Aquatic Biomes | (Taken fromThe Online Biology Book is hosted by Estrella Mountain Community College, in sunny Avondale, Arizona.- http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookTOC.html)

Conditions in water are generally less harsh than those on land. Aquatic organisms are buoyed by water support, and do not usually have to deal with desiccation. Despite covering 71% of the Earth's surface, areas of the open ocean are a vast aquatic desert containing few nutrients and very little life. Clearcut biome distinctions in water, like those on land, are difficult to make. Dissolved nutrients controls many local aquatic distributions. Aquatic communities are classified into: freshwater (inland) communities and marine (saltwater or oceanic) communities.

Species diversity and salt concentration. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

The Marine Biome

The marine biome contains more dissolved minerals than the freshwater biome. Over 70% of the Earth's surface is covered in water, by far the vast majority of that being saltwater. There are two basic categories to this biome: benthic and pelagic. Benthic communities (bottom dwellers) are subdivided by depth: the shore/shelf and deep sea. Pelagic communities (swimmers or floaters suspended in the water column) include planktonic (floating) and nektonic (swimming) organisms. The upper 200 meters of the water column is the euphotic zone to which light can penetrate.

Coastal Communities

Estuaries are bays where rivers empty into the sea. Erosion brings down nutrients and tides wash in salt water; forms nutrient trap. Estuaries have high production for organisms that can tolerate changing salinity. Called "nurseries of the sea" because many young marine fish develop in this protected environment.

Seashores

Rocky shorelines offer anchorage for sessile organisms. Seaweeds are main photosynthesizers and use holdfasts to anchor. Barnacles glue themselves to stone. Oysters and mussels attach themselves by threads. Limpets and periwinkles either hide in crevices or fasten flat to rocks.

Sandy beaches and shores are shifting strata. Permanent residents therefore burrow underground. Worms live permanently in tubes. Amphipods and ghost crabs burrow above high tide and feed at night.

Coral Reefs

Areas of biological abundance in shallow, warm tropical waters. Stony corals have calcium carbonate exoskeleton and may include algae. Most form colonies; may associate with zooxanthellae dinoflagellates. Reef is densely populated with animal life. The Great Barrier Reef of Australia suffers from heavy predation by crown-of-thorns sea star, perhaps because humans have harvested its predator, the giant triton.

Oceans

Oceans cover about three-quarters of the Earth's surface. Oceanic organisms are placed in either pelagic (open water) or benthic (ocean floor) categories. Pelagic division is divided into neritic and three levels of pelagic provinces. Neritic province has greater concentration of organisms because sunlight penetrates; nutrients are found here. Epipelagic zone is brightly lit, has much photosynthetic phytoplankton, that support zooplankton that are food for fish, squid, dolphins, and whales. Mesopelagic zone is semi-dark and contains carnivores; adapted organisms tend to be translucent, red colored, or luminescent; for example: shrimps, squids, lantern and hatchet fishes. Bathypelagic zone is completely dark and largest in size; it has strange-looking fish. Benthic division includes organisms on continental shelf (sublittoral), continental slope (bathyal), and the abyssal plain.

Sublittoral zone harbors seaweed that becomes sparse where deeper; most dependent on slow rain of plankton and detritus from sunlit water above. Bathyal zone continues with thinning of sublittoral organisms. Abyssal zone is mainly animals at soil-water interface of dark abyssal plain; in spite of high pressure, darkness and coldness, many invertebrates thrive here among sea urchins and tubeworms.

Thermal vents along oceanic ridges form a very unique community. Molten magma heats seawater to 350^oC, reacting with sulfate to form hydrogen sulfide (H₂S). Chemosynthetic bacteria obtain energy by oxidizing hydrogen sulfide. The resulting food chain supports a community of tubeworms and clams.

Zones within the marine biome. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

The Freshwater Biome

The freshwater biome is subdivided into two zones: running waters and standing waters. Larger bodies of freshwater are less prone to stratification (where oxygen decreases with depth). The upper layers have abundant oxygen, the lowermost layers are oxygen-poor. Mixing between upper and lower layers in a pond or lake occurs during seasonal changes known as spring and fall overturn.

Lakes are larger than ponds, and are stratified in summer and winter. The epilimnion is the upper surface layer. It is warm in summer. The hypolimnion is the cold lower layer. A sudden drop in temperature occurs at the middle of the thermocline. Layering prevents mixing between the lower hypolimnion (rich in nutrients) and the upper epilimnion (which has oxygen absorbed from its surface). The epilimnion warms in spring and cools in fall, causing a temporary mixing. As a consequence, phytoplankton become more abundant due to the increased amounts of nutrients.

Lake overturn. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Life zones also exist in lakes and ponds. The littoral zone is closest to shore. The limnetic zone is the sunlit body of the lake. Below the level of sunlight penetration is the dark profundal zone. At the soil-water interface we find the benthic zone. The term benthos is applied to animals that live on the bottom.

Rapidly flowing, bubbling streams have insects and fish adapted to oxygen-laden water. Slow moving streams have aquatic life more similar to lake and pond life.

Community Density and Stability | Back to Top

Communities are made up of species adapted to the conditions of that community. Diversity and stability help define a community and are important in environmental studies. Species diversity decreases as we move away from the tropics. Species diversity is a measure of the different types of organisms in a community (also referred to as species richness). Latitudinal diversity gradient refers to species richness decreasing steadily going away from the equator. A hectare of tropical rain forest contains 40-100 tree species, while a hectare of temperate zone forest contains 10-30 tree species. In marked contrast, a hectare of taiga contains only a paltry 1-5 species. Habitat destruction in tropical countries will cause many more extinctions per hectare than it would in higher latitudes.

Environmental stability is greater in tropical areas, where a relatively stable/constant environment allows more different kinds of species to thrive. Equatorial communities are older because they have been less disturbed by glaciers and other climate changes, allowing time for new species to evolve. Equatorial areas also have a longer growing season.

The depth diversity gradient is found in aquatic communities. Increasing species richness with increasing water depth. This gradient is established by environmental stability and the increasing availability of nutrients.

Community stability refers to the ability of communities to remain unchanged over time. During the 1950s and 1960s, stability was equated to diversity: diverse communities were also stable communities. Mathematical modeling during the 1970s showed that increased diversity can actually increase interdependence among species and lead to a cascade effect when a keystone species is removed. Thus, the relation is more complex than previously thought.