2 - Primary Production in Tropical Streams and Rivers

Abstract

Net primary production is a fundamental ecological process that reflects the amount of carbon synthesized within an ecosystem, which is ultimately available to consumers. It reveals that aquatic algae are a significant contributor to food webs in tropical rivers and streams. It also shows that the rates of in-stream primary production in tropical regions are typically at least an order of magnitude greater than comparable temperate systems. Although nutrient status can significantly modify rates, the ultimate driver of aquatic primary production is light availability. Flood plains are important for fodder and for nursery habitat for fish, which re-invade main channels when floods recede. Tropical river systems undoubtedly sit at the high end of the global productivity spectrum, and the wet and dry forest catchments they drain have the highest rates of primary production of any terrestrial biome. They are characterized by climatic conditions that often promote year-round growth, with less seasonal variation in solar irradiance than temperate latitudes. Much of the primary production within the channels of streams and rivers, particularly those with fast flowing water, is typically restricted to benthic habitats involving algae and cyanobacteria. Rivers draining undisturbed tropical catchments are low in nutrients and consequently aquatic primary production is low. Macrophytes appear to be the major primary producers within floodplains. Primary production in tropical rivers is variable and typically only a component of overall productivity.