We characterize stratification patterns over diel, seasonal, and annual time scales in inshore and offshore regions of Lake Victoria, East Africa; determine conditions leading to horizontal exchanges; and, using surface energy budgets derived from local meteorological stations and two reanalysis products, address whether stratification depends on advective as opposed to local processes. The largest change in the surface energy budget occurred when winds intensified at the end of the long rains, with the wind's intensification, duration, and spatial extent dependent on El Niñ o-Southern Oscillation cycles. These winds flush inshore waters and cause cross-basin upwelling similar to that observed in the deep African Great Lakes. Wedderburn numbers indicated mixing and cross-basin within-thermocline transport. The internal wave-induced mixing and enhanced latent heat fluxes of 2300 to 2400 W m 22 contributed to the loss of seasonal stratification. Advection of cool water was required to balance the heat budget of northern offshore waters in the latter half of the southeast monsoon except in an El Niñ o year. Northern waters became weakly stratified after the southeast monsoon, with nocturnal winds contributing to heat transport and ventilation of the lower water column. Following the rainy season, downwelling by sustained southerly albeit low winds is a likely cause of the seasonal thermocline. Inshore waters are 0.2-1.5uC warmer than those offshore, conditions conducive to horizontal convective circulation except during onshore winds. The seasonal cycle of stratification and inshore-offshore and cross-basin exchanges are moderated by differential heating, cooling, and basin-scale thermocline tilting.The diel, seasonal, and annual variations in stratification within lakes influence the magnitude of vertical mixing, inshore-offshore exchanges, persistence of hypoxia, fluxes of greenhouse gases and nutrients, and the timing and duration of phytoplankton growth, with consequences for higher trophic levels. The variations in temperature and stratification depend on the surface energy budget, that is, the net shortwave and longwave radiation and sensible and latent heat fluxes combined with advection and the input of momentum from winds. Surface energy budgets are powerful tools for understanding controls on latitudinal variations in lake thermal structure and for quantifying how the energy inputs and water losses from lakes vary with changes in climate (Lenters et al.

27We review alternative hypotheses and associated mechanisms to explain Lake Victoria's Nile 28 perch takeover and concurrent reduction in haplochromines through a (re)analysis of long term

Possible causes of the increased algal blooms in Lake Victoria in the 1980s have been disputed by several authors; some suggested a topdown effect by the introduced Nile perch, whereas others suggested a bottom-up effect due to eutrophication. In this article the potential impact is established of grazing by fish on phytoplankton densities, before the Nile perch upsurge and the concomitant algal blooms in the Mwanza Gulf. The biomass and trophic composition of fish in the sublittoral area of the Mwanza Gulf were calculated based on catch data from bottom trawls, and from gill nets covering the whole water column. Estimates of phytoplankton production in the same area were made from Secchi values and chlorophyll concentrations. The total phytoplankton intake by fish was estimated at 230 mg DW m -2 day -1 . The daily gross production ranged from 6,200 to 7,100 mg DW m -2 day -1 and the net production from 1,900 to 2,200 mg DW m -2 day -1 . Thus, losses of phytoplankton through grazing by fish were about 3-4% of daily gross and 10-12% of daily net phytoplankton production. As a consequence it is unlikely that the phytoplankton blooms in the second half of the 1980s were due to a top-down effect caused by a strong decline in phytoplankton grazing by fish.Handling editor: I.A. Nagelkerken