Tropical lakes: how latitude makes a difference

Tropical lakes are far less numerous than temperate lakes because lakes of glacial origin are rare in the tropics. Also, the mixture of lake types differs from tropical to temperate latitudes; lakes of riverine origin are probably more common than any other type in the tropics. For lakes of a given type and size, latitudinal trends can be traced mainly to latitudinal gradients in minimum monthly solar irradiance. Higher monthly minima for irradiance in the tropics are responsible for higher minimum water temperatures, higher mean temperatures for the water column, and a smaller thermal differentiation between upper and lower water column. Tropical lakes of moderate to great depth are predominantly warm monomictic, and show great regularity in seasonal mixing, which typically coincides with the hemispheric winter. Stratification is seasonally persistent, but is less stable than at higher latitudes, and the amount of heat exchange required to cause important changes in stability is also smaller than at higher latitudes. For these reasons, as well as a geostrophic factor that can magnify the variability of wind-generated mixing, tropical lakes show much more intraseasonal variation in thickness of the mixed layer than morphometrically similar temperate lakes. Episodes of mixed layer thickening alternating with subsequent restoration of a thinner mixed layer affect nutrient cycling and plankton dynamics in tropical lakes. Efficiency of nutrient cycling is high at low latitudes because of intraseasonal deep mixing combined with high mean temperatures, which support higher nutrient regeneration rates. Primary production is about twice as high on a given nutrient base as it would be at higher latitudes, primarily because of efficient nutrient cycling combined with higher mean temperatures and greater stability in solar irradiance. Nitrogen is more often a limiting element for autotrophs in the tropics than at temperate latitudes. Phytoplankton, zooplankton, and benthic communities are no more complex at low latitudes than at high latitudes, in contrast to terrestrial communities. Phytoplankton communities show broad overlap in species composition with temperate communities and have little endemism. Zooplankton also show overlap, but have more endemism, contain more taxa that are characteristically tropical, and are composed of smaller species than temperate communities. Efficiency of energy transfer appears to be low in many tropical lakes. Zooplankton fishes are neither abundant nor diverse in many tropical lakes. Zooplankton herbivores often show suppressed growth rates indicating nutritional blockage of energy transfer even when food is abundant, and transfer of energy to fishes may be intercepted or suppressed by Chaoborus, which is abundant and widespread in the tropics.