Most of the planet is covered by ocean waters whose average depth is four times the average elevation of the land, making the open sea by far the largest ecosystem on Earth. Despite this volume, marine net primary production remains similar to or less than that on land because photosynthesis in the sea is carried out by microscopic bacteria and algae restricted to the sunlit surface layers (plants are virtually absent).

The diversity of major lineages (phyla and classes) is much greater in the sea than on land or in freshwaters, and many phyla of invertebrate animals occur only in marine waters. Species diversity appears to be far lower, perhaps because marine waters are physically much less variable in space and time than the terrestrial environment.

Marine fisheries are the largest source of wild protein, derived from fishes, molluscs and crustaceans. The world catch from capture fisheries has grown five-fold over the past five decades, but appears to have declined during the 1990s despite increased fishing effort. More than half of the world's major fishery resources are now in need of remedial management, mainly because of excess exploitation.