uths of large rivers and higher in the depths of the ocean; however, the relative proportions of dissolved salts vary little across the oceans. The most abundant solid dissolved in seawater is sodium chloride. The water also contains salts of magnesium, calcium, potassium, and mercury, among other elements, some in minute concentrations. A wide variety of organisms, including bacteria, protists, algae, plants, fungi, and animals live in various marine habitats and ecosystems throughout the seas. These range vertically from the sunlit surface and shoreline to the great depths and pressures of the cold, dark abyssal zone, and in latitude from the cold waters under polar ice caps to the warm waters of coral reefs in tropical regions. Many of the major groups of organisms evolved in the sea and life may have started there.

The ocean moderates Earth's climate and has important roles in the water, carbon, and nitrogen cycles. The surface of the water interacts with the atmosphere, exchanging properties such as particles and temperature, as well as currents. Surface currents are the water currents that are produced by the atmosphere's currents and its winds blowing over the surface of the water, producing

wind waves, setting up through drag slow but stable circulations of water, as in the case of the ocean sustaining deep-sea ocean currents. Deep-sea currents, known together as the global conveyor belt, carry cold water from near the poles to every ocean and significantly influence Earth's climate. Tides, the generally twice-daily rise and fall of sea levels, are caused by Ear