Fresh water habitats, depending upon the local geology, may have total ion concentrations ranging from less than 0.1 mM to more than 10 mM in NaCl equivalents. Moreover, salt concentrations can vary widely due to evaporation. In addition to total salinity, the relative amounts of particular ions may vary over a considerable range. This is markedly true in waters with unusual substrates such as limestone. Likewise, salt water habitats may be extremely variable in ionic composition.
Brackish water occurs in coastal regions where sea water and fresh water are mixed.
Tidal flows can vary salinities of such areas over a wide range of concentrations. Because environments are variable, we achieve far greater insight into physiological adaptations of aquatic insects if we know the range of salinity that insects are adapted to tolerate. Organisms that can tolerate wide variations in salt concentrations in the water they live in are called euryhaline from the Greek words for wide and salt. An extreme example is the brine fly Ephydra which lives in the Great Salt Lake. In low environmental salt concentrations the larvae are able to hyperosmoregulate, that is, able to maintain extracellular fluid compartments at higher osmotic concentrations than the medium.