15.9: Chapter Summary

Communities and Niches.

Species within an ecosystem are distributed nonuniformly, but they often cluster into communities with common characteristics. Each species has a survival niche that is defined by the ranges of environmental variables, such as salinity, temperature, turbidity, and nutrient concentrations, within which it can survive. Within its survival niche, each species has a fundamental niche within which it can survive and reproduce successfully. In some instances, a species may not occupy all of its fundamental niche, as a result of competition by other species.

Coral Reefs.

Reef-building corals grow only on the seafloor in the photic zone of tropical waters between about 30°N and 30°S. These corals house photosynthetic zooxanthellae within their tissues. Reef-building corals obtain some food from their zooxanthellae. Productivity is higher in coral reefs than in other, nutrient-poor tropical marine areas because nutrients are recycled between coral and zooxanthellae, the reefs cause some upwelling, and most organisms in the reef community are residents and do not export nutrients. High turbidity adversely affects corals by reducing photosynthesis by zooxanthellae and requiring the corals to expend energy to clear away deposited particles.

Primary production in coral reef communities is performed by zooxanthellae and by benthic microalgae and macroalgae, many of which are calcareous algae whose hard parts help to build the reef. A typical coral reef has a sheltered lagoon where coral growth is patchy because of variable salinity and turbidity and where many detritus, suspension, and deposit feeders are present. At the lagoon’s seaward edge is a reef flat swept generally free of sediment by waves where coral growth is active and consists mainly of encrusting forms. Invertebrates live in holes and grooves cut in the reef flat.

Farther seaward, there is sometimes a low sandy island and usually an irregular ridge formed from broken coral thrown up periodically by waves and cemented by calcareous algae. Seaward of the ridge is the buttress zone, which may have a gradual to nearly vertical downward slope. It is scoured by waves to about the 20 m depth, characterized by massive robust corals, and often cut across by grooves in which there is little coral growth. Farther seaward, at depths below the reach of wave action, delicate forms of coral, including soft corals, grow in abundance.

Kelp Forests.

Kelp forests grow where the seafloor is stable, preferably rocky, and within the photic zone, and where the water is cold and rich in nutrients. Kelp fronds grow as much as half a meter a day, and kelp primary productivity is very high. Kelp reproduce both vegetatively and by releasing spores to the water column. Kelp forests provide shelter and habitat for many species of fishes and invertebrates. Only a few of these species eat kelp itself, but kelp releases large quantities of detritus that enter the food chain when it is consumed by detritus feeders.

Sea urchins eat kelp, and sea otters eat sea urchins. In the eighteenth and nineteenth centuries, hunting of sea otters off California reduced sea otter populations and thus their predation on sea urchins. The increased sea urchin populations ate more kelp and destroyed the forest in many areas. Sea otters are now protected, and kelp is slowly returning.

Rocky Intertidal Communities.

Species of the rocky intertidal zone are exposed to the atmosphere part of the time. They are subjected to variable conditions of temperature, water and air exposure, salinity, oxygen concentration, and pH, and they are vulnerable to birds and land predators. The rocky intertidal community is separated into four zones, distinguished by degree of exposure to air. The supralittoral zone is above high water and exposed permanently to air but is reached periodically by spray. It supports lichens, encrusting algae, grazers (including marine snails and limpets), and scavengers, primarily isopods. The high-tide zone is covered in water only during high tides and supports encrusting algae, tough attached macroalgae, filter-feeding barnacles, and periwinkle and limpet species different from those in the supralittoral zone. The middle-tide zone is covered and uncovered by water during most or all tidal cycles, has sparse macroalgae because of competition from mussels and barnacles, and supports a diverse community of invertebrates. The low-tide zone is uncovered only during the lowest tides and supports macroalgae and many species of invertebrates and fishes. The upper limit of each zone is generally determined by tolerance of the species to air exposure and other environmental factors, whereas the lower limit is generally determined by competition with other species.

Tide pools undergo substantial changes in temperature, salinity, and other factors because they are small and isolated from mixing with ocean water for part of the tidal cycle. Because small tide pools tend to have greater changes than large tide pools, they support fewer species, and these species are more tolerant.

Sargasso Sea.

Extensive rafts of Sargassum, a macroalga, float on the surface of the Sargasso Sea, which is the interior of the North Atlantic Gyre. Nutrients are extremely limited in this region, and the large Sargassum biomass develops because it is very long-lived and currents tend to concentrate and retain it within the center of the gyre. A variety of small fish and invertebrate species live in the Sargassum, many of which are unique to this community. The animal biomass is very small in relation to the algal biomass, and there are few grazers because primary productivity is low and any food that is grazed is therefore replaced slowly.

Polar Regions.

Polar regions have extreme seasonal variation in light availability and ice cover, and generally cold surface waters. River runoff or ice exclusion creates strong haloclines in places, but in all other polar waters, vertical mixing due to storms is intense and nutrients are abundant. Microscopic ice algae are important in both polar regions. They grow rapidly during the ice-melting season. Phytoplankton bloom during only a few weeks of summer when sufficient light is available. Many animal species are adapted to the short primary production period and the large year-to-year variability by having a long life span, maturing late, bearing only a few offspring each year, and storing food energy as fat to survive the winter.

Beyond the Sun’s Light.

Below the photic zone, food sources are limited to particulate detritus, carcasses that fall through the water column, and prey species. However, many species migrate vertically from the aphotic zone to the mixed layer, usually at night, to feed. Species that migrate vertically become less common with increasing depth. Many deep-sea animals are adapted to survive on very infrequent meals, and some are able to swallow prey larger than themselves. High pressures inhibit bacterial decomposition, which helps ensure that detrital food particles remain available.

Hydrothermal Vents.

Hydrothermal vents located along oceanic ridge axes and on submerged volcanoes of volcanic island arcs discharge seawater that has percolated through the seafloor and been heated by magma or cooling magmatic rock. The effluent from black smokers, which discharge the hottest water (270 to 380°C), has no oxygen and high concentrations of hydrogen sulfide and metal sulfides. Chemosynthetic bacteria use the sulfides to fuel primary production. Away from the ridge axis vents discharge cooler water and are called white smokers because they discharge much lower concentrations of metal sulfides, and instead contain abundant precipitated white particles of silica, and calcium and magnesium sulfates and carbonates. 

Communities at hydrothermal vents are composed of many species that are unique to these environments. At many Pacific vents, the community closest to the vent is dominated by giant tube worms and clams that feed on chemosynthetic bacteria that they cultivate within their bodies. Various species of other invertebrates occupy zones at different distances from the vent. Most are filter feeders that live on clumps of bacteria and archaea grown at the vent or beneath the seafloor and sloughed off to become suspended particles. Food availability declines rapidly with distance from the vent. Many questions remain about these communities, including how they cross large distances of abyssal ocean to colonize new vents.

White smoker vents lie on the oceanic ridge flank, discharges fluids with little metal sulfide but instead particles rich in calcium and magnesium. Additionally, chemosynthetic communities that use methane or other hydrocarbons as their energy source are present in a few locations in the oceans.