Life on Earth first appeared in an aquatic environment around 3 billion years ago. Over the next 2.6 billion years, life evolved and diversified in the many aquatic environments around the world before moving onto land. Water now covers approximately ¾ of the Earth's surface and aquatic ecosystems are important in sustaining ecosystems on Earth. The oceans are important in regulating climate patterns and the abundance of algae near the surface is important in producing oxygen and removing carbon dioxide from the atmosphere.
Aquatic ecosystems vary in type and size from a small pond to the open oceans where some of the largest mammals are found.
Freshwater ecosystems are found inland and do not have high salt concentrations. There are two classes of freshwater ecosystems:
Lotic: are aquatic ecosystems which have a directional flow of water, and include rivers and streams. The presence of moving water is a challenge for aquatic life because they need to have adaptations that prevent them from being washed downstream.
Lentic: are aquatic ecosystems which are relatively still with no directional movement of water, and include lakes and ponds. Lakes and ponds are classified based on the amount of nutrients found in the system. Oligotrophic lakes have low nutrient concentrations and eutrophic lakes have high nutrient concentrations.
More than 70% of the Earth's surface is covered by oceans. Considering the size of the oceans, very little diversity of life is supported. This is in part due to the deep oceans receiving no sunlight, the low availability of nutrients, and a temperature that remains constant year round at around 6° C. The ocean depths do contain some unique species adapted to the conditions, but biodiversity is quite low deep in the ocean. In comparison, marine ecosystems found in shallow waters are thriving with life and coral reefs are home to a diverse group of organisms.
Aquatic ecosystems are primarily limited by the amount of sunlight penetrating the surface waters. The more sunlight that penetrates the surface, the more plant growth there is to provide food and shelter for the many aquatic organisms ranging from insect larvae to small fish. The sun provides energy for photosynthesis and heat that provides suitable temperatures for many aquatic organisms. Most aquatic life forms are ectotherms, cold blooded organisms whose body temperature varies with the temperature of the environment, including plants, plankton, snails, fish, amphibians, and reptiles.
The temperature also affects the solubility of oxygen as cooler temperatures will have a higher ability to dissolve oxygen in the water. Since oxygen is required by most aquatic animals, the availability of oxygen will limit the species diversity.
Nutrients are important in the production of many internal structures of plants and animals. In calcium (Ca) deficient ecosystems, snails and clams will not be found because the Ca is needed for the development of their shells. As with terrestrial ecosystems, nitrogen is also important and aquatic plants first take up the nitrogen which is then consumed by animals. Nitrogen deficient ecosystems will have a decrease in plant and animal life; these are called oligotrophic ecosystems. Too much nitrogen may also be detrimental to the ecosystem resulting in an overgrowth of algae; these are called eutrophic ecosystems.
The acidity (pH) of aquatic ecosystems is generally between 6 and 9. Acidity will vary depending on the type of rock nearby. For example, lakes near limestone rock will have a higher pH (more basic), volcanoes will produce a lower pH (more acidic), and acid rain may result in lake pH levels dropping to around 4. High acidity (low pH) affects the ability of gills to properly absorb oxygen from the water and only species that can tolerate low pH values will be found.
Oil spills are a problem for ecosystems because the oil interferes with the plumage of birds reducing the insulation ability, which increases the bird's vulnerability to variable temperatures. Oiled birds are also less buoyant and often drown. Ingested oil interferes with kidney and liver function. Similar effects are also observed in marine mammals. Other than the direct effect on animals, the other issue is that since oil covers the surface of the water, less sunlight penetrates thus reducing photosynthesis and plant life. With less plant life, the availability of food and habitat for consumers decreases, as does the oxygen content of the water.
Fertilizers (inorganic soil) were produced during the agricultural revolution to increase agricultural performance; however, runoff of nutrients enriches the surface waters causing eutrophication in which an algal bloom covers the surface preventing sunlight from penetrating the waters.
Pesticides are used to kill any unwanted “pests”. These chemical may include herbicides (weeds), insecticides (insects), and fungicides (fungi). Pesticides were extensively used in the 1950s and 1960s. Rachel Carson first warned of the effects on ecosystems in a book titled “Silent Spring” published in 1962. In the 1970s, when populations of top predatory birds began to decline, controls were immediately put into place and today there are strict guidelines to be followed with the production and use of pesticides.
Aquatic ecosystems across the world have been altered with the introduction of new species (purple loosestrife, zebra mussel, round goby, lamprey) capable of out competing native species. Food webs, species interactions, abiotic features, and the overall composition of ecosystems have been affected as a result.