In late summer and autumn, before the vertical circulation reaches the thermocline, the phytoplankton population of Lake Zurich is dominated by the red-coloured filamentous cyanobacterium Planktothrix rubescens. which stratifies in the metalimnion at depths close to the photosynthetic compensation point. 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, which raised or lowered the thermocline by up to 2 m over 36 h, and (2) flotation 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-production, Sigma Sigma (NP), by the population of P. rubescens over a period of four cloudless days. The estimates were calculated from measurements of surface irradiance (at 5-min intervals), vertical light attenuation, temperature, filament volume concentration and the photosynthesis/irradiance (P/I) curves of filaments concentrated from the metalimnion. Despite the similar, high 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 distribution of the filaments. Measurements of P/I curves of lakewater samples incubated at a depth of 11 m showed changes in the photosynthetic coefficients during the day. These also generated large changes in calculated values of Sigma Sigma (NP). The computer spreadsheet used to calculate Sigma Sigma (NP) was modified to incorporate time-based changes in the photosynthetic 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 the light field in the metalimnion, where it outcompetes other phytoplankton. [KEYWORDS: Lake Zurich; stratification; Planktothrix rubescens;diel photosynthesis changes Phytoplankton photosynthesis; oscillatoria-agardhii; buoyancy regulation; gas vacuoles; light; cyanobacteria; irradiance; adaptation; vesicles; growth]
Original languageEnglish
JournalAquatic Sciences
Journal publication date2001

ID: 114500