<P> The most important variable characteristics of estuary water are the concentration of dissolved oxygen, salinity and sediment load . There is extreme spatial variability in salinity, with a range of near zero at the tidal limit of tributary rivers to 3.4% at the estuary mouth . At any one point the salinity will vary considerably over time and seasons, making it a harsh environment for organisms . Sediment often settles in intertidal mudflats which are extremely difficult to colonize . No points of attachment exist for algae, so vegetation based habitat is not established . Sediment can also clog feeding and respiratory structures of species, and special adaptations exist within mudflat species to cope with this problem . Lastly, dissolved oxygen variation can cause problems for life forms . Nutrient - rich sediment from man - made sources can promote primary production life cycles, perhaps leading to eventual decay removing the dissolved oxygen from the water; thus hypoxic or anoxic zones can develop . </P> <P> Estuaries provide habitats for a large number of organisms and support very high productivity . Estuaries provide habitats for many fish nurseries, depending upon their locations in the world, such as salmon and sea trout . Also, migratory bird populations, such as the black - tailed godwit, make essential use of estuaries . </P> <P> Two of the main challenges of estuarine life are the variability in salinity and sedimentation . Many species of fish and invertebrates have various methods to control or conform to the shifts in salt concentrations and are termed osmoconformers and osmoregulators . Many animals also burrow to avoid predation and to live in the more stable sedimental environment . However, large numbers of bacteria are found within the sediment which have a very high oxygen demand . This reduces the levels of oxygen within the sediment often resulting in partially anoxic conditions, which can be further exacerbated by limited water flux . </P> <P> Phytoplankton are key primary producers in estuaries . They move with the water bodies and can be flushed in and out with the tides . Their productivity is largely dependent upon the turbidity of the water . The main phytoplankton present are diatoms and dinoflagellates which are abundant in the sediment . </P>

Where does the fresh water in the estuary come from