto di Sanita, Viale R. Elena 299, I-00161 Rome, Italy

ABSTRACT: The northern Adriatic Sea has been historically subjected to phosphorus and nitrogen loading. Recent signs of increasing eutrophication include oxygen deficiency 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 polysaccharides is suspected. In order to study the effect of different nutrient (nitrogen:phosphorus:silicon) ratios on phytoplankton production, biomass, polysaccharides, and species succession, 4 land-based enclosure experiments were performed with northern Adriatic seawater. During 2 of these experiments the importance of zooplankton grazing 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, with simultaneous high phosphorus additions, phytoplankton biomass declined. Nitrogen deficiency induced the highest production of polysaccharides 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 Adriatic, 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 Adriatic, and since 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 eutrophication in the northern Adriatic Sea.

KEY WORDS: Adriatic Sea · Nitrogen · Silicon · Limitation · Phytoplankton · Phosphorus · Polysaccharide · Chemical composition

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