<P> Estuaries form a transition zone between river environments and maritime environments . They are subject both to marine influences -- such as tides , waves , and the influx of saline water -- and to riverine influences -- such as flows of fresh water and sediment . The mixing of sea water and fresh water provide high levels of nutrients both in the water column and in sediment , making estuaries among the most productive natural habitats in the world . </P>
what is the main source of nutrients in an estuary
sea water fresh water