14.2: Habitats

The major habitats available to marine organisms are in the water column, on the seafloor surface, and in seafloor sediment (Chap. 12). For photosynthetic organisms (CC14), only the first two habitats are suitable, and only within the photic zone. Consumer and decomposer species are present in each of the three habitats: pelagic species in the water column, benthic epifauna and epiflora on the seafloor, and benthic infauna within the sediment.

The three habitats offer different advantages and challenges. Each species has selected its own unique balance between those opportunities and challenges. Many species take advantage of different balances at successive stages of their life cycle by changing their habitat between stages.

In shallow water, photosynthesis may take place at the seafloor if it is within the photic zone, and chemosynthetic communities are present at least in restricted areas of the seafloor below the photic zone. In both locations, primary production by benthic organisms creates some food. This primary production can be substantial locally, particularly in coastal waters and at hydrothermal vents, but in most of the ocean the food supply is produced by photosynthesis by phytoplankton that include algae and cyanobacteria. Some archaea utilize light energy to supplement their energy needs, so they are phototrophs, but they use a mechanism other than photosynthesis and may use simple organic compounds instead of carbon dioxide. In the deep ocean, nutrient depleted, high oxygen waters and deep sea sediments, certain species of archaea are the dominant “primary producers” (Chap. 12, CC14).

Pelagic Habitat

Phytoplankton and other phototrophic microorganisms in the pelagic environment are effectively grazed by zooplankton and single-celled protists that, in turn, are eaten by carnivores or omnivores. During this process, most of the biomass produced by primary producers is used by photic-zone pelagic consumers to fuel their energy needs (Chap. 12). Consequently, only a small fraction of primary producer biomass eventually sinks or is transported below the photic zone, and food is scarce in the environments below the photic zone.

Most pelagic organisms must actively seek food and avoid or fight off predators at the same time. Whatever strategy an organism uses for these activities, it uses up energy that must be obtained from food. Hence, one penalty for living in the pelagic habitat is that more food must be consumed than is needed for growth and reproduction. The need to avoid sinking also carries an energy penalty for pelagic organisms. Buoyancy can be maintained by swimming, which uses energy directly, or with pockets of gas or low-density oil in the body structure, each of which requires energy to produce.

All pelagic organisms must evade predators, compete for food, and counteract the tendency to sink. Below the photic zone, predators are less abundant and easier to avoid because they cannot hunt visually in the darkness. Offsetting this advantage is the scarcity of food below the photic zone. Many species, especially of zooplankton and squid, migrate between the photic zone and the aphotic zone to take advantage of this trade-off between the zones. These organisms feed in the photic zone at night and then descend to the darkness below in the day, in a strategy known as diel vertical migration. Nevertheless, even this behavior has an energy cost, because energy must be expended to move vertically between layers.

Benthic Epifaunal Habitat

Organisms that live on the seafloor do not need to expend energy to control buoyancy and generally require less energy to find and capture food than pelagic organisms require. Hunting is reduced to a two-dimensional problem, and because many benthic prey are sedentary or nearly so, hunting movement can be slow and energy-efficient. Even more energy is saved by benthos that remain sedentary and wait for prey to walk, swim, or drift by.

The energy savings of living in the benthic epifaunal environment may be offset by the scarcity of food and the difficulty of evading predators. Organisms that move slowly and sit on the sediment may be easy to find and capture. Hence, many benthic epifaunal species have developed defensive mechanisms such as poisons, spines, and shells or tubes into which they withdraw. Such mechanisms require energy that otherwise could be used for growth and reproduction. Where food is readily available, epifauna may be abundant and the competition for available space may be a significant disadvantage. Finding living space is a problem, particularly for epifauna that attach to rocks, because most of the seafloor is covered by sediment and rocky substrates are very limited.

Although the benthic epifaunal environment in shallow water is similar to that in deeper water, benthic algae enhance the food supply in many shallow areas. Coral reefs sustain exceptionally diverse (CC17) and abundant encrusting algae communities. Also, in some shallow areas the kelp in kelp forests (Fig. 15-8) provides abundant hiding places for predator avoidance. 

Benthic Infaunal Habitat

Even though food is scarce in most of the benthic infaunal environment, some organic detritus accumulates continuously in sediments. Many animals, such as worms and certain crustaceans, feed by sifting sediment grains to obtain organic particles or by digesting organic matter as sediment is passed through the gut. Other benthic infaunal species live in the sediment primarily for protection from predators and feed on particles in the water above or prey on passing animals.

The relatively poor supply of food available to benthic infauna is offset by the energy savings of relatively sedentary lifestyles, by the lack of need for buoyancy control, and by the reduced need for defenses against predators. In estuarine and other coastal environments, much of the salinity and temperature variation that can occur in overlying waters does not occur in the sediment, so infauna are not subject to these environmental stresses. Intertidal benthic infauna also avoid exposure to air during low tides.

Benthic infauna must expend energy to dig into or move through sediments or rock. They must also cope with an environment that varies because of biochemical and chemical processes within the substrate. The most important of these processes consume dissolved oxygen and produce toxic hydrogen sulfide, so most infaunal species can live only in the oxygenated surface layer of sediment. In areas where detritus inputs are high and water movements slow, the oxygenated layer is thin or absent, but elsewhere it ranges from a few millimeters to several meters in thickness. Where sulfide is present, the biota consists primarily of bacteria, archaea, and fungi adapted to the sulfide environment and animals that build burrows or tubes through which they can obtain oxygenated water from above.

Other Ocean Habitats

Special challenges and opportunities are also found in other ocean habitats, including the surface microlayer, the intertidal zone, and hydrothermal vents. The surface microlayer, which is only a few molecules thick, concentrates a variety of natural and contaminant organic compounds. A few species attach themselves to the surface microlayer to keep from sinking. Surface tension tends to prevent small particles within the microlayer from sinking, even if their density is higher than that of seawater. Many species distribute their eggs and larvae by placing them in the surface microlayer, where currents distribute them until they grow large enough to feed and swim independently. Placement in the surface microlayer reduces the need to use energy to provide the eggs and larvae with oils or other means of buoyancy control.

Although the intertidal zone and hydrothermal vents are sites of very high primary productivity and food is abundant, each of these habitats imposes substantial offsetting challenges due to variations in environmental conditions. These two unique environments are discussed in Chapter 15.