Diurnal changes of stratification and photosynthesis in some tropical African waters

Abstract

A description is given of diurnal changes, due to the photosynthesis of phytoplankton, in three shallow African waters. Their vertical distribution was largely controlled by changes of thermal stratification; examples corresponding to different degrees of the latter are illustrated from four series of observations. Thermal stratification was generally intermittent, with marked stratification during the day, ended by isothermal mixing at night. An effect of these changes upon the distribution of a blue-green alga,Anabaena, and its photosynthetic activity, is also illustrated. Experimental determinations of the rates of photosynthesis (as milligrams of oxygen produced per unit volume of water, or of algal cells, per hour), and their distribution with time and depth, were obtained using the classic method of suspended light and dark bottles. Photosynthetic behaviour generally resembled that found in temperate waters, but maximum rates of photosynthesis calculated per unit volume of algal cells were unusually high. The preceding observations are used to calculate rates of photosynthesis below unit area of water surface, both from the experimental results and from the diurnal variation of oxygen content in the open water. In the latter case the observed increase of oxygen content in a water column during daytime is corrected for sources of loss, due to respiration and exchange with the atmosphere, using a mean rate of oxygen depletion measured during the night. Although the method is subject to many sources of error, which are discussed, the conclusion is reached that it can provide useful, if often rough, estimates of the rate of primary production. The experimental estimates were calculated both for the experimental periods, and for longer daily periods, using some general equations previously proposed. The calculations from diurnal changes and from experimental data are in agreement con­cerning the order of magnitude of production, but close conformity was obtained for only one series of observations. The areal values of production estimated from diurnal changes lay in a limited range of 4 to 11 g oxygen (equivalent to approximately 1⋅5 to 4 g carbon) m-2day-1, despite considerable variation in individual factors such as population density and under­water light penetration. These values are similar to other estimates for phytoplankton in very productive waters.