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.
Estuaries are some of the most productive fisheries in the world.
The Chesapeake Bay is a good example. Until recently, there has been a longstanding debate whether high animal productivity is supported by the supply of carbon from river water or from autochthanous production. By measuring the d 34S and d 13C of the major potential producers, upland plants, plankton, and Spartina , they determined that these potential sources of carbon had unique and distinguishable signatures. Land plants discriminate in favor of the lighter 12C much more than either plankton or Spartina . Then, by measuring the S and C isotope composition of marine animals, they found that the animals signatures fell in between that of the estuary C sources, indicating that the richness of marine animal life is supported by autochthanous production in the estuary, and not river transport.