Biological communities
Some organisms are adapted for living in freshwater, others for living in salt water. In general, freshwater biota do not extend into saline or slightly saline water. Consequently, as salinity increases, the species richness and growth of freshwater biota is reduced (Hart et al . 1991). Freshwater is generally defined as water in which salinity is less than 3000 mg L–1 and sea water as 35 000 mg L–1 . These are the world average values for those systems (Boulton and Brock 1999); 3000 mg L–1 is often considered the lower limit for saline waters (Hart et al . 1991). Water between 3000 mg L–1 and 10 000 mg L–1 can be defined as saline as biotic effects are well known within this range. Animals are divided on the basis of their ability to regulate their internal osmotic concentrations against the external environment: those that regulate internal salt concentr ations well can adapt to a wide range of salinities (euryhaline regulators), whereas those that are poor regulators cannot and are restricted only to a narrow range of salinities (stenohaline regulators). Salt-tolerant plants (halophytes) tend to prefer brackish or saline conditions rather than freshwater, whereas most freshwater plants (non-halophytes) do not tolerate increasing salt concentration.