Reduced top–down control of phytoplankton in warmer climates can be explained by continuous fish reproduction

Large herbivorous zooplankton and spring clear-water phases occur rarely in tropical lakes. It has been hypothesized that this can be explained by latitudinal differences in the structure of the trophic cascade. Many tropical fish species reproduce throughout the year and are omnivorous. Both properties could reduce the chances for large zooplankton to cause a clear-water phase. Continuous reproduction leads to permanent presence of planktivorous young individuals, and thus a continuously high predation pressure on large zooplankton. On the other hand, omnivory could allow fish to be more abundant, leading to a higher predation pressure on zooplankton. Data to distinguish between effects of continuous reproduction and omnivory boosted abundance are scarce. Here, we use a simple model to indicate that continuous fish reproduction in the tropics by itself can reduce the chances for top–down control of phytoplankton by Daphnia even if the carrying capacity for small fish would not be boosted by omnivory. The model predicts that only at very low fish densities, clear-water phases are possible, but these tend to occur at irregular times of the year. We also analyzed the effect of change in the mean annual temperature. This model analysis suggests that in temperate regions where fish reproduction is pulsed, climatic warming would tend to promote the chances for top–down induced clear-water phases.