Phytoplankton primary productivity characteristics in response to photosynthetically active radiation in three Kenyan Rift Valley saline-alkaline lakes

Phytoplankton photosynthetic characteristics in the Kenyan Rift Valley lakes Bogoria, Nakuru and Elmentaita were studied between November 2003 and February 2005. In these world-famous saline-alkaline lake systems, long-term continuous monitoring and photoautotrophic primary productivity modelling have been done for the first time. High light attenuation coefficients were observed with lakes means around 13 m(-1) reflecting the huge phytoplankton biomass. No photoinhibition was observed in the primary productivity field measurements. High values of the photosynthesis-irradiance curve initial slope (alpha) up to 0.85 (mg O-2 mg Chl a(-1) h(-1))(mu mol photons m(-2) s(-1))(-1) and a low onset of productivity saturation (Ek) down to 11.4 mu mol photons m(-2) s(-1) as an acclimation to poor light supply were found. For the trophogenic zone, high mean net primary production (NPP) rates of 6.8, 10.7 and 8.5 g O-2 m(-2) day(-1) were recorded for Bogoria, Nakuru and Elmentaita. For the whole water column, NPP decreased to -1.4, 1.6 and 7.2 g O-2 m(-2) day(-1) because of high community respiration. Modelling of the gross primary production (GPP, Chlorophyll a, light supply, initial slope alpha, maximum production rates considered) gave annual values of 4.9, 6.8 and 4.2 kg O-2 m(-2) year(-1), respectively, for Bogoria, Nakuru and Elmentaita, annual NPP values down to the compensation depth were 70, 65 and 55% of the GPP.