Marine ecosystem

Marine ecosystems are the largest aquatic systems on our planet. They exist in waters with a high salt content. This distinguishes them from freshwater ecosystems, which have lower salinity levels. These salty waters cover more than 70% of the Earth's surface. They account for more than 97% of the total water supply on Earth. Furthermore, they provide 90% of the habitable space on our planet.

The physical environment of the ocean is divided into several distinct zones. The oceanic zone is the vast, open part of the sea. This area is home to large animals like whales, sharks, and tuna. Below this lies the benthic zone, which consists of the substrates or surfaces on the sea floor. Many invertebrates live in this bottom layer. There is also the intertidal zone, which is the area between high and low tides.

Coastal environments create specialized habitats with unique biological communities. Coral reefs are famous examples, such as the Great Barrier Reef. These reefs are made of large coral colonies living in symbiotic relationships. Mangroves are trees or shrubs that grow in low-oxygen soil near tropical coastlines. They use adaptations like salt excretion and root aeration to survive. Their dense roots help protect coasts from erosion caused by waves and tides.

Other specialized habitats include seagrass meadows and kelp forests. Seagrasses are plants that evolved from marine algae that returned to the ocean 100 million years ago. These meadows provide food and refuge for species like seahorses and turtles. Kelp forests are formed by brown macroalgae found in temperate and polar waters. These forests can influence coastal oceanographic patterns. However, they are threatened by overfishing, which can lead to overgrazing by herbivores.

Estuaries and lagoons represent different ways water interacts with the land. Estuaries occur where rivers meet the ocean, creating brackish water. This water is a mix of fresh and salt water. Estuaries are highly productive and support many human activities. In fact, 22 of the 32 largest cities in the world are located on estuaries. Lagoons are areas separated from the ocean by sandbars or coral reefs. They can be coastal or atoll lagoons, which are circular reefs surrounding deep water.

Salt marshes serve as a vital transition between the ocean and the land. The soil in these marshes often contains peat, which is decomposing plant matter. This peat can create hypoxic conditions, or low oxygen levels, in the water. These conditions allow specific bacteria to grow, often creating a sulfurous smell. Salt marshes are divided into the low marsh, high marsh, and the upland border. They are incredibly productive, supporting more than 75% of fishery species.

Human activity presents significant challenges to the stability of these systems. Unsustainable exploitation, such as overfishing, unbalances the food web. Pollution and climate change are also major threats. The ocean absorbs much of the carbon dioxide that causes global warming. This leads to ocean acidification, which changes the chemistry of the water. Because of these risks, the United Nations established Sustainable Development Goal 14. This goal, titled "Life below water," aims to conserve and sustainably use marine resources.