The continental shelf is the submerged extension of a continent, a gently sloping area extending from the shoreline out into the ocean basin. This relatively shallow region is a hub of biological activity, accounting for a disproportionately large share of the ocean’s life. Although continental shelves only comprise about 7% to 8% of the total ocean area, they are the most biologically productive parts of the marine environment worldwide. The abundance of life here supports vast fisheries and complex food webs. Understanding the unique physical conditions that make this environment so productive is necessary to appreciate the density and variety of animals that thrive here.
Defining the Continental Shelf Habitat
The continental shelf begins at the low-water mark and extends seaward until it reaches the shelf break, where the slope dramatically increases and descends into the deep ocean. The average depth is about 150 meters, and the shelf break is commonly defined at around 200 meters. The mild incline allows for an expansive, shallow water column, known as the neritic zone.
This shallowness is the primary factor driving high productivity, ensuring sunlight penetrates to the seafloor across much of the habitat. This places the water column within the photic zone, allowing for widespread photosynthesis by phytoplankton, which forms the base of the food chain. Proximity to land also results in a significant influx of nutrients through river runoff, delivering dissolved nitrogen and phosphorus.
Water movement, driven by tides, currents, and wind, causes extensive mixing, preventing stratification and recycling nutrients upward. Upwelling of deep, cold, nutrient-rich water also contributes to fertile conditions. The seabed is highly varied, composed of terrigenous sediments ranging from fine silts and muds to coarse sand, gravel, and rock outcrops. These substrates determine the types of bottom-dwelling organisms that colonize the area.
The Benthic Community: Life on the Seafloor
The animals that live on, in, or attached to the continental shelf seafloor collectively form the benthic community. The substrate composition dictates how organisms interact with this environment, leading to a division between those living within the sediment (infauna) and those living on its surface (epifauna).
Infauna burrow into the seabed and are abundant in areas with soft sediments like mud and fine sand. These filter-feeders and deposit-feeders include polychaete worms, clams, and burrowing crustaceans. Bivalve mollusks, such as clams and scallops, thrive by filtering organic matter from the water or consuming detritus within the sediment.
Epifauna live on the surface or are fixed to a hard substrate. In rocky areas, diverse life attaches itself, including sponges, sea anemones, and colonial cnidarians. Mobile epifauna, such as sea stars, sea cucumbers, crabs, and lobsters, move across the seafloor in search of prey or decaying matter.
Specialized benthic habitats, such as kelp forests and oyster beds, provide complex three-dimensional structures for shelter and feeding. Rocky reefs attract organisms like agile rockfish and octopus, which use crevices for protection and hunting. The density of this bottom-dwelling life attracts pelagic predators, illustrating the link between the seafloor and the waters above.
The Pelagic Community: Life in the Water Column
Suspended in the water column above the shelf is the pelagic community, sustained by the high primary productivity of the neritic zone. This community begins with zooplankton, minute animal drifters that graze on abundant phytoplankton. Zooplankton populations are dominated by small crustaceans like copepods and krill, along with the eggs and larvae of numerous fish and invertebrates.
These small organisms form a foundational food source, supporting vast schools of coastal pelagic fish. These agile swimmers are adapted to life in the sunlit, shallower waters. Examples include forage fish like herring, sardines, and mackerel, which travel in dense aggregations to feed on plankton. Larger fin fish, many of which are commercially important, also inhabit this zone, including species such as cod, haddock, flounder, and whiting.
The high biomass attracts larger, more transient predators and marine mammals. Seals, sea lions, and dolphins are common residents or visitors, capitalizing on the reliable supply of schooling fish. Larger species like baleen whales, including humpback and minke whales, are drawn to the shelf to feed on massive quantities of krill and other plankton.
Apex predators, such as various species of sharks, patrol the water column, preying on the abundant fish populations. These animals, whether permanent residents or seasonal migrants, demonstrate the energy transfer from the base of the food web upward.