Photosynthesis and nitrogen fixation in a cyanobacterial bloom in the Baltic Sea

L.J. Stal, A.E. Walsby

    Research output: Contribution to journal/periodicalArticleScientificpeer-review


    Dairy integrals of photosynthesis by a cyanobacterial bloom in the Baltic Sea, during the summer of 1993, were calculated from the vertical distributions of light, temperature and the organisms in the water column and from photosynthesis/irradiance curves of picoplanktonic and diazotrophic cyanobacteria isolated from the community. The distribution of chlorophyll a in size-classes <20 mu m and > 20 mu m was monitored over 9 days that included a deep mixing event followed by calm. Picocyanobacteria formed 70% of the cyanobacterial biomass and contributed 56% of the total primary production. Of the filamentous diazotrophs that formed the other 30%, Aphanizomenon contributed 28% and a Nodularia- containing fraction 16% of the primary production. For the whole population there was little change in standardized photosynthetic O-2 production which remained at about 31 mmol m(-2) before and after the mixing event. There were differences, however, between the classes of cyanobacteria: in picocyanobacteria primary production hardly changed, while in Aphanizomenon it increased by 2.6 and in Nodularia it fell below zero. Total phytoplankton photosynthesis was strongly dependent on total daily insolation with the compensation point at a photon insolation of 22.7 mol m(-2) d(-1). Similar analyses of N-2 fixation showed much less dependence on depth distribution of light and biomass: Aphanizomenon fixed about twice as much N-2 as Nodularia their; their fixation exceeded their own N demand by about 12%. Together, these species contributed 49% of the total N demand of: the phytoplankton population. Computer models based on the measured light attenuation and photosynthetic coefficients indicate that growth of the cyanobacterial population could occur only in the summer months when the critical depth of the cyanobacteria exceeds the depth of mixing. [KEYWORDS: Baltic Sea; bloom; cyanobacteria; nitrogen fixation; primary production Phytoplankton photosynthesis; planktonic cyanobacteria; gas vesicles; picoplankton; dynamics; buoyancy]
    Original languageEnglish
    Pages (from-to)97-108
    JournalEuropean Journal of Phycology
    Issue number2
    Publication statusPublished - 2000


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