Reduced rainfall increases metabolic rates in upper mixed layers of tropical lakes

Ecosystem-level metabolism is a good sentinel for human and natural disturbances in freshwater systems, responding from local changes (for example, land use) to regional and global changes (for example, climate). Despite the increasing understanding of metabolic processes in tropical lakes, our knowledge on how morphometric and catchment characteristics affect metabolic responses to those changes in tropical lakes is still very scarce. We investigated how metabolic rates in the upper mixed layer of twelve Brazilian tropical lakes responded to reduced rainfall, considering their lake area and drainage area ratio and the percentage of native forest cover in their drainage area. An 80% reduction in the 2013 rainy season rainfall, compared with 2012, resulted in a reduction of approximated 1 m in the water column depth, a 1–2 m deepening of the upper mixed layer, a 50% reduction in mean light availability, and a doubling in total phosphorus concentrations. These changes were associated with 38% increases in gross primary production (GPP), stimulated by higher nutrient concentrations and reduced photoinhibition. These effects of reduced rainfall were strongest in lakes with a small volume in relation to their catchment areas. Our results show that climatic-related reductions in precipitation in this tropical region will reduce lake volumes, affect temperature, water mixing, and nutrient supply with pronounced effects on lake metabolic processes and carbon cycling in this region.