Climate change in tropical fresh waters (comment on the paper 'Plankton dynamics under different climatic conditions in space and time' by de Senerpont Domis et al., 2013)

1. de Senerpont Domis etal. (2013, Freshwater Biology, 58, 463-482) forecasted changes in plankton dynamics in temperate, polar and tropical regions resulting from climate change. For tropical regions, they predicted an increase in precipitation intensity that would increase nutrient loading, increasing phytoplankton biomass and select for plankton adapted to flushing. 2. We do not agree with these predictions, as regional projections from the IPCC did not forecast a major increase in precipitation in tropical regions. The only regions where a slight increase in precipitation was projected were eastern Africa and South-East Asia. In eastern Africa, the major freshwater bodies are large, deep lakes that have very long residence times and are unlikely to be affected by flushing. Moreover, nutrient inputs from their catchment represent a small fraction of their total nutrient loading. 3. Several independent studies carried out in this region have provided evidence of a decrease in primary productivity in some of these large tropical lakes due to climate change. The major process providing nutrients to the euphotic layer is internal loading, which has been reduced as warming of the surface waters has increased the temperature gradient and the water column stability. Moreover, reduced velocity of trade winds during the dry season has affected the mixed layer depth and decreased internal nutrient fluxes. Therefore, the trend for large tropical lakes in a warming climate is oligotrophication, not eutrophication. 4. In tropical South America, the rainfall increase is not the dominant scenario; thus, the predicted changes in plankton dynamics do not stand. 5. Therefore, we believe that the predictions presented in the paper for tropical systems under a changing climate are invalid for most tropical systems.