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

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 ¹⁴C 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, reproducible, 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 comparable 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 season (e.g. the eutrophied Gulf of Finland), although terrestrial loading ratios for the Baltic Sea exceed the Redfield ratio several-fold. The commonly vigorous N ₂-fixing cyanobacterial blooms in the southern 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 modification and loss, emphasizing the role of coastal zones in global nutrient cycles and stoichiometry.