Photo credit Tim Nicholson
There are many different types of marine ecosystems, ranging from estuaries, lagoons, rocky shores, mangrove forests, saltmarshes and coral reefs to coastal, open ocean, benthic (seafloor) and hydrothermal vent ecosytems. Most of these ecosystems rely on primary producers, such as phytoplankton (free-floating unicellular plants) and, to a lesser extent, macrophytes (seaweed), that utilise the sun's energy for photosynthesis. These organisms are at the base of very complex food webs, which include animals ranging in size from zooplankton (e.g. krill) and invertebrates (e.g. crabs) to fish and marine mammals. Various physical and biogeochemical conditions (temperature, light, mixing, turbulence, nutrients, trace metals...) control the rate of primary production, which will in turn have an impact on the entire food web and determine, for example, fisheries productivity.