Aquatic life | Wikipedia audio article

An aquatic ecosystem is an ecosystem in a
body of water. Communities of organisms that are dependent
on each other and on their environment live in aquatic ecosystems. The two main types of aquatic ecosystems are
marine ecosystems and freshwater ecosystems.==Types=====Marine===Marine ecosystems, the largest of all ecosystems,
cover approximately 71% of the Earth’s surface and contain approximately 97% of the planet’s
water. They generate 32% of the world’s net primary
production. They are distinguished from freshwater ecosystems
by the presence of dissolved compounds, especially salts, in the water. Approximately 85% of the dissolved materials
in seawater are sodium and chlorine. Seawater has an average salinity of 35 parts
per thousand of water. Actual salinity varies among different marine
ecosystems. Marine ecosystems can be divided into many
zones depending upon water depth and shoreline features. The oceanic zone is the vast open part of
the ocean where animals such as whales, sharks, and tuna live. The benthic zone consists of substrates below
water where many invertebrates live. The intertidal zone is the area between high
and low tides; in this figure it is termed the littoral zone. Other near-shore (neritic) zones can include
estuaries, salt marshes, coral reefs, lagoons and mangrove swamps. In the deep water, hydrothermal vents may
occur where chemosynthetic sulfur bacteria form the base of the food web. Classes of organisms found in marine ecosystems
include brown algae, dinoflagellates, corals, cephalopods, echinoderms, and sharks. Fishes caught in marine ecosystems are the
biggest source of commercial foods obtained from wild populations.Environmental problems
concerning marine ecosystems include unsustainable exploitation of marine resources (for example
overfishing of certain species), marine pollution, climate change, and building on coastal areas.===Freshwater===Freshwater ecosystems cover 0.78% of the Earth’s
surface and inhabit 0.009% of its total water. They generate nearly 3% of its net primary
production. Freshwater ecosystems contain 41% of the world’s
known fish species.There are three basic types of freshwater ecosystems: Lentic: slow moving water, including pools,
ponds, and lakes. Lotic: faster moving water, for example streams
and rivers. Wetlands: areas where the soil is saturated
or inundated for at least part of the time.===Lentic===Lake ecosystems can be divided into zones. One common system divides lakes into three
zones (see figure). The first, the littoral zone, is the shallow
zone near the shore. This is where rooted wetland plants occur. The offshore is divided into two further zones,
an open water zone and a deep water zone. In the open water zone (or photic zone) sunlight
supports photosynthetic algae, and the species that feed upon them. In the deep water zone, sunlight is not available
and the food web is based on detritus entering from the littoral and photic zones. Some systems use other names. The off shore areas may be called the pelagic
zone, the photic zone may be called the limnetic zone and the aphotic zone may be called the
profundal zone. Inland from the littoral zone one can also
frequently identify a riparian zone which has plants still affected by the presence
of the lake—this can include effects from windfalls, spring flooding, and winter ice
damage. The production of the lake as a whole is the
result of production from plants growing in the littoral zone, combined with production
from plankton growing in the open water. Wetlands can be part of the lentic system,
as they form naturally along most lake shores, the width of the wetland and littoral zone
being dependent upon the slope of the shoreline and the amount of natural change in water
levels, within and among years. Often dead trees accumulate in this zone,
either from windfalls on the shore or logs transported to the site during floods. This woody debris provides important habitat
for fish and nesting birds, as well as protecting shorelines from erosion. Two important subclasses of lakes are ponds,
which typically are small lakes that intergrade with wetlands, and water reservoirs. 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 the watershed, the volumes
of sediment entering the lake can accelerate this process. The addition of sediments and nutrients to
a lake is known as eutrophication.====Ponds====
Ponds are small bodies of freshwater with shallow and still water, marsh, and aquatic
plants. They can be further divided into four zones:
vegetation zone, open water, bottom mud and surface film. The size and depth of ponds often varies greatly
with the time of year; many ponds are produced by spring flooding from rivers. Food webs are based both on free-floating
algae and upon aquatic plants. There is usually a diverse array of aquatic
life, with a few examples including algae, snails, fish, beetles, water bugs, frogs,
turtles, otters and muskrats. Top predators may include large fish, herons,
or alligators. Since fish are a major predator upon amphibian
larvae, ponds that dry up each year, thereby killing resident fish, provide important refugia
for amphibian breeding. Ponds that dry up completely each year are
often known as vernal pools. Some ponds are produced by animal activity,
including alligator holes and beaver ponds, and these add important diversity to landscapes.===Lotic===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. The food base of streams within riparian forests
is mostly derived from the trees, but wider streams and those that lack a canopy derive
the majority of their food base from algae. Anadromous fish are also an important source
of nutrients. Environmental threats to rivers include loss
of water, dams, chemical pollution and introduced species. A dam produces negative effects that continue
down the watershed. The most important negative effects are the
reduction of spring flooding, which damages wetlands, and the retention of sediment, which
leads to loss of deltaic wetlands.===Wetlands===
Wetlands are dominated by vascular plants that have adapted to saturated soil. There are four main types of wetlands: swamp,
marsh, fen and bog (both fens and bogs are types of mire). Wetlands are the most productive natural ecosystems
in the world because of the proximity of water and soil. Hence they support large numbers of plant
and animal species. Due to their productivity, wetlands are often
converted into dry land with dykes and drains and used for agricultural purposes. The construction of dykes, and dams, has negative
consequences for individual wetlands and entire watersheds. Their closeness to lakes and rivers means
that they are often developed for human settlement. Once settlements are constructed and protected
by dykes, the settlements then become vulnerable to land subsidence and ever increasing risk
of flooding. The Louisiana coast around New Orleans is
a well-known example; the Danube Delta in Europe is another.==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. Aquatic ecosystems are also used for human
recreation, and are very important to the tourism industry, especially in coastal regions.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 of 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. There are many examples of excessive stresses
with negative consequences. Consider three. 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.==Abiotic characteristics==
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.==Biotic characteristics==
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.==See also==Aquatic plant
Freshwater Hydrobiology
Limnology Marine ecosystem
Stephen Alfred Forbes – one of the founders of aquatic ecosystem science
Stream metabolism Terrestrial ecosystem==Notes

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