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.