FIG. 48.14 Representative marine biomes.

Intertidal

The part of the neritic (shallow-water) system closest to shore is the intertidal zone, with its distinctive species associations. The intertidal zone lies along coastlines between mean high and low tides, meaning that organisms in this biome are exposed to the atmosphere on a daily basis. Intertidal organisms are thus challenged by periodic desiccation as well as by temperature extremes that accompany exposure to direct sunlight. In sandy intertidal zones, or tidal flats, many animals burrow into the wet sand to escape exposure at low tide. In rocky intertidal zones, organisms may close their shells tightly—for example, mussels and barnacles. Waves can cause mortality and so, especially along rocky coastlines, both algae and animals are securely attached to the substrate. Nutrient levels can be high, favoring strong algal growth. Consumers include a variety of sea stars, sea urchins, mollusks, barnacles, and corals. Intertidal biomes exhibit the greatest diversity where tidal exposure is relatively short. Tidal pools can harbor diverse animal and algal species.

Coral reefs

Coral reefs form another neritic biome, indeed, the most diverse biome in the oceans. The best-known reefs occur in shallow, tropical to subtropical environments where there is little water movement or accumulation of sand or mud, with the accumulating skeletons of corals building a structure above the seafloor. Nutrient levels are commonly low, but primary production is high and mostly tied to dinoflagellate algae that live within the tissues of corals. Rapid recycling of nutrients between the corals and their algal symbionts ensures high levels of primary production. Free-living algae occur in reefs, but are kept at low abundance by grazing fish. Many species of invertebrates live within the reefs, often in nooks and crannies among coral heads that provide shelter from predators. Fish diversity can be especially high. Coral reefs also occur in the deep sea, but build gradually, as coral growth is slow in the absence of symbiotic algae.

Pelagic realm

The pelagic realm—the part of the ocean that is neither close to shore nor close to the seafloor—forms the bulk of the oceanic system. Organisms live within the water column, either as plankton or nekton. In the upper 200 m or so of the pelagic realm, sunlight permits photosynthesis, with diverse algae and cyanobacteria as primary producers. In deeper waters, however, sunlight is absent and life is sustained by the rain of organic particles from surface waters. Minute arthropods dominate the zooplankton, while fish and cephalopods are key components of the nekton. This is true in both surface and deep waters, but animals are less abundant at depth. Fish and squids are conspicuous consumers, but recent research shows that, in the upper water column, heterotrophic protists may actually be more abundant and diverse. Bacteria and archaeons occur throughout the pelagic realm.

Deep Sea

Kilometers below the surface, deep seawaters are cold and dark, but they are not sterile. Sinking detritus supports sparse populations of animal consumers, as well as bacteria and archaeons that feed on organic particles. Despite their limited biomass, deep-sea biomes exhibit high species diversity, as high as those of shallow marine communities. However, the composition of communities in deep-sea biomes differs relatively little from place to place, and so the total diversity of these environments is lower than that found within the photic zone. Dense animal populations occur locally on the deep seafloor, where hydrothermal vents expel fluids containing high abundances of hydrogen, hydrogen sulfide, and methane. These compounds support locally high levels of primary production by chemosynthetic bacteria, many of them present as symbionts within the tissues of vent animals.