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An aquatic ecosystem is an ecosystem found in and around a body of water, in contrast to land-based terrestrial ecosystems. Aquatic ecosystems contain communities of organisms—aquatic life—that are dependent on each other and on their environment. The two main types of aquatic ecosystems are marine ecosystems and freshwater ecosystems. Freshwater ecosystems may be lentic (slow moving water, including pools, ponds, and lakes); lotic (faster moving water, for example streams and rivers); and wetlands (areas where the soil is saturated or inundated for at least part of the time).

Types

Marine ecosystems

Marine coastal ecosystem

Marine surface ecosystem

Freshwater ecosystems

Lentic ecosystem (lakes)

Lotic ecosystem (rivers)

River ecosystems are flowing waters that drain the landscape, and include the biotic (living) interactions amongst plants, animals and micro-organisms, as well as abiotic (nonliving) physical and chemical interactions of its many parts. River ecosystems are part of larger watershed networks or catchments, where smaller headwater streams drain into mid-size streams, which progressively drain into larger river networks. The major zones in river ecosystems are determined by the river bed's gradient or by the velocity of the current. Faster moving turbulent water typically contains greater concentrations of dissolved oxygen, which supports greater biodiversity than the slow-moving water of pools. These distinctions form the basis for the division of rivers into upland and lowland rivers.

Wetlands

Functions

Aquatic ecosystems perform many important environmental functions. For example, they recycle nutrients, purify water, attenuate floods, recharge ground water and provide habitats for wildlife. The biota of an aquatic ecosystem contribute to its self-purification, most notably microorganisms, phytoplankton, higher plants, invertebrates, fish, bacteria, protists, aquatic fungi, and more. These organisms are actively involved in multiple self-purification processes, including organic matter destruction and water filtration. It is crucial that aquatic ecosystems are reliably self-maintained, as they also provide habitats for species that reside in them.

In addition to environmental functions, aquatic ecosystems are also used for human recreation, and are very important to the tourism industry, especially in coastal regions. They are also used for religious purposes, such as the worshipping of the Jordan River by Christians, and educational purposes, such as the usage of lakes for ecological study.

Biotic characteristics (living components)

The biotic characteristics are mainly determined by the organisms that occur. For example, wetland plants may produce dense canopies that cover large areas of sediment—or snails or geese may graze the vegetation leaving large mud flats. Aquatic environments have relatively low oxygen levels, forcing adaptation by the organisms found there. For example, many wetland plants must produce aerenchyma to carry oxygen to roots. Other biotic characteristics are more subtle and difficult to measure, such as the relative importance of competition, mutualism or predation. There are a growing number of cases where predation by coastal herbivores including snails, geese and mammals appears to be a dominant biotic factor.

Autotrophic organisms

Autotrophic organisms are producers that generate organic compounds from inorganic material. Algae use solar energy to generate biomass from carbon dioxide and are possibly the most important autotrophic organisms in aquatic environments. The more shallow the water, the greater the biomass contribution from rooted and floating vascular plants. These two sources combine to produce the extraordinary production of estuaries and wetlands, as this autotrophic biomass is converted into fish, birds, amphibians and other aquatic species.

Chemosynthetic bacteria are found in benthic marine ecosystems. These organisms are able to feed on hydrogen sulfide in water that comes from volcanic vents. Great concentrations of animals that feed on these bacteria are found around volcanic vents. For example, there are giant tube worms (Riftia pachyptila) 1.5 m in length and clams (Calyptogena magnifica) 30 cm long.

Heterotrophic organisms

Heterotrophic organisms consume autotrophic organisms and use the organic compounds in their bodies as energy sources and as raw materials to create their own biomass.

Euryhaline organisms are salt tolerant and can survive in marine ecosystems, while stenohaline or salt intolerant species can only live in freshwater environments.

Abiotic characteristics (non-living components)

An ecosystem is composed of biotic communities that are structured by biological interactions and abiotic environmental factors. Some of the important abiotic environmental factors of aquatic ecosystems include substrate type, water depth, nutrient levels, temperature, salinity, and flow. It is often difficult to determine the relative importance of these factors without rather large experiments. There may be complicated feedback loops. For example, sediment may determine the presence of aquatic plants, but aquatic plants may also trap sediment, and add to the sediment through peat.

The amount of dissolved oxygen in a water body is frequently the key substance in determining the extent and kinds of organic life in the water body. Fish need dissolved oxygen to survive, although their tolerance to low oxygen varies among species; in extreme cases of low oxygen, some fish even resort to air gulping. Plants often have to produce aerenchyma, while the shape and size of leaves may also be altered. Conversely, oxygen is fatal to many kinds of anaerobic bacteria.

Nutrient levels are important in controlling the abundance of many species of algae. The relative abundance of nitrogen and phosphorus can in effect determine which species of algae come to dominate. Algae are a very important source of food for aquatic life, but at the same time, if they become over-abundant, they can cause declines in fish when they decay. Similar over-abundance of algae in coastal environments such as the Gulf of Mexico produces, upon decay, a hypoxic region of water known as a dead zone.

The salinity of the water body is also a determining factor in the kinds of species found in the water body. Organisms in marine ecosystems tolerate salinity, while many freshwater organisms are intolerant of salt. The degree of salinity in an estuary or delta is an important control upon the type of wetland (fresh, intermediate, or brackish), and the associated animal species. Dams built upstream may reduce spring flooding, and reduce sediment accretion, and may therefore lead to saltwater intrusion in coastal wetlands.

Freshwater used for irrigation purposes often absorbs levels of salt that are harmful to freshwater organisms.

Threats

The health of an aquatic ecosystem is degraded when the ecosystem's ability to absorb a stress has been exceeded. A stress on an aquatic ecosystem can be a result of physical, chemical or biological alterations to the environment. Physical alterations include changes in water temperature, water flow and light availability. Chemical alterations include changes in the loading rates of biostimulatory nutrients, oxygen-consuming materials, and toxins. Biological alterations include over-harvesting of commercial species and the introduction of exotic species. Human populations can impose excessive stresses on aquatic ecosystems. Climate change driven by anthropogenic activities can harm aquatic ecosystems by disrupting current distribution patterns of plants and animals. It has negatively impacted deep sea biodiversity, coastal fish diversity, crustaceans, coral reefs, and other biotic components of these ecosystems. Human-made aquatic ecosystems, such as ditches, aquaculture ponds, and irrigation channels, may also cause harm to naturally occurring ecosystems by trading off biodiversity with their intended purposes. For instance, ditches are primarily used for drainage, but their presence also negatively affects biodiversity.

There are many examples of excessive stresses with negative consequences. The environmental history of the Great Lakes of North America illustrates this problem, particularly how multiple stresses, such as water pollution, over-harvesting and invasive species can combine. The Norfolk Broadlands in England illustrate similar decline with pollution and invasive species. Lake Pontchartrain along the Gulf of Mexico illustrates the negative effects of different stresses including levee construction, logging of swamps, invasive species and salt water intrusion.

See also

• Aquatic plant – Plant that has adapted to living in an aquatic environment
• Hydrobiology – Science of life and life processes in water
• Hydrosphere – Total amount of water on a planet
• Limnology – Science of inland aquatic ecosystems
• Ocean
• Stephen Alfred Forbes – American naturalist - one of the founders of aquatic ecosystem science
• Stream metabolism

References

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v t e Aquatic ecosystems
General components and freshwater ecosystems General Acoustic ecology Algal bloom Anoxic waters Aquatic adaptation Aquatic animal Insect Mammal Water bird Aquatic biomonitoring Aquatic plant Aquatic population dynamics Aquatic predation Aquatic respiration Aquatic science Aquatic toxicology Benthos Bioluminescence Biomass Cascade effect Colored dissolved organic matter Dead zone Ecohydrology Eutrophication Fisheries science Food chain Food web GIS and aquatic science Hydrobiology Hypoxia Macrobenthos Meiobenthos Microbial ecology Microbial food web Microbial loop Nekton Neuston Particle Pelagic zone Photic zone Phytoplankton Plankton Pleuston Productivity Ramsar Convention Sediment trap Shoaling and schooling Siltation Spawn Stable isotope analysis in aquatic ecosystems Substrate Thermal pollution Trophic level Underwater camouflage Water column Zooplankton Freshwater Freshwater biology Freshwater biome Freshwater environmental quality parameters Freshwater fish Hyporheic zone Limnology Lake ecosystem Lake stratification Macrophyte Pond Fish pond Rheotaxis River ecosystem Stream bed Stream pool Trophic state index Upland and lowland Water garden Wetland Bog Brackish marsh Fen Freshwater marsh Freshwater swamp forest Ecoregions List of freshwater ecoregions (WWF) Africa and Madagascar Latin America and the Caribbean List of marine ecoregions Specific examples Everglades Maharashtra North Pacific Subtropical Gyre San Francisco Estuary
Marine ecosystems General Deep scattering layer Diel vertical migration f-ratio Iron fertilization Large marine ecosystem Marine biology Marine chemistry Marine food web Marine primary production Marine snow Ocean fertilization Oceanic physical-biological process Ocean turbidity Photophore Thorson's rule Upwelling Viral shunt Whale fall Marine life Census of Marine Life Deep-sea community Deep-water coral Marine fungi Marine invertebrates Marine larval ecology Seagrass Seashore wildlife Wild fisheries Microorganisms Marine bacteriophage Marine prokaryotes Marine protists Marine viruses Paradox of the plankton Vertebrates Marine mammal Marine reptile Saltwater fish Coastal fish Coral reef fish Deep-sea fish Demersal fish Pelagic fish Seabird Marine habitats Bay mud Marine coastal ecosystem Coastal biogeomorphology Cold seep Coral reef Davidson Seamount § Ecology Estuary Intertidal ecology Intertidal wetland Kelp forest Hydrothermal vent Lagoon Mangrove Marine biomes Mudflat Oyster reef Rocky shore Salt marsh Salt pannes and pools Seagrass meadow Sponge ground Sponge reef Tide pool Conservation Coral bleaching Ecological values of mangroves Fisheries and climate change HERMIONE Human impact on marine life Marine conservation activism Marine pollution Marine protected area
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v t e Ponds, pools, and puddles
Ponds	Ash pond Balancing lake Ballast pond Beel Cooling pond Detention pond Dew pond Evaporation pond Facultative lagoon Garden pond Ice pond Immersion pond Infiltration basin Kettle pond Log pond Melt pond Mill pond Polishing pond Raceway pond Retention pond Sag pond Salt evaporation pond Sediment pond Settling pond Solar pond Stepwell Stew pond Tailings Tarn Waste pond Waste stabilization pond
Pools	Anchialine pool Brine pool Infinity pool Natural pool Ocean pool Plunge pool Reflecting pool Spent fuel pool Stream pool Swimming pool Tide pool Vernal pool
Puddles	Bird bath Coffee ring effect Puddle Puddles on a surface Seep puddle
Biome	Beaver dam Duck pond Fish pond Goldfish pond Koi pond
Ecosystems	Aquatic ecosystem Freshwater ecosystem Lake ecosystem
Related	Aerated lagoon Bakki shower Big fish–little pond Body of water Constructed wetland Full pond Hydric soil Phytotelma Pond of Abundance Pond liner Ponding Puddle (M C Escher) Spring Swimming hole Water aeration Water garden Water Lilies (Monet) Well

• Aquatic ecology
• Ecosystems
• Aquatic plants
• Fisheries science
• Systems ecology
• Water