Sea

A sea is a large body of salty water that is usually smaller than an ocean and partly enclosed by land. Seas are connected to the global ocean, although the boundary between a sea and an ocean is not always sharply defined. Some seas lie along the edges of continents, while others are almost enclosed by land and connected to the ocean through narrow passages. Their size, depth, temperature and salinity vary from one region to another.

Seawater contains dissolved salts, mainly sodium chloride, as well as many other substances. Waves, tides and currents keep seawater moving and help shape coastal landscapes. Rivers bring freshwater and sediments into seas, while rainfall, evaporation and the formation or melting of ice influence water volume and salinity. Over long periods, marine processes can form and modify beaches, cliffs, bays, islands and other coastal features.

Seas support complex ecosystems that range from microscopic plankton to large marine animals. Fish, mollusks, crustaceans, algae, marine plants and many other organisms depend on coastal or open-water habitats. These ecosystems also provide important resources for people and contribute to the movement of heat and carbon that influences Earth's climate. Human societies have long relied on seas for fishing, transportation, trade and recreation.

However, marine environments can be disturbed by plastic waste, chemical pollution, overfishing, habitat destruction, warming waters and ocean acidification. Protecting coastal habitats, reducing pollution, managing fisheries responsibly and lowering greenhouse gas emissions can help preserve marine biodiversity and ecosystem functions. Although a sea may appear to be a separate body of water, it remains connected to the wider global ocean system.