The effects of diel changes in photosynthetic coefficients and depth of Planktothrix rubescens on the daily integral of photosynthesis in Lake Zurich

In late summer and autumn, before the vertical circulation reaches the ther mocline, the phytoplankton population of Lake Zurich is dominated by the re d-coloured filamentous cyanobacterium Planktothrix rubescens. which stratif ies in the metalimnion at depths close to the photosynthetic compensation p oint. The filament volume concentration reached a maximum of 12 cm(3) m(-3) ; the depth of the maximum varied from 10.5 to 12.5 m. Changes in the depth distribution were attributed to a combination of (1) seiche movements, whi ch raised or lowered the thermocline by up to 2 m over 36 h, and (2) flotat ion by the buoyant filaments relative to the isotherms, by up 0.4 m d(-1). These changes caused a 2-fold change in insolation at the Planktothrix peak . Estimates were made of the daily integral of photosynthetic O-2-productio n, Sigma Sigma (NP), by the population of P. rubescens over a period of fou r cloudless days. The estimates were calculated from measurements of surfac e irradiance (at 5-min intervals), vertical light attenuation, temperature, filament volume concentration and the photosynthesis/irradiance (P/I) curv es of filaments concentrated from the metalimnion. Despite the similar, hig h insolation on each of the four days, the calculated values of Sigma Sigma (NP) varied from 9 to 53 mmol m(-2) d(-1), owing to the changing depth dis tribution of the filaments. Measurements of P/I curves of lakewater samples incubated at a depth of 11 m showed changes in the photosynthetic coeffici ents during the day. These also generated large changes in calculated value s of Sigma Sigma (NP). The computer spreadsheet used to calculate Sigma Sig ma (NP) was modified to incorporate time-based changes in the photosyntheti c coefficients and vertical distribution of the organism. These refinements provide a more accurate description of photosynthesis by the deep-living P . rubescens, which adjusts its position by buoyancy regulation to exploit t he light field in the metalimnion, where it outcompetes other phytoplankton .