The main ecosystem of many estuaries is composed of plankton and bottom organisms. In most estuaries, freshwater streams mix with the saltwaters that the tide exchanges with the open sea. This chapter discusses the main features of many Florida estuaries—a freshwater stream flowing in from the left and the sea on the right exchanging waters through an inlet in the outer barrier island of beaches. The zones of salinity that slide in and out of the estuary with the tide are also dependent on the velocity of the freshwater streams. Some of the high productivity of estuaries is due to their good physical circulation. High levels of organic food develop in estuaries. In the panhandle, the Apalachicola River, draining much of Georgia, reaches the Gulf of Mexico carrying heavy loads of clay sediments and is thus quite different from other Florida rivers.