freshwater ecosystems are a subset of earth's aquatic ecosystems. they include lakes and ponds, rivers, streams, springs, bogs, and wetlands. they can be contrasted with marine ecosystems, which have a larger salt content. freshwater habitats can be classified by different factors, including temperature, light penetration, nutrients, and vegetation.
freshwater ecosystems can be divided into lentic ecosystems (still water) and lotic ecosystems (flowing water).
limnology (and its branch freshwater biology) is a study about freshwater ecosystems. it is a part of hydrobiology.
original attempts to understand and monitor freshwater ecosystems were spurred on by threats to human health (ex. cholera outbreaks due to sewage contamination). early monitoring focused on chemical indicators, then bacteria, and finally algae, fungi and protozoa. a new type of monitoring involves quantifying differing groups of organisms (macroinvertebrates, macrophytes and fish) and measuring the stream conditions associated with them.