Estuaries, bays, lagoons, and coastal rivers.

Estuaries are partially enclosed bodies of water with a connection to open ocean in which seawater enters according to the rhythm of the tides. Seawater entering the estuary is diluted with freshwater flowing from rivers and streams, and the pattern of dilution depends on the volume of freshwater, tidal amplitude range, and evaporation rate. These ecosystems are known to thrive where the inflow of both seawater and freshwater provide high levels of nutrients in both the water column and sediments. The high concentration of nutrients supports phytoplankton, which is the primary producer in the ecosystem. Each estuary is unique with respect to its physical, chemical, and biological characteristics. Classification into general groups is made according to the geological processes that formed their embayment or, alternatively, according to their water circulation and mixing characteristics. Dynamics inside an estuary depend on its geographic location, the shape of the coastline and ocean floor, the depth of the water, tidal patterns and currents, local winds, freshwater input, and any restrictions to water flow. Additionally, there is a constant fluctuation in dissolved oxygen, salinity, temperature, and sediment load. At any point, salinity and temperature can vary considerably over time and season. Sediment often settles in intertidal mudflats, and dissolved oxygen variations can limit livability for many organisms. The wide range of salinity, temperature, and other chemical conditions, coupled with high primary productivity, creates environments that support many species of fishes and invertebrates.