They are some of the most biologically diverse communities in the world, because these spaces offer hiding places for shrimp, fish, and other marine life from predation by crocodiles and birds. Many mangrove and other estuarine ecosystems are being destroyed by land use changes in coastal areas. For example, along the gulf coast of the US, Southeast Asia, and Latin America, the use of estuaries for managed shrimp aquaculture has all but destroyed the species diversity, and probably the functioning, of the native estuary. Check out the following article about the rise of shrimp farming and the loss of mangroves . Estuaries supply a significant amount of N and P to open waters.
Anaerobic sediments promote denitrification.
Moving further offshore to the open water habitat of the estuary, where freshwater gives way to ocean saltwater, biogeochemical cycling is strongly dependent on the mixing of fresh and salt water. This figure shows how salinity in ppt, changes in a typical cross section of an estuary. Remember that the suspended load in rivers is composed of mineral sediments and DOC such as organic acids. The result is that regions around the salt wedge provide a large supply of nutrients and carbon for abundant marine life. This figure shows a conceptual spatial model of biogeochemical processes across a cross section of an estuary from a river, through the estuary, and to the ocean overlying the continental shelf.