MEPS 336:15-27 (2007)  -  doi:10.3354/meps336015

ABSTRACT: In a 3 wk Baltic Sea coastal experiment artificial N and P limitation were studied in 9 mesocosms, 51 m³ 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, N₂-fixing filamentous cyanobacteria. Stable C:N and N:P ratios of organic particles and the increase of total nitrogen indicated an average N₂-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 N₂-fixers Anabaena spp. and Nodularia spumigena; and (3) indifferent species, e.g. the N₂-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.

KEY WORDS: Anabaena · Aphanizomenon · Nodularia · Baltic Sea · Cyanobacteria · Nitrogen fixation · Nutrient limitation · Phytoplankton