Anaquatic ecosystem is anecosystem found in and around abody of water, in contrast to land-basedterrestrial ecosystems. Aquatic ecosystems containcommunities oforganisms—aquatic life—that are dependent on each other and on their environment. The two main types of aquatic ecosystems aremarine ecosystems andfreshwater ecosystems.[1] Freshwater ecosystems may belentic (slow moving water, includingpools,ponds, andlakes);lotic (faster moving water, for examplestreams andrivers); andwetlands (areas where the soil is saturated or inundated for at least part of the time).[2]
Alake ecosystem or lacustrine ecosystem includesbiotic (living)plants,animals andmicro-organisms, as well asabiotic (non-living) physical and chemical interactions.[9] Lake ecosystems are a prime example of lentic ecosystems (lentic refers to stationary or relatively stillfreshwater, from theLatinlentus, which means "sluggish"), which includeponds,lakes andwetlands, and much of this article applies to lentic ecosystems in general. Lentic ecosystems can be compared withlotic ecosystems, which involve flowing terrestrial waters such asrivers andstreams. Together, these two ecosystems are examples offreshwater ecosystems.Lentic systems are diverse, ranging from a small, temporary rainwater pool a few inches deep toLake Baikal, which has a maximum depth of 1642 m.[10] The general distinction between pools/ponds and lakes is vague, but Brown[9] states that ponds and pools have their entire bottom surfaces exposed to light, while lakes do not. In addition, some lakes become seasonally stratified. Ponds and pools have two regions: thepelagic open water zone, and thebenthic zone, which comprises the bottom and shore regions. Since lakes have deep bottom regions not exposed to light, these systems have an additional zone, theprofundal.[11] These three areas can have very different abiotic conditions and, hence, host species that are specifically adapted to live there.[9]
Two important subclasses of lakes areponds, which typically are small lakes that intergrade with wetlands, and waterreservoirs. Over long periods of time, lakes, or bays within them, may gradually become enriched by nutrients and slowly fill in with organic sediments, a process called succession. When humans use thedrainage basin, the volumes of sediment entering the lake can accelerate this process. The addition of sediments and nutrients to a lake is known aseutrophication.[12]
River ecosystems are flowing waters that drain the landscape, and include thebiotic (living) interactions amongst plants, animals and micro-organisms, as well asabiotic (nonliving) physical and chemical interactions of its many parts.[13][14] Riverecosystems are part of largerwatershed networks or catchments, where smallerheadwater 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 ofdissolved oxygen, which supports greater biodiversity than the slow-moving water of pools. These distinctions form the basis for the division of rivers intoupland and lowland rivers.
The food base of streams within riparian forests is mostly derived from the trees, but wider streams and those that lack acanopy derive the majority of their food base fromalgae.Anadromous fish are also an important source ofnutrients. Environmental threats to rivers include loss of water, dams, chemical pollution andintroduced species.[15] Adam produces negative effects that continue down the watershed. The most important negative effects are the reduction ofspring flooding, which damageswetlands, and the retention ofsediment, which leads to the loss of deltaic wetlands.[16]Aquatic ecosystems perform many important environmental functions. For example, theyrecycle nutrients, purify water, attenuate floods, recharge ground water and provide habitats for wildlife.[19] 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.[20]
In addition to environmental functions, aquatic ecosystems are also used for human recreation, and are very important to thetourism industry, especially in coastal regions.[21] 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 forecological study.[22]
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 produceaerenchyma 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.[23] There are a growing number of cases where predation by coastal herbivores including snails, geese and mammals appears to be a dominant biotic factor.[24]
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.[25] 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 onhydrogen sulfide in water that comes fromvolcanic vents. Great concentrations of animals that feed on these bacteria are found around volcanic vents. For example, there aregiant tube worms (Riftia pachyptila) 1.5 m in length and clams (Calyptogena magnifica) 30 cm long.[26]
Heterotrophic organisms consume autotrophic organisms and use the organic compounds in their bodies as energy sources and as raw materials to create their ownbiomass.[25]
Euryhaline organisms are salt tolerant and can survive in marine ecosystems, whilestenohaline or salt intolerant species can only live in freshwater environments.[27]
An ecosystem is composed ofbiotic communities that are structured by biological interactions andabiotic environmental factors. Some of the important abiotic environmental factors of aquatic ecosystems include substrate type, water depth, nutrient levels, temperature, salinity, and flow.[23][19] 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.[28] Plants often have to produceaerenchyma, while the shape and size of leaves may also be altered.[29] Conversely, oxygen is fatal to many kinds ofanaerobic bacteria.[25]
Nutrient levels are important in controlling the abundance of many species of algae.[30] The relative abundance of nitrogen and phosphorus can in effect determine which species of algae come to dominate.[31] 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.[32] Similar over-abundance of algae in coastal environments such as the Gulf of Mexico produces, upon decay, a hypoxic region of water known as adead zone.[33]
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 ofwetland (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.[23]
Freshwater used forirrigation purposes often absorbs levels of salt that are harmful to freshwater organisms.[25]
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.[19] 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.[34] 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.[35]
There are many examples of excessive stresses with negative consequences. The environmental history of theGreat Lakes of North America illustrates this problem, particularly how multiple stresses, such aswater pollution, over-harvesting andinvasive species can combine.[32] The Norfolk Broadlands in England illustrate similar decline with pollution and invasive species.[36]Lake Pontchartrain along the Gulf of Mexico illustrates the negative effects of different stresses including levee construction, logging of swamps, invasive species andsalt water intrusion.[37]
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