Marine coastal ecosystem

Marine coastal ecosystems are complex biological systems located where the land meets the ocean.

These environments exist along the margins of the continental shelves. They occupy approximately 7 percent of the total ocean surface area. These regions are defined by high levels of biodiversity and productivity. This means they support a vast number of different species and produce a lot of biological energy. Because they sit at the boundary of two worlds, they facilitate the movement of nutrients and elements. They also act as a natural buffer for the planet by sequestering carbon.

One of the most important areas is the intertidal zone. This is the part of the coast that is exposed to air during low tide and covered by saltwater during high tide. Scientists divide this zone into four distinct physical sections. The spray zone is a damp area only reached by high tides or storms. The high intertidal zone is submerged at high tide but stays dry for long periods in between. This area is home to resilient wildlife like barnacles, mussels, and hermit crabs. The middle intertidal zone experiences tides twice a day and holds more variety. Finally, the low intertidal zone is submerged most of the time, allowing life to be more abundant.

Estuaries represent another vital habitat type. These occur where freshwater from rivers meets the saltwater of the ocean. This meeting creates brackish water, which is a specific mixture of fresh and salt water. Estuaries are extremely productive and serve as economic hubs for many human communities. In fact, 22 of the 32 largest cities in the world are located on estuaries. These ecosystems provide essential services like water filtration, erosion control, and nutrient cycling. They also act as crucial nurseries for many species of fish and shellfish.

Vegetated coastal ecosystems are found in various climates around the globe.

Mangrove forests are found in tropical or subtropical intertidal zones. They consist of salt-tolerant trees that protect habitats for crabs, shrimp, and fish. Salt marshes are coastal wetlands found in temperate and high-latitude areas. They are populated by plants like cordgrass that can handle salt. Seagrass meadows are underwater grasslands made of flowering plants. These meadows provide food and shelter for sea turtles and dugongs. In colder, nutrient-rich waters, kelp forests dominate. These are made of large brown algae that grow several meters tall to create dense underwater forests.

Coral reefs are among the most famous and diverse ecosystems on Earth. They are built by colonies of tiny animals called coral polyps. These polyps secrete hard calcium carbonate skeletons over long periods. As these skeletons accumulate, they create massive and complex underwater structures. These reefs thrive in high-energy shorelines where waves agitate the water. While they are highly productive, they are also very vulnerable to global warming. Marine heatwaves can cause a decline in these structures and the life they support. The Great Barrier Reef is a notable example of this massive ecosystem.

Lagoons provide a different type of coastal habitat. These are bodies of water separated from the larger ocean by natural barriers like sandbars or coral reefs. There are two main types: coastal lagoons and atoll lagoons. Coastal lagoons are separated by a simple barrier, while atoll lagoons are surrounded by circular coral reefs. Most lagoons are quite shallow, meaning they are heavily affected by wind and evaporation. This causes the salinity and temperature to vary widely. Lagoons are found on every continent except Antarctica. They support diverse species including birds, fish, and plankton.

Human activity creates significant pressure on these delicate systems. Over half of the world's population lives within 200km of a coastline. This proximity makes humans very susceptible to the changes in these ecosystems. Anthropogenic pressures, which are caused by humans, include pollution, overfishing, and coastal development. Climate change introduces further risks through sea level rises and ocean acidification. These changes can shift the entire ecological baseline of a region. To combat this, the United Nations established 17 Sustainable Development Goals in 2015. Their 14th goal, "Life Below Water," aims to conserve and sustainably use marine resources by 2030.