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

Dairy integrals of photosynthesis by a cyanobacterial bloom in the Baltic S ea, during the summer of 1993, were calculated from the vertical distributi ons of light, temperature and the organisms in the water column and from ph otosynthesis/irradiance curves of picoplanktonic and diazotrophic cyanobact eria 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 cya nobacterial biomass and contributed 56% of the total primary production. Of the filamentous diazotrophs that formed the other 30%, Aphanizomenon contr ibuted 28% and a Nodularia-containing fraction 16% of the primary productio n. For the whole population there was little change in standardized photosy nthetic O-2 production which remained at about 31 mmol m(-2) before and aft er the mixing event. There were differences, however, between the classes o f cyanobacteria: in picocyanobacteria primary production hardly changed, wh ile in Aphanizomenon it increased by 2.6 and in Nodularia it fell below zer o. 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 o n 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 li ght attenuation and photosynthetic coefficients indicate that growth of the cyanobacterial population could occur only in the summer months when the c ritical depth of the cyanobacteria exceeds the depth of mixing.