Subtidal Zone Life on the Continental Shelf Subtidal

Subtidal Zone: Life on the Continental Shelf

Subtidal Zone Part of the continental shelf that is NEVER exposed at low tide (Sublittoral zone) n Extends from the low tide level on shore to the shelf break (the outer edge of the continental shelf). n

Subtidal Zone n Physical factors that affect subtidal organisms are linked to 2 of the shelf’s fundamental characteristics: 1. 2. Relatively shallow water Proximity to land

Subtidal Zone n Tides and waves stir up the water column and prevent stratification n n i. e. , temperature and salinity do not differ that much from the bottom to the surface Tides and waves also make the shelf water murkier than that in the open ocean

I. Soft-Bottom Subtidal Communities Dominant substrate of continental shelves n Dominated by infauna (organisms living in the sediment), but there a few epifauna (organisms that live on top of sediment) n n Because the soft bottom offers nothing to hold on to sessile (attached) organisms are rare.

Soft-Bottom Organisms

I. Soft-Bottom Subtidal Communities n There is a close relationship between particle size and the distribution of organisms Size of sediment particles Less More Turbulence Small and closely Larger and more packed porous Oxygen and detritus concentration Less oxygen, more detritus Type of animal Deposit feeders More oxygen, less detritus Suspension feeders

Soft-Bottom Subtidal Communities A. Unvegetated Soft-Bottom Communities n n n Largest majority of the soft-bottom communities Main primary producers = diatoms, microscopic algae, and bacteria Inhabited by deposit and suspension feeders

Soft-Bottom Subtidal Communities B. Seagrass Beds n n n Develop best in sheltered, shallow water along the coast Limited by the penetration of light through the water column The roots and stem keep them anchored and stabilize the soft bottom Leaves cut down wave action and currents…reduced turbulence also improves water clarity The plants are not heavily grazed, but produce a lot of detritus

Eelgrass “Community”

II. Hard-Bottom Subtidal Communities Usually the submerged extensions of rocky shores n Often called reefs n n NOTE: THIS DOES NOT INCLUDE CORAL REEFS!

Hard-Bottom Subtidal Communities A. Rocky Bottoms n n Biggest inhabitants are seaweeds and their biggest problem is finding a place to attach Rich in epifauna and poor in infauna (reverse of soft-bottom communities) Many colonies outcompete other species by growing over them Grazers and predators strongly influence the composition of the communities

Example of an influential grazer: n Sea Urchins…graze on seaweeds and on some of the encrusting animals…

Hard-Bottom Subtidal Communities B. Kelp Communities n n Kelp are a group of large, fast-growing brown seaweeds that live in relatively cold water and are restricted to temperate and subpolar regions Kelp beds = large, dense patches of kelp Kelp forests = when the fronds (leaf-like blades) float on the surface in a thick mat Temperature is the major limiting physical factor

Geographic Distribution of Kelps

Hard-Bottom Subtidal Communities B. Kelp Communities n n Kelp are fragile and don’t do well in heavy wave action…they can also be limited by grazing sea urchins, warm currents, and pollution Drift kelp and understory seaweeds are a major food source to other organisms, but NOT the live kelp themselves