Species-specific responses of a cyanobacteriadominated phytoplankton community to artificial nutrient limitation in the Baltic Sea

In a 3 wk Baltic Sea coastal experiment artificial N and P limitation were studied in 9 mesocosms, 51 m 3 each, filled with a natural summer plankton community.The initial situation (molar dissolved N:P ratio of 0.9; zooplankton to phytoplankton biomass ratio of 2.8) was boosted with 5 daily N and P additions in Redfield ratio, to increase algal biomass and demand for nutrients.Thereafter, either N or P limitation was induced by cutting one element, while supplying the other.Discontinuing N supply resulted in lower phytoplankton biomass (as chlorophyll a and wet weight), but more bloom-forming, N 2 -fixing filamentous cyanobacteria.Stable C:N and N:P ratios of organic particles and the increase of total nitrogen indicated an average N 2 -fixation rate of 3.3 to 7.4 mmol N m -2 d -1 , which by the end of the experiment equalised the total N in the mesocosms with and without N addition.Discontinuing P supply resulted in higher algal biomass, and the particulate organic matter with increasing molar C:P (from 90 to 300) and N:P ratios (from 16 to 50) suggested bulk level P shortage.The biomass response distinguished 3 groups of phytoplankton, each containing filamentous cyanobacterial species that are considered: (1) nitrogen limited, e.g.chlorophytes, non-heterocystous filamentous cyanobacteria Pseudanabaena spp.and colonial cyanobacteria (e.g.Snowella sp., Cyanodictyon sp.); (2) phosphorus limited, e.g. the N 2 -fixers Anabaena spp.and Nodularia spumigena; and (3) indifferent species, e.g. the N 2 -fixer Aphanizomenon sp. and heterotrophic and opportunistic flagellates.Aphanizomenon sp.-the dominant species in the experiment -increased equally in all mesocosms at the expense of fixed N and internal P reserves.Our results highlight the diverse nutritional and growth strategies of the bloom-forming cyanobacteria and suggest that eutrophication management decisions in the Baltic Sea should inevitably consider simultaneous reduction of both N and P.