Measurements were made of the rates of nitrogenase activity (acetylene reduction) by cyanobacteria collected from the Baltic Sea at 2-h intervals, over a period of 24 h, and incubated under natural light. By relating the chlorophyll-specific rate of N2 fixation (PN) to the mean photon irradiance (I) at different periods, a PN/I curve was constructed. A mathematical description of this relationship was used in the calculation of rates of N2 fixation at different depths and times of day from continuous measurements of surface irradiance and light attenuation. By relating these calculations to the vertical distribution of chlorophyll due to Aphanizomenon flos-aquae, an estimate of the daily integral of N2 fixation by the population of this cyanobacterium was obtained: it varied from 0·39 to 0·71 mmol m−2 with a mean value of 0·53 mmol−2 over 9 d. Comparisons with similar calculations of the daily integral of photosynthesis over the same period indicated the atomic ratio of N/C fixed is about 0·22, not very different from the N/C ratio expected for the elemental composition of these cyanobacteria. It is demonstrated that when buoyant gas-vacuolate colonies float up during calm periods the increased irradiance that they experience supports an increased rate of N2 fixation. This increase is less than that seen for carbon fixation because photosynthesis is negated by respiration at low irradiances and is less inhibited at high irradiances.

It has been suggested that the populations of planktonic cyanobacteria that occupy the metalimnion of stratified lakes during the summer months may be aestivating between the main periods of growth during entrainment in the epilimnion in spring and summer. We determined the vertical distribution of the biomass and daily integral of photosynthesis of the population of Planktothrix (Oscillatoria) rubescens in Lake Zürich for 136 d from July to November 1995. The population showed an 80-fold increase during the stratified period but it only doubled over the subsequent period of entrainment. During the first eight days, part of the increase was attributed to recruitment of filaments floating up from greater depths but all of the subsequent production could be accounted for by photoautotrophic growth. On sunny days the biomass-specific photosynthesis of this population reached some of the highest values over the whole period despite its depth (>13 m). On very cloudy days, however, primary productivity was very low and on 4 days, when the mean depth of the population exceeded 15 m, there was no net production. Over the whole period of the study, the accumulated photosynthetic production exceeded the increase in biomass of the population by a factor of 9·5. Although much of this production occurred during the period of entrainment only a small proportion was translated into growth of the population. It is concluded that the growth that takes place in the period of stratification in the metalimnion is essential to subsequent production.

Observations were made on the vertical distribution of colonies of Aphanizomenon flos-aquae for 9 d at a drift-station east of Bornholm Island in the Baltic Sea. The buoyant colonies were dispersed in the upper layers of the water column during periods of wind-induced mixing but floated up during calm periods. From measurements of the vertical light extinction, surface irradiance and the photosynthesis versus irradiance curve, calculations were made of the changes in the daily integral of photosynthesis with respect to time and depth throughout the water column. From these calculations it is demonstrated that net photosynthesis by the population of Aphanizomenon flos-aquae increased nearly threefold by floating up after a deep mixing event. It is estimated that, averaged over alternating periods of calm and mixing, the buoyancy provided by gas vesicles in this organism will result in a nearly twofold increase in photosynthesis. A quantitative analysis has been made of the relationship of the daily integral of photosynthesis by the Aphanizomenon population with the mean depth of the population in the water column and the daily insolation. The analysis shows that the integral decreases linearly with respect to mean depth.