Light conditions and photosynthetic efficiency of phytoplankton in Murchison Bay, Lake Victoria, Uganda

Abstract

Eutrophication has increased in the tropical Lake Victoria during the last decades, leading to problems of toxic algal blooms, oxygen depletion, and reduced water visibility. To better understand the mechanism behind this negative development, more knowledge on the coupling between optical properties and photosynthetic efficiency is needed. Here we present such data from Murchison Bay during the period December 2009–March 2010. Photosynthetic efficiency was measured as photochemical energy quantum conversion efficiency ( F ′ q / F ′ m ) and functional absorption cross-section ( σ ′ PSII ) by applying the Fast Repetition Rate Fluorometry (FRRF) technique. With chlorophyll- a concentrations between 100 and 174 μg L −1 , the vertical attenuation coefficient of photosynthetic active radiation (PAR) was found to be in the range 1.93–2.25 m −1 . The FRRF parameters F ′ q / F ′ m and σ ′ PSII , as well as the dissolved oxygen, were found to vary with the physiochemical properties of the water column. The spectral light characteristics of the deeper parts of the water in Murchinson Bay were found to be favourable for growth of cyanobacteria due to the presence of phycobilins, which has typical absorption peaks between 565 and 620 nm. The light utilisation coefficient α B derived from the FRRF parameters was found to be within the same range as that obtained earlier using conventional methods for other bays in Lake Victoria. We conclude that eutrophication is still increasing in Murchison Bay, and that nitrogen limitation together with deep penetration of light within the 550–650 nm wavelength band seems to favour the dominance of cyanobacteria.