Ocean and freshwater biomes

Sunlight rarely reaches farther than 200 m into the ocean, yet the average ocean floor lies about 4,267 m down.

Water covers most of the planet

Ocean ecosystems cover more than 70% of Earth's surface, and freshwater ecosystems only about 1.8%, according to OpenStax. Phytoplankton, the tiny drifting algae and bacteria of the sea, perform about 40% of all photosynthesis on Earth, so ocean food webs matter to every breathing animal. Aquatic biomes are not sorted by rainfall. In water, the key factors are how deep light reaches, how salty the water is, temperature, and the movement of water by currents and tides.

Zones of light and depth

Water absorbs light, even when it is perfectly clear. NOAA explains that light may be detected as far as 1,000 m down, but there is rarely any significant light below 200 m. The top 200 m is the sunlight zone, where almost all photosynthesis happens and where most commercial fisheries operate. From 200 m to 1,000 m is the twilight zone, too dim for photosynthesis. Below 1,000 m it is dark: the midnight zone to 4,000 m, the abyss from 4,000 m to 6,000 m and the hadal zone in the trenches beyond. The deepest point, the Challenger Deep in the Mariana Trench, is about 11,000 m down.

Life in the dark depends on food from above. When plankton and the animals that eat them die, their bodies sink. The ocean floor, or benthic realm, is rich in nutrients for that reason, and sea cucumbers, worms, sponges and fishes live on the falling material. Around hydrothermal vents, mostly in the abyssal zone, chemosynthetic bacteria form the base of a food web of their own.

Reefs and estuaries

Coral reefs grow in warm, shallow, sunlit water within 30° of the equator. Reef-building corals live in partnership with photosynthetic algae inside their tissues, which supply most of their food, and more than 4,000 species of fish live on reefs. OpenStax warns that warmer water makes corals expel these algae, which is called bleaching, and that more carbon dioxide makes the ocean more acidic, which makes it harder for corals to build skeletons. Estuaries form where rivers meet the sea. Their brackish water changes saltiness with every tide, once or twice a day, and they are nurseries for young fish, crabs and shellfish.

Lakes, rivers and dead zones

In lakes and ponds the limiting nutrients are usually nitrogen and phosphorus. Add too much, from sewage or fertiliser, and algae bloom; when the algae die and decay, bacteria use up the oxygen and fish die. The same thing happens at sea. NOAA measures the summer dead zone in the Gulf of America each year. The record, 8,776 square miles, came in 2017. In 2025 NOAA forecast 5,574 square miles and scientists measured 4,402. In 2026 they measured 1,332 square miles, the second smallest in 40 years of measurement, but Tropical Storm Bertha had stirred oxygen-rich water down to the seabed just before the survey. The goal of 1,900 square miles by 2035 is for the five-year average, which NOAA put at 3,745 square miles in 2026, almost twice the goal. One small year does not settle the trend.