All freshwater eventually passes through or over sedimentary habitats on the way to oceanic or atmospheric domains. These zones of freshwater sediments contain an enormous variety of species that produce and process organic carbon. Some species perform specific functions such as fixing or recycling nitrogen. Others break down contaminants or mix sediments so as to alter the rates of ecosystem processes Approximately 175 000 species have already been described and yet still, new species continue to be discovered. Their diversity and distributions range greatly along gradients of depth, dissolved oxygen, latitude, and altitude in wetlands, lakes, rivers, and groundwaters. Deep, isolated habitats contain unique, endemic species especially among those organisms with limited dispersal ability. Competent scientists must be trained in many parts of the globe to accelerate studies of this species-rich biota and essential ecosystem relationships.