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

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

Kersti Kangro1, Kalle Olli1,*, Timo Tamminen2, Risto Lignell3

1Institute of Botany and Ecology, Tartu University, Lai 40, Tartu 51005, Estonia
2Finnish Environment Institute, PO Box 140, 00251 Helsinki, Finland
3Finnish Institute of Marine Research, PO Box 2, 00561 Helsinki, Finland
*Corresponding author. Email:

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

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