<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> <P> It is important to remember that a primary source of food for many organisms on estuaries, including bacteria, is detritus from the settlement of the sedimentation . </P> <P> Of the thirty - two largest cities in the world in the early 1990s, twenty - two were located on estuaries . </P>

How can you explain estuary as an ecosystem