The daily integral of nitrogen fixation by planktonic cyanobacteria in the Baltic Sea

Abstract

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 (P_N) to the mean photon irradiance (I) at different periods, a P_N/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.