Marine Communities

Several marine communities are important or interesting enough to merit special attention.

Estuaries. Estuaries have high productivity because they receive a constant nutrient input from rivers, and tidal motions provide an energy subsidy that improves oxygen transfer

(b)

Figure 15.28 Adaptation of two species of the copepod genus Oithona to viscosity differences due to temperature: (a) warm-water species; (b) cold-water species. (From Garrison, 1993. # Wadsworth Publishing Co. Used with permission.)

and mass transfer of nutrients from the sediment. They receive detritus inputs from adjacent salt marshes. The organisms that populate estuaries are similar to those already described for salt marshes.

Rocky Intertidal Communities. This group constitute one type of littoral zone; others include the sand beach and the salt marsh. The wave energy that strikes these ecosystems would seem to make it an inhospitable place. Besides the crushing wave energy, they are subjected to sharp swings in moisture and temperature. In fact, they are highly productive ecosystems and have one of the highest species richness and diversities of any marine ecosystem. For this reason, and also because they are among the most accessible of the high-richness ecosystems, they are among the most interesting. There are few other environments where the casual observer could find so many different animals in one place.

Several reasons explain why. The wave energy provides mixing, gas transfer, and flushing of wastes. It also eliminates competition from species not specially adapted for this environment. The tides add to the energy subsidy. Detritus is brought in by the tides. Tides naturally create a range of habitats, classified by the fraction of the time that each is exposed. Areas near the low-tide mark are submerged almost all the time, high-tide areas only briefly. Since the intertidal zone spends time both above and below the water, organisms there are subjected to attack by predators from the land and the sea. Heavy predation is one of the factors that contributes directly to species diversity. Finally, the rocky intertidal environment itself provides many diverse habitats and niches: tide pools, crevices, exposed rock, and gravel beds.

Algae encrust rocks or grow in the form of large, tough, elastic, and slippery plants, such as kelp, to resist wave energy. Sessile filter-feeding animals such as barnacles and mussels attach themselves to rocks. Snails scrape algae from the rocks. Clams, octopi, starfish, crabs, sea urchins, anemones, sponges, shrimp, fish, and shorebirds are all present.

Coral Reefs. Coral reefs are unique and important communities formed from colonial types of coral. Corals are cnidarians with a polyp body plan that sits in a cuplike exoske-leton made of calcium carbonate. The opening at the top of the polyp has tentacles with stinging cells for capturing prey. The outer layer of the coral tissue has embedded within it thousands of dinoflagellates called zooxanthellae. There may be as many as 30,000 cells per milliliter of coral tissue, up to 75% by mass, and they provide the bright and varied colors for which coral reefs are famous. The coral and the zooxanthellae exist in a symbiotic relationship in which the coral provide shelter and nutrients and the algae provide food and removal of the coral's waste products (which are nutrients to them). The zooxanthellae also seem to give the coral its ability to secrete large amounts of calcium carbonate. The coral-algae association seems a most intimate form of nutrient recycling, which must be essential since coral tends to grow in nutrient-depleted waters.

Coral grows on a solid substrate. When the coral dies, the exoskeleton remains, and new organisms can grow on top of this. Over many hundreds and thousands of years the skeletons accumulate, forming reef structures. Coral can only grow in warm, shallow waters. If the land subsides or sea level increases (as has been happening over the last dozen millennia or so), the reef grows to maintain its position relative to the surface, forming offshore barrier reefs or, where a central island has become submerged, a ring-shaped atoll with a lagoon. Reef-forming coral grows almost exclusively in waters with a seasonal temperature minimum above 20°C.

The massive reef structure creates numerous ecological niches. The zooxanthellae, plus filamentous green algae and other algae, form the base of the coral reef food pyramid (Figure 15.29). The corals themselves are primary consumers, as well as fish, clams, sea urchins, crustaceans, and other invertebrates. Secondary consumers include sea urchins, sea anemones, sea stars, and fish. The top carnivores are eels, octopi, and barracuda. The animal diversity of coral reefs is the highest of all marine communities. Despite their high productivity, coral reefs do not support large fisheries. It is thought that high predation rates limit production at each level of the food chain. The overall effect is a low transfer efficiency.

Coral reefs will grow at depths no greater than 150 m, where light levels in the clear water they require falls below 4% of the surface levels. Too high a nutrient level hurts coral because other benthic plants and suspension feeders such as clams will outcompete them. Higher nutrient levels also brings increased plankton concentrations and resulting turbidity, which also inhibits coral growth.

Abyssalpelagic and Abyssal Benthos. With the exception of hot vents described below, the deep-sea bottom is sparsely populated. While the nearshore sediments may have 5000 g of biomass per square meter and the continental shelf could have 200, the abyssal

Figure 15.29 Food web of a coral reef with a biomass budget. B is average annual biomass in kg/ km2; P is the production in kg/m2 ■ yr. (Based on Barnes and Mann, 1991; original source is R. W., Grigg, J. J. Polovina, and M. J. Atkinson, 1984, Coral Reefs, Vol. 3, pp. 32-37).

benthos typically has less than 1 mg. Furthermore, the deeper it is, the less biomass there will be. This is because it is entirely a detritus-based food web, the energy input comes from sedimentation from the euphotic zone, and this input has increasing chances of being intercepted by pelagic organisms on its way down.

The environment is permanently dark, cold, and currents are weak. Adaptations include low rates of metabolism and growth, and enzymes that are specially optimized to function at the high pressures found on the ocean floor. Most organisms are blind, but some, such as the lanternfish, use bioluminescence (the biological production of light) to find and lure prey. All the animals are predators or scavengers. There are no plants, so there are no herbivores.

Mesopelagic fish have swim bladders, which makes them difficult to harvest without rupturing them and killing the fish. Fish from below 1000 m depth typically lack swim bladders. They and other organisms from this depth can survive in aquaria at the surface if the temperature and other physical factors are favorable. Some fish, such as the lantern-fish, migrate diurnally to the euphotic zone at night to feed, then descend to between 700 and 900 m during the day. More than 2000 species of animals live in the aphotic zone, including copepods, ostracods, jellyfish, prawns, mysids, amphipods, swimming worms, and a group of strange-looking fish. The fish are typically small but have huge mouths. Since prey are scarce, they need not to have to reject larger victims.

Feeding strategies of deep-sea creatures are often bizarre. Some bury themselves in the sediment with their mouths open at the surface. Other creatures mistake them for caves and crawl in for shelter, only to be forced to crawl right into the stomach by downward-projecting spines. Others can smell dead organisms from kilometers away, then spend weeks crawling to the source of the scent. Some organisms feed less than once per year and live for hundreds of years.

Surprisingly, species diversity at the bottom of the sea is very high. This may be explained by the principle of competitive exclusion, which states that high predation pressure prevents any single species from dominating.

Hot Vent Communities. In 1977, scientists in the submersible Alvin discovered a new ecosystem northeast of the Galapagos Islands while searching for a source of heated water at a depth of 3000 m. There, at the rift where two of the tectonic plates that form Earth's crust are spreading apart, hydrothermal vents were spewing water at 350°C, laden with dissolved minerals, including H2S.

But the astonishing thing about the vents were the abundance of benthic invertebrates surrounding them, many of them huge in size and previously unknown to science. Besides large crabs, clams, shrimp, and anemones, there were huge "tube worms,'' contained in parchmentlike tubes the diameter of a human arm and about 3 to 4 m. Three species have been found so far, forming a new genus called Riftia, in the phylum Pogonophora. Altogether, about 100 species live in vent communities.

The vent community had no plants and was too dense to subsist on detritus from above. What was the source of the energy to maintain it? Furthermore, the worms had no mouth, digestive tract, or anus. Instead, the worms and some clams contained "feeding bodies'' packed with chemoautotrophic bacteria. The worms would absorb hydrogen sulfide from the water and transport it to the feeding bodies. These would then produce the carbohydrates to sustain the ecosystem. Thus, this unique ecosystem is based not on the energy from sunlight, as is every other ecosystem on Earth, but on geochemical energy from deep within the Earth itself.

Hot vent communities have been discovered in seas throughout the world, including sites on the mid-Atlantic ridge, the Gulf of California, offshore from the state of Washington, and near Okinawa. Similar communities have been found where hydrogen sulfiderich water seeps from the base of the continental shelf off Florida, Oregon, and Japan. These communities also have tube worms. In a third type of deep ocean chemoautotrophic ecosystem, sites were found in the Gulf of Mexico and Baffin Bay, where ecosystems subsist on hydrogen sulfide and methane seeping from natural hydrocarbon deposits.

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