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Contents

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	Introduction
	The Coastal Environment
	The Open Ocean Environment
	Marine Communit= ies and Human Activities
	Further Reading=
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Figures

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	Figure 1=
	Figure 2=
	Figure 3=
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Tables

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	Table 1<= /a>
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How to c= ite

Permissi= ons

= Marine Communities

Paul= = Snelgrove Memorial University<= /span> of Newfoundlan= d, St JohnR= 17;s, Canada

3DIntroductory <= /span>

Introductory

= doi:10.1038/npg.els.0003175

Marine comm= unities are collections of plants and animals within an area of the ocean that interact with one another more than with other such collections.

Introductio= n=

Marine communities are broadly grouped into neritic communities, or those that occur between the edge of the continental shelf and the land–sea border, and pelagic communitie= s, or those that occur in the open ocean beyond the edge of the continental shelf. For the most part these terms are used in reference to the water column between the ocean floor and the atmosphere (Figure 1). Organisms that live on the bottom are referred to as benthos (e.g. mussels, seastars), whereas those that live above the bottom in the water column are called plankton if they cannot make significant headway against currents (e.g. jellyfish, fish eggs) or nekton (e.g. whales, fishes) if they can make significant headway (Figure 2).

Figure 1
Schematic representation of the ocean floor where scales are greatly exaggerated and not drawn to proportion (particularly the horizontal). See text for explanation of terms. ...

Figure 2
Some of the organisms commonly found in marine systems. A and B are planktonic whereas C–F are benthic. A is a copepod, a type of crustacean that is often very abundant in surface waters; B is a small jellyfish, which can also be very numerous in surface waters; C is a cumacean; D is a polychaete; E is a sipunculid; and F is a nematode. = All of these benthic taxa live in bottom sediments. For ...

The Coastal Environment

The rocky intertidal

Rocky intertidal environments are alternately exposed to air during low tide and flooded with seawater du= ring high tide, and are therefore relatively harsh environments in terms of = the physiological adaptations that organisms must possess in order to survi= ve. Exposure to air can result in significant loss of moisture, and through heating and evaporation, organisms that reside in tide pools can experi= ence very high salinity (salt content of water) and temperatures. Wave action tends to dislodge organisms that are not firmly attached, and organisms= may experience considerable drag. At higher latitudes, abrasion by pack ice= in the winter also occurs. Organisms that reside in this environment must therefore be well adapted to these conditions, and many have evolved st= rong means of attachment, mechanisms of either sealing themselves off during= low tide or tolerating substantial water loss, or a physiological capacity = to tolerate fluctuating temperature and salinity.

The advantage of rocky intertidal habi= tats is that hard substrate is available in conjunction with abundant light = for photosynthesis. In subtidal habitats, water and the materials in it fil= ter out light, and as depth increases, the capacity for photosynthesis that drives the food chain declines. Rocky intertidal habitats are therefore= an ideal environment for seaweeds and other marine plants. Because attached plants are abundant, other components of the food chain also benefit, n= ot only from the abundance of food but also from the abundance of habitats created by the plants. Because wave action is strong, water moves quick= ly past sessile (fixed, nonmobile) organisms and a common feeding mode (ca= lled suspension feeding) in this habitat is to filter food particles out of = the water as it moves by. Because intertidal areas offer many advantages to those organisms that can tolerate the harsh conditions, space is often a limiting variable, and competition for space is an extremely important factor in regulating abundances and species composition. See also: Algal ecology; Brown algae

Rocky intertidal communities have been studied more than any other marine community as a result of their ease = of access, ease of manipulation and modest numbers of species. Unlike many other marine environments, it is possible to physically remove a specie= s or set of species without altering the habitat itself. Experiments of this type have established some important ecological paradigms. Early studie= s of intertidal habitats established strong patterns of zonation, where band= s of organisms dominated by different suites of species are observed at different levels of tidal exposure. The widths of these bands vary depending on the specific region and slope of the rock face. Organisms = that reside in the upper intertidal are typically very hardy, and capable of withstanding very harsh conditions. They do not have to compete intensi= vely with other species because relatively few species are able to tolerate = the extreme environmental conditions. At mid-tidal levels, competition by animals for space can be very intense. At exposure levels near the low-= tide mark, environmental harshness is less severe and predation is important. The keystone predators, or predators that have an effect on the communi= ty that is disproportionately great relative to their abundance, are less robust in terms of tolerating exposure to air, and their influence is therefore most significant near the low tide mark. Generally speaking, = species composition in the upper intertidal is regulated largely by physical processes whereas in the lower intertidal it is regulated more by biological interactions.

Sandflats and mudflats

Some intertidal regions are covered wi= th sediment rather than exposed bedrock, and to some extent the organisms = that reside in these sediments face similar challenges to those that confront organisms residing in rocky intertidal habitat. Temperature and salinity can fluctuate quite widely and aerial exposure is again an issue, but in this case the physically dynamic nature of the sediment can contribute = to the instability of the environment. Organisms must therefore be able to cope with shifting sediments that may move around with storms or even changing tides. The issues of temperature and salinity variation are somewhat reduced relative to rocky intertidal habitats because the sedimentary environment provides some buffer against the changes brough= t on by flooding and ebbing tides. These are, nonetheless, harsh habitats, a= nd the species numbers in sandflats and mudflats are relatively modest. Benthic diatoms (a type of algae) may form mats on the sediment that ac= t as a food source for some organisms. A variety of invertebrates such as molluscs and polychaete worms also live in these sediments, providing a potential food source to larger species that utilize sand- and mudflats. Some species are deposit feeders, defined as organisms that obtain their nutrition by ingesting sediment particles and organic matter associated with the particles. Others suspension feed as described for rocky intertidal organisms above. As is true for rocky intertidal areas, migratory species such as birds will feed in exposed sand- and mudflats when the tide is low, and these areas can therefore be of great ecologi= cal significance to a broad spectrum of organisms. Predators, herbivores and scavengers may also be present, as is true for most marine communities. See also: Mollusca (molluscs);&nbs= p; Diatoms

Salt marshes and mangroves

In many relatively sheltered coastal areas, often with some freshwater input, vascular plants may occupy the area between low and high tide (Figure 3). In tropical environments, plants called mangroves can form dense habitat where the upper canopy of the tree provides habitat for terrestrial taxa including insects and birds, and = the lower trunk and roots reside in mud submerged in salt water. Invertebra= tes such as mussels and crabs live upon and within the muddy sediments, and some fishes move among the vegetation, particularly during high tide. In temperate and boreal environments, salt marshes fill a similar ecologic= al role. Again vascular plants are present, but in this case cord grasses typically dominate. As with mangroves, insects and birds utilize the pa= rts of the plant that are above the water, and invertebrates and fish utili= ze submerged components. The invertebrates obtain their nutrition by sever= al mechanisms. Relatively few organisms feed directly on the living plant material but various organisms utilize the plant as a substrate. When p= lant material is broken off, it is colonized by fungi and microbes that play= a critical role in the decomposition of organic matter. These microbes may alter plant material so that it is more attractive to some species or t= hey may themselves be an important food source.

Figure 3
Global distribution of corals, mangroves and salt marshes superimposed over map of seafloor bathymetry. Red areas are continental shelf, abo= ve which are found neritic communities, whereas blue areas are open ocean where oceanic communities occur. Green areas show plate spreading regions, along which vents and seeps occur. Bathymetry image from Wal= ter Smith at National Oceanic and Atmospheric ...

Clearly, these plants are tolerant to salt, allowing them to utilize habitat at the land–sea interface = that is unsuitable for many other plant species that might otherwise compete= for the space. Typically, the organisms that reside in salt marshes are low= in diversity because of the harshness of the environment. Not only are there similar issues with respect to exposure as described for other intertidal habit= ats, but salt marshes and mangroves are also extremely productive environmen= ts where a great deal of plant material is produced. Much of this material= may sink to the sediments and decay, but because the plants are vascular and contain structural materials such as lignin, they do not decompose easi= ly and they cannot be used by many nonmicrobial organisms. Sediments are therefore organic rich but often with reduced levels of oxygen as a res= ult of the microbial breakdown of all the organic material. Because they are very productive, salt marshes and mangroves are important habitats for a number of species, and there are a number of ecologically and economica= lly important species that utilize these areas (e.g. striped bass in marshe= s, shrimp in mangroves). The abundance of potential food resources and the structural habitat that offers hiding places from predators makes it an ideal environment for many juveniles, although many species will comple= te their life cycle in other habitats, such as the adjacent shelf environm= ent. In some instances, substantial amounts of the decaying organic material from marshes and mangroves may be transported into the adjacent environ= ment such as the continental shelf, where it can be an important constituent= of food webs. See also: Plant salt stress

Estuaries

Estuaries are defined as coastal areas where seawater is measurably diluted by freshwater runoff. The salt mar= shes and mangroves described above are typically confined to estuarine areas, but there are other types of estuarine habitat as well. Seagrass beds o= ccur in some estuaries, whereas other estuaries lack rooted vegetation and a= re dominated by phytoplankton, which are single-celled and chain-forming organisms that are photosynthetic. Many estuaries are very productive because they are rich in nutrients that allow plant material (including phytoplankton) to grow, and thereby form a strong base for the food cha= in. As a result, many organisms utilize estuarine habitats for some phase of their life cycle; use of estuaries by juvenile life history stages of m= any fishes, for example, is very common. This high level of productivity al= so supports fisheries, since a number of estuaries have high secondary production. See also: Phytoplankton=

Many estuaries exhibit substantial variation in salinity, and the numbers of species that are found in a particular estuary at a given time are relatively modest. Estuaries are, nonetheless, of great ecological importance.

Neritic environments

The water column in many coastal regio= ns is very productive, and phytoplankton provide the photosynthetic base of the food chain. Phytoplankton are fed on by zooplankton such as copepod= s, the small crustaceans that numerically dominate the zooplankton in most neritic environments. Gelatinous forms (jellyfish, comb jellies) can al= so be very abundant in some instances. Zooplankton are then preyed upon by organisms at higher trophic levels, such as fishes. The productivity of coastal environments is typically limited by light, nutrient availabili= ty, and water column stability. In strongly seasonal environments at temper= ate and boreal latitudes, winter storms mix the water column and bring nutrients up from deeper waters. The warming of surface waters in spring gives the water column increased stability through stratification, and = in warming waters the phytoplankton are able to utilize the nutrients and = the increased availability of sunlight for photosynthesis. A spring bloom ensues, where high concentrations of phytoplankton known as diatoms dominate. Nutrients become reduced as cells divide and multiply, eventu= ally sinking out of the euphotic zone, which is the upper layer of the water column where light is strong enough to permit photosynthesis. This decr= ease in nutrient availability also results in shifts in phytoplankton species composition, and a succession of species occurs from spring through sum= mer. In some areas, a second bloom may occur in the autumn as additional nutrients are mixed up from below the surface layer as autumn storms be= gin and water column stratification begins to break down. Another factor th= at may cause the decline in phytoplankton abundance is grazing by herbivor= ous zooplankton, which can respond rapidly to enhanced phytoplankton abunda= nce. In tropical nearshore environments, this seasonal cycle is greatly dampened, and productivity is typically relatively low and dominated by= a more diverse assemblage of small species of phytoplankton. Some of the = most productive coastal regions in the world are upwelling regions, where wi= nd patterns are such that they push coastal surface waters offshore and al= low cold, nutrient-rich waters to be upwelled to the surface. Many lucrative fisheries are associated with these upwelling regions, and indeed many = of the world′s major fisheries are associated with coastal regions characterized by high productivity. See also: Biogeography of marine algae; Algal carbon dioxide concentrating = mechanisms; Algal photosynthesis;&nb= sp; Harmful algal blooms;&nb= sp; Fisheries management

Coastal sedimentary environmen= ts

Most of the bottoms of the bays and co= ntinental shelf habitat outside of the bays are covered in sediments ranging from gravel mixtures to coarse sands to fine-grained muds. Typically the composition of the sediment has a strong influence on the types of organisms that are associated with it. Ecologists working in sedimentary environments routinely subdivide organisms into megafauna (those identifiable in bottom photographs), macrofauna (those organisms retain= ed on a 500 μm sieve), meiofauna (organisms retained on 44 μm sieve), and microbes (organisms too small to be retaine= d on a 44 μm sieve). Because of the taxonomic difficulties and the differences in sampling approach that are appropriate to the different = size groupings, most scientists focus on just one of these size groupings.

Most of the organisms that reside in t= he sediment are invertebrates and microbes that live between the sediment grains (infauna) confined to the upper few centimetres close to the sediment–water interface. At depths within the sediment greater t= han a few centimetres, oxygen becomes extremely limited and biomass and densities of organisms are greatly reduced and limited to a low diversi= ty anaerobic community composed primarily of bacteria and some meiofauna. = Some organisms live on the sediment surface (epifauna) or just above the sediment (hyperbenthos). Most of these organisms depend on material settling to the bottom, although some suspension feed and others deposit feed.

The organisms that reside in the sedim= ents are linked to the water column above them by several different mechanis= ms. As mentioned above, they typically depend on the water column above to provide food. Benthic organisms in turn can be an important food source= for many organisms such as flatfish and cod, and thereby contribute to the success of commercial species and other components of the planktonic fo= od web. Some taxa, including types of crab, shrimp and molluscs, are themselves the target of major fisheries. Sedimentary organisms also pl= ay an important role in the cycling of nutrients. Microbes in particular a= re critical in converting plant material, which may be in the form of phytodetritus or other types of plant material, into nutrients such as nitrate. Macrofauna also play a role, not only in terms of digesting organic matter but also in oxygenating the sediments via the bioturbati= on that results from them moving through the sediments or moving the sedim= ent particles as they feed. Bioturbation results in substantially greater penetration of oxygen into the sediments than would occur by diffusion alone. The nutrients that are released by this process may be mixed up = into the water column through diffusion or storm events, and can thereby contribute to primary production in surface waters. See also: Mollusca (molluscs);&nbs= p; Crustacea (crustaceans)<= o:p>

Another important linkage between bent= hic organisms and those that live in the water column are the planktonic la= rval stages that are part of the reproductive cycle of many invertebrates. Larvae may be planktonic for minutes to months, depending on the specie= s in question, and can sometimes dominate the zooplankton.=

Coral reefs<= /b>

Coral reefs represent one of the most visually spectacular habitats in the ocean and are characterized by a r= ich diversity of fishes and invertebrates. The reefs themselves are compose= d of living corals that contain single-celled photosynthetic algal symbionts called zooxanthellae. Light is required by the symbionts for photosynthesis, and reef-building corals are therefore confined to relatively clear waters at shallow depths (tens of metres to <100 m) where light is abundant. Coral reefs are also geographically limited to regions where ocean temperatures are approximately 20°C or warmer on a year-round basis and they are therefore largely tropical in distribution. One of the reasons that ree= fs are so species rich is that the corals themselves create a highly compl= ex habitat with many microhabitats for different species to occupy. Like r= ocky intertidal habitats, coral reefs have vertical zonation as a function of depth. This zonation is regulated not only by tidal exposure and high levels of wave disturbance at shallow depths, but also by light penetra= tion at greater depth. See also: Algal symbioses

Although nutrient levels in the waters around coral reefs are low, reefs are very productive environments. To = some extent this is because nutrients are tightly cycled. Important primary producers on reefs include the symbiotic algae, but also macrophytic al= gae, calcareous algae and seagrasses. This high primary productivity supports high secondary productivity, and many reefs support active fisheries. Indeed, it has been estimated that coral reefs could support some 12% of world fisheries and in many tropical areas these fisheries provide the = vast majority of protein in human diets. Coral reefs are also an important p= art of the tourism economy of many tropical countries, not only because they attract scuba divers but also because they provide a natural breakwater that helps to preserve sandy beaches. Coral reefs are presently conside= red to be among the most threatened marine habitats (discussed below), and scientists have argued that urgent conservation measures are needed.

The = Open Ocean Environment

Oceanic environments