Ocean Ecosystems: Soft Bottoms
Where water movements are not strong enough to wash them away, sediments coat much of the benthic environment. Soft bottoms are common along coasts, along continental margins, and in the deep sea. Plants and animals that attach to sandy or rocky surfaces are called benthic epifauna; those that bury themselves in soft sediments or bore into the rocky bottom or shells of other animals are called benthic infauna. Few benthic organisms can live in shifting sediments, as on a beach exposed to wave action, and many more are found in sediments in protected bays or estuaries.
The plants of soft bottoms are marine angiosperms, seed-bearing vascular plants with true roots. So that they can photosynthesize, benthic plants live only in the photic zone. Grasses are the only marine plants that live on soft bottoms. They catch sediments and organic matter in their roots, protecting the shoreline from erosion and providing shelter, substrate, and food for a diverse group of animals. Much organic material from these wetland ecosystems is carried offshore, where it provides nutrients for organisms living beneath the photic zone.
The animals found on the shore or in near-shore sediments live primarily on plankton and organic debris from land. Suspension feeders attach themselves to hard or sandy bottoms and strain water for food. Filter feeders are similarly attached but actively pump large amounts of water through their bodies to get food. Many benthic infauna are deposit feeders who eat sediments, extracting the organic matter trapped between the grains. Predators and scavengers on soft bottoms include starfish, snails, cephalopods, and crustaceans. Bacteria are an important protein source and play a major role in decomposition.
The benthic environment is extremely diverse in water depth, temperature, salinity, substrate type, and predation and competition. The most important factor determining the distribution of near-shore benthic infauna is grain size. Large-grained particles, such as sands, are fairly porous. They gain and lose water, gases, and organic material quickly. Filter feeders attach to sands, since smaller sediments are easily swept up by water and can clog the animals' mucus-lined filtration systems. Deposit feeders prefer to live in the top 1 to 2 centimeters of organic-rich, fine-grained mud. This is the aerobic zone, where dissolved oxygen permeates. Below the oxygenated layer is the black, oxygen-depleted, or anaerobic , zone, where only anaerobic bacteria can live fully. Anaerobic bacteria produce hydrogen sulfide, the rotten egg smell of black mud. Some animals, like clams, live in the anaerobic zone to avoid predators but extend siphons into the aerobic zone to obtain food and oxygen.
The deep sea is uniformly cold and dense, and sediment particles are small and relatively uniform in size. The number of benthic species increases from the near shore to the deep ocean but the number of individuals and total biomass decreases. All major groups of shallow water benthos have deep ocean counterparts. But shortage of food causes the deep-sea organisms to be smaller, live longer, and reproduce less frequently. Most deep-sea organisms are deposit feeders with a few conspicuous filter feeders and predators.
Kunzig, Robert. The Restless Sea: Exploring the World Beneath the Waves. New York: W. W. Norton and Company, 1999.
Ricketts, Edward Flanders, et al. Between Pacific Tides, 5th ed. Stanford, CA: Stanford University Press, 1986.