6 Marine Ecosystems

Marine habitats and their conditions

Marine ecosystems are communities of organisms interacting with one another and with their physical and chemical surroundings. Their conditions vary with depth, light, temperature, salinity, oxygen, nutrients, currents, and the nature of the seafloor. Because water and organisms move between habitats, these ecosystems are connected rather than isolated.

Coastal and intertidal habitats

Coastal and intertidal habitats include estuaries, salt marshes, mangrove forests, mudflats, rocky shores, sandy beaches, seagrass meadows, kelp forests, and coral reefs. Estuaries mix freshwater and seawater; their changing salinity and nutrient supply support distinctive communities.

Marshes and mangroves shelter young fish and invertebrates. Mangrove roots also slow water and help stabilize shorelines. Seagrass beds provide food and shelter, and their plants stabilize sediments. Coral reefs create complex structures that support many species.

Open-ocean habitats

Open-ocean habitats include the water column, or pelagic zone, and the seafloor, or benthic zone. Light generally supports photosynthesis in the upper ocean, especially in the sunlit zone; below it, photosynthesis is not possible. Deep-sea animals depend largely on organic matter sinking from above, although some communities, such as those around hydrothermal vents, use chemical energy instead.

Habitat conditions shape which species can survive. Intertidal organisms must tolerate repeated exposure to air and changing conditions, while deep-sea organisms live without sunlight and under high pressure.

Biodiversity and species interactions

Biodiversity includes the variety of species, genes, and habitats in an ecosystem. Marine life ranges from microscopic plankton and microbes to corals, mollusks, fish, seabirds, and marine mammals.

Habitat structure can create many niches: the spaces and resources organisms use in different ways. For example, a reef’s crevices provide hiding places and feeding sites for many organisms. Coastal habitats can also connect life stages: a fish may use a sheltered estuary as a nursery before moving offshore as an adult.

Species interact through competition, predation, grazing, filtration, and symbiosis. In symbiosis, different species live in close association. At hydrothermal vents, some animals host chemosynthetic microbes that supply food made using chemical energy. These relationships help determine how energy and nutrients move through a community.

Food webs and energy flow

A food web represents who eats whom and how energy moves among organisms. Producers make organic matter; in most marine ecosystems, these include photosynthetic phytoplankton, seaweeds, and seagrasses. Consumers obtain energy by eating producers or other consumers. Decomposers and microbes break down dead material and waste, returning nutrients to the environment.

Arrows in a food web point from the organism being eaten toward the organism that eats it. A simplified open-ocean pathway is:

phytoplankton → zooplankton → small fish → large fish or marine mammals

Real food webs have many overlapping pathways. Some animals eat at several trophic levels, and many species are prey for more than one predator. As a result, a change in one population, such as a decline in a predator or an increase in nutrients, can affect other parts of the web.

Deep-sea vents show that food webs do not always begin with sunlight. There, chemosynthetic microbes use energy from chemical reactions to make organic matter. Grazers and animals hosting these microbes can then support predators in a localized vent community.

Productivity and nutrient supply

Primary productivity is the rate at which producers make organic matter. In sunlit waters, phytoplankton use sunlight, carbon dioxide, and nutrients to grow. Productivity depends on both light and the supply of nutrients such as nitrogen and phosphorus.

Nutrients can be recycled as organisms produce waste and decompose, or supplied from elsewhere by mixing, currents, rivers, and upwelling. In upwelling, deep, nutrient-rich water rises toward the surface and can stimulate phytoplankton growth.

Productivity varies across the ocean. Coastal waters and regions affected by upwelling can receive abundant nutrients, while other surface waters may have ample light but fewer nutrients. Productivity also changes seasonally as light, mixing, and nutrient availability shift.

Ecosystem processes and connections

Marine ecosystems continually move energy and matter. Energy enters mainly through photosynthesis, or through chemosynthesis at certain deep-sea sites, and passes through food webs. Matter, including carbon and nutrients, is recycled: organisms take it up, transfer it through feeding, and return it through respiration, waste, and decomposition.

Some organic material sinks from surface waters to the deep ocean, where it feeds organisms and contributes to nutrient cycling. These processes connect different parts of the ocean.

Coastal habitats also influence their surroundings. Estuaries and wetlands receive nutrients from watersheds, and marshes can filter some pollutants and excess nutrients from flowing water. Mangroves and seagrasses store carbon in plants and sediments, provide habitat, and help protect shorelines. These processes link marine ecosystems to nearby land, the atmosphere, and human communities.

• niche