Seasonal phytoplankton nutrient limitation patterns as revealed by bioassays over Baltic Sea gradients of salinity and eutrophication

Abstract

The in situ P versus N limitation of northern Baltic Sea phytoplankton was studied over 3 seasonal cycles at 6 locations, encompassing the large-scale salinity and eutrophy gradients of the Baltic Sea. Limitation patterns were inferred from 170 time-series (3 d) experiments with a replicated factorial experimental design, analyzed using a novel model selection-based classification. Seven limitation classes describe exclusive and primary limitation by N, P, combined N and P, or none. Response parameters were chlorophyll a (chl a) and primary productivity. Although chl a responses reported more N-limited, and 14 C responses more P-limited cases, the responses of both parameters were highly similar. Distinct seasonal patterns of in situ P versus N limitation were evident, repro- ducible, and dissimilar even in closely related environments. The most pristine low-saline areas were dominantly P-limited, while more eutrophied areas showed predominant N limitation even at compa- rable salinities. The typical Baltic Sea surface salinity regions (5 to 6 PSU) were clearly N-limited either for summer months (e.g. the mildly eutrophied Bothnian Sea), or throughout the growth sea- son (e.g. the eutrophied Gulf of Finland), although terrestrial loading ratios for the Baltic Sea exceed the Redfield ratio several-fold. The commonly vigorous N2-fixing cyanobacterial blooms in the south- ern basins do not alleviate the basic N-limitation pattern, and do not appear in the summertime N- limited Bothnian Sea. Management strategies should include toning down the N-limited spring bloom in eutrophied areas, which fuels summertime P release from sediments, in turn favoring cyanobacteria. Terrestrial nutrient loading does not reach offshore areas without significant modifi- cation and loss, emphasizing the role of coastal zones in global nutrient cycles and stoichiometry.