Among tropical marine ecosystems estuaries are one of the highly productive areas and act as a nursery to large number of organisms. The primary production in most of the estuaries is less during the monsoon period. Post-monsoon period shows proliferation of phytoplankton causing increased production. Short pulses of higher primary production are a common feature. Light is the most limiting factor for primary production. Succession of phytoplankton is controllEd. by various environmental factors such as salinity, level of nutrients etc. Mangroves found in some estuarine regions are more productive than the backwaters and estuaries. Estuaries and mangrove environments influence the coastal waters to a great extent. The coastal regions are atleast five times more productive than the oceanic realms. When the planktonic and macrophyte systems are considered together, the coastal waters beyond doubt, have a higher primary production than that of the open ocean. Various processes occurring at the interface of land and sea tend to influence phytoplankton production. The contribution of phytoplankton to the primary production in coral reefs is small when compared to the total production of the sea grass bed