Determining the photosynthetic productivity of a stratified phytoplankton population

During the summer, a stratified population of the red-coloured cyanobacteri um Planktothrix rubescens dominates the phytoplankton of Lake Zurich. The o rganism hovers in the metalimnion by regulating its buoyancy, provided by g as vesicles. The daily integral of primary production by the organism has b een calculated from its P/I curve and measurements of the vertical distribu tions of Planktothrix sp., irradiance and temperature in the lake, using nu merical integration with Excel spreadsheets. The basic methods are describe d and the particular problems of analysing production by stratified populat ions are discussed. Daily integrals of primary production based on measured growth/irradiance (mu /I) curves of P. rubescens produce a closer correspo ndence to the changes in the natural population than do integrals based on P/I curves.