Benthic Zone - MicrobeWiki Benthic Zone This student page has not been curated. Contents 6 References Introduction The benthic zone is the lowest level of a marine or freshwater system and includes the sediment surface, the water just above it, and some sub-surface layers. Benthic zones exist all over the world in every appreciable water system, be it an ocean, lake, pond, river, or stream. The benthic zone starts at the shore and extends down along the bottom of the lake or ocean. This means that it could be as shallow as a few inches at its start, but may reach depths of 6,000 meters as it coincides with the abyssal plain at the bottom of the ocean. Because of the depths it can reach, the benthic zone is often characterized by low sunlight and low temperatures (Alldredge 1988). This usually attributes to little life and biodiversity, so one would be inclined to assume that it is a vast waste. However, the presence of sediment layers at the benthic zone provides many nutrients and adds greatly to species richness. Because the benthic zone has such a range of depths, many kinds of organisms may live there.

Crustaceans, sponges, bivalves, snails, sea stars, polychaetes, fish, and many others can inhabit the zone (Konhauser 2007). This page will focus more on deep oceanic benthic zones, but will touch on other zones as well. Benthic zone of the sea floor. Diagram showing the different zones of an ocean. Note the wide range of the benthic zone. Although specific for an ocean, this is how the benthic zone is characterized across all water systems. Physical environment Physical Characteristics Because the benthic zone can occur in such a wide range of environments, physical and chemical characteristics vary largely and are almost always context dependent. Nutrients are not available in the way they are in much higher depths in the water column, so organisms in the benthic zone are nutrient limited. Temperature Temperature in the benthic zone ranges from warmer temperatures at shallow depths because of close proximity to the water surface, but may drop to 2 °C to 3 °C at the most extreme depths of the abyssal zone. At such cold temperatures, abundance of life is quite low and organisms move at a very slow pace.

Warmer waters at shallower depths provide much more energy and can host more complex systems (Alldredge 1988). Chemical Environment The nutrients at the sediment layer are usually the limiting source of energy for the organisms that inhabit the benthic zone at great depths. These nutrients actually originate from dead or decaying organic matter from higher up in the water column. This organic matter drifts down continuously from shallower depths and settles on the bottom, where it is consumed and is the main driver of the benthic zone food chain (Gooday et al. 1990). Most areas do not have sufficient heat or sunlight to sustain much life, so organic matter is very important to the chemical environment. Pressure Pressure ranges from very little in a few inches of water to tremendously great at the bottom of the ocean. For an extreme example, the deepest spot in the Mariana Trench (the deepest trench in the ocean) is around 36,000 feet below sea level and experiences pressures that are more than 1000 times greater than standard surface pressure ("Deepsea Challenge"). High pressure in zones of great depth make for a very homogenous environment that produces a number of peculiar traits among organisms.

One such peculiarity is the enhanced size of certain organisms. At great depths in arctic and antarctic waters, dissolved oxygen content is quite high. This allows organisms to grow much larger when compared to close relatives in temperate zones (De Broyer and Danis 2005). Light Availability In water, light becomes very limited in its significance at depths below about 200 meters. Between 200 and 1000 meters, the intensity of light quickly dissipates. This zone is termed the dysphotic zone and photosynthesis is no longer possible. At 1000 meters, the aphotic zone starts and light availability becomes negligible (NOAA). Microbial communities At shallower depths, there are many more photosynthetic microbes. At greater depths, microbes do not have access to light and heat and must rely on detritus for energy. The organisms in the benthic zone are classified into those which dwell on the surface and those which burrow into the sea floor. They are the epifauna and infauna, respectively. Organisms inhabiting the benthic zone are collectively called the benthos.

The name benthos comes from the Greek noun βένθος, which means “depths of the sea” (Alldredge 1988). Benthic microorganisms are almost exclusively microalgae and bacteria, but other others include: ciliates, amoebae, and flagellates. In general, most organisms there are detritivores and scavengers because of the abundance of dead or decaying organic matter. In addition to these heterotrophs, there are chemoautotrophs present that use the substrate to make biomass. There are also photoautotrophs present at shallower depths where sunlight is abundant. The impacts of benthic microorganisms spread, much like the impacts of all other microbes, across the community as a whole. They provide essential energy and nutrients at the lower levels of the food chain and are imperative to the success of other forms of life. Microbial processes Nutrient Flow Benthic microorganisms are key to controlling nutrient flow between sediment layers and the water column. As nutrients are taken in and stored by microorganisms, their flow through the system is greatly slowed. This process can act as a buffer.

Buffers are essential in all natural systems and prevent excessive flux of elements. One such example is seen when microalgae buffer nutrients to prevent phytoplankton overexploitation, which often results in eutrophication ("Benthic Microalgae"). EPS Formation In benthic areas where sunlight does penetrate, microalgae are ubiquitous. When these algae photosynthesize, they take up carbon dioxide and other micronutrients. Much of the carbon taken up is later released as extracellular polymeric substances (EPS or “slime”). EPS is thought to play an important role in the sediment layer, as it contributes both to sediment structure and the local food web. As the name “slime” implies, EPS is a sticky organic material that constitutes biofilms and allows cellular attachment to surfaces ("Benthic Microalgae"). Its stickiness helps keep sediment particles together and prevents resuspension, which keeps dissolved oxygen levels stable. EPS further contributes because it is rapidly metabolized by bacteria and thereafter provides nutrients to the local food web ("Sources of Pollution: Sediment Resuspension"). Current Research Microbial Response to Elevated Hydrocarbon Levels Some researchers have investigated the impacts of the Deepwater Horizon oil spill on microbial community composition.

The spill that started on April 20th, 2010 lasted for 87 days and released 210 million US gallons of oil into the surrounding environment. A research group used taxonomic and parallel marker gene approaches to determine the microbial community composition before and after the spill. As expected, diversity and richness declined greatly. In the sites studied, nematodes dominated naturally, but after the spill, fungi were the overwhelmingly dominant group. The spill caused certain opportunistic and resilient species to take hold of communities. Fungi that could take advantage of high levels of hydrocarbons, specifically those of the genera Apergillus, Acremonium, Acarospora, Rhodocollybia, and Rhizopus, were found at very high abundances post-spill, but were rare pre-spill. Interestingly, fungi of the genus Altenaria demonstrated increase activity of lignocellulose-degrading enzymes. Those enzymes have actually been used for the breakdown of industrial toxins. A complete restructuring of microbial communities like this causes an effect that precipitates into all forms of marine life and will be felt for a considerable amount of time (Bik et al. 2010).

Diagram showcasing the processess over time of whale fall communities. Note the long lasting effects from microbial activity. From Nature Publishing Group Photos showing microbial boring marks into whale fossilized whale bone. Microbes break down the bones and produce compounds that greatly affect the surrounding chemical composition. From Danise, et al. 2012. Whale Fall An interesting process relatively new to marine biology centers around the carcasses of whales that have sunken to the ocean floor. This large mass, termed “whale fall”, provides unprecedented amounts of nutrients for organisms in the benthic zone. Many types of micro- and macrofauna will accumulate around whale fall and can feed for months or years at a time. This direct feeding is more temporary, but a long lasting change is produced by the microfauna that feed. When microbes feed on the lipid-rich bones, they excrete hydrogen sulfide and other chemical compounds that restructure the chemical composition of the surrounding water. Oxygen is also depleted because of the high metabolic rates of microbes present. Because of low oxygen concentrations, microbes that perform sulfate reduction and methanogenesis are favored. These biotic and chemical changes can greatly alter microbial and macrofaunal community structure (Danise, et al. 2012).

Marine Snow Because of their reliance on it, microbes may become spatially dependent on detritus in the benthic zone. Columns where marine snow (detritus) consistently falls will be more heavily populated by microbes. Bacteria and protozoa, specifically flagellates and foraminifers, colonize quite rapidly on detritus matter. After colonies are established, larger grazing organisms feed on them. Areas where these processes take place have a markedly higher abundance of life and are more complex systems (Gooday et al. 1990). Detritus levels vary seasonally as well, which has an effect on the life that they support. Population dynamics, microbial community structure, and even reproductive cycles may be greatly influenced by marine snow when nutrients are a limiting factor. Opportunistic species are often first to colonize these areas and affect the way niches are filled, which has a cascading effect in communities (Gooday et al. 1990). These processes all combine to create great variation and uniqueness in benthic zone habitats. References Alldredge, Alice; Silver, Mary W. (1988). "Characteristics, dynamics and significance of marine snow".

Progress in Oceanography 20: 41–82. "Benthic Microalgae." Benthic Microalgae. Virginia Institute of Marine Science, n.d. Bik HM, Halanych KM, Sharma J, Thomas WK (2012) Dramatic Shifts in Benthic Microbial Eukaryote Communities following the Deepwater Horizon Oil Spill. PLoS ONE 7(6): e38550. doi:10.1371/journal.pone.0038550. S. Danise, B. Cavalazzi, S. Dominici, F. Westall, S. Monechi, S. Guioli (2012).

Evidence of microbial activity from a shallow water whale fall (Voghera, northern Italy), Palaeogeography, Palaeoclimatology, Palaeoecology, Volumes 317–318, 1 February 2012, Pages 13-26, ISSN 0031-0182, 10.1016/j.palaeo.2011.12.001. "Deepsea Challenge" Deepsea Challenge. National Geographic. A. J. Gooday, Carol M. Turley and J. A. Allen (1990). Responses by Benthic Organisms to Inputs of Organic Material to the Ocean Floor: A Review. 19 June 1990. The Royal Society 331: 1616 119-138 Konhauser, K. 2007. Introduction to Geomicrobiology. Blackwell Publishing. "Light May Be Detected as Far as 1,000 Meters down in the Ocean, but There Is Rarely Any Significant Light beyond 200 Meters." How Far Does Light Travel in the Ocean?

NOAA, n.d. C. De Broyer and B. Danis (2005)"Polar Regions: Witnesses to the Future." Polar Regions: Witnesses to the Future. Royal Belgian Institute of Natural Sciences, May 2004. "Sources of Pollution: Sediment Resuspension." Sediment Resuspension. Barataria Terrebonne National Estuary Program, n.d. Edited by Michael Schock, a student of Angela Kent at the University of Illinois at Urbana-Champaign. Benthic zone - New World Encyclopedia Benthic zone Next (Bentonite) Seagrass growing off the coast of the Florida Keys . The benthic zone is one of the ecological regions of a body of water. It comprises the bottom—such as the ocean floor or the bottom of a lake —the sediment surface, and some sub-surface layers. Organisms living in this zone—that is, on or in the bottom of the body of water—are called benthos. The adjective benthic refers to something connected with or occurring on the bottom of a body of water (CSC 2008).

In contrast, the pelagic zone is the descriptive term for the ecological region above the benthos, including the water-column up to the surface. The terms benthic and benthos are derived from the Greek for "depths of the sea" (CAML 2008), but the terms are also used in freshwater biology to refer to the zone and organisms at the bottoms of freshwater bodies of water, including both standing and running waters, including lakes, rivers, and streams (NABS 2008). The benthic zone provides many valuable products and ecological services. Benthic organisms play an important role in food chains , including as food for humans, and some play a critical role in the breakdown of organic matter (CSC 2008). Some benthic zones, such as areas of coral reefs , kelp forests, and eelgrass beds, help to buffer wave action along coastlines, providing storm protection (CSC 2008).

The benthic zone provides an area for spawning, foraging, and refuge for various fish species, and benthic habitats function in nutrient cycling and removal of contaminants from the water, such as with the removal by filter feeders ( scallops , mussels , and so on) of pollutants, organic matter, and sediments (CSC 2008). The diversity of these habitats and their organisms also add to the delight and mystery of nature for humans, whether it is through the beauty of a coral reef, the observation of a sea anemone on the ocean floor, or the sighting of a new creature in the exploration of a deep ocean habitat. Contents 4 Credits Description and types The benthic zone refers to both freshwater, brackish, and saltwater environments. Depending on the water body, the benthic zone may include areas that are only a few inches below water, such as a stream or shallow pond, or may be at the bottom of an ocean below a 4,000 meter (13,000 foot) column of water.

Benthic habitats are very diverse, depending upon their depth and location and have distinct biological, physical, and geochemical characteristics (CSC 2008). The superficial layer of the soil lining the given body of water is an integral part of the benthic zone, as it influences greatly the biological activity that takes place there. Examples of contact soil layers include sand bottoms, rock outcrops, coral , and bay mud. Marine benthic environments are divided further into zones based on their depth. From the shallowest to the deepest, these include (CSC 2008): The nearshore and estuarine zones (less than 200 meters deep) The bathyal zone (200 to 2,000 meters) The abyssal zone (2,000 to 6,000 meters) The hadal zone (over 6,000 meters deep) Estuarine and nearshore benthic habitats include coral reefs, kelp forests, shellfish beds, shallow submerged mudflats, rocky hard-body habitats, and rippled sandflats (CSC 2008).

The abyssal zone—the pelagic zone, from 2,000 to 6,000 meters in depth—remains in perpetual darkness and never receives daylight. It contains the very deep benthic communities near the bottom of oceans . The abyssal zone has temperatures generally around 0 to 3°C, but 13-15°C in the Mediterranean sea. Abyss is from the Greek word meaning bottomless sea. The bottoms deeper than the abyssal zone occupy the sparsely inhabited hadal zone, with the name derived from Hades, the classical Greek underworld. The bottom areas lying at lesser depths than the abyssal zone occupy the bathyal zone. These three zones belong to the deep-sea realm. The hadal, abyssal, and bathyal zones are in such relatively deep, high pressure areas of the ocean, that they and their organisms are difficult to observe and map. The deep trenches or fissures that plunge down thousands of feet below the ocean floor—for example, the midoceanic trenches such as the Mariana Trench in the Pacific —are almost unexplored. Only the bathyscaph Trieste and the remote control submarine Kaiko have been able to descend to these depths.

The pelagic zone, the open-ocean zone that includes the water-column above the benthos to the surface, is similarly divided into subzones, such as the epipelagic (surface to around 100 meters), mesopelagic (twilight zone from 200 meters to around 1,000 meters), bathypelagic (about 1,000 meters to 4,000 meters), abyssopelagic (4,000 meters to above the ocean floor), and hadopelagic (deep water in trenches). In contrast to the organisms of the benthic zone, which tend to be constrained to remain close to the bottom, organisms in the pelagic zone tend to move much more freely and widely. These include the various types of Plankton , which move passively as they are carried by water currents, and the various types of nekton, which can swim against the current and control their position. Benthos Giant clam along the Great Barrier Reef .

Benthos are the organisms living in the benthic zone—that is on or in the bottom (substatum) of water bodies, and some would add those near the bottom (NABS 2008). These organisms generally live in close relationship with the substrate bottom; many such organisms are permanently attached to the bottom. Many organisms adapted to deep-water pressure cannot survive in the upper parts of the water column. The pressure difference can be very significant (approximately one atmosphere for each ten meters of water depth). The benthos can be categorized in various ways. One division classifies the primary producers (algae, aquatic plants) living on the bottom as "phytobenthos" and all consumers (protozoa and benthic animals) living on or near the bottom as "zoobenthos" (NABS 2008). The term epibenthos is used for those organisms living on top of the sediment and hyperbenthos for those living just above the sediment.

Benthos also can be categorized according to size: Macrobenthos, size greater than one mm Meiobenthos, size less than one mm but greater than 32 µm (µm is a thousandth of a millimeter) Microbenthos, size less than 32 µm These dimensions vary from researcher to researcher, with some accounts using a cutoff of 45 μm between meiobenthos and microbenthos and some even one tenth of a millimeter (NHPTV 2008). For example, some define meiobenthos as those organisms that can pass through a one mm mesh but will be retained by a 45 μm mesh. Whether an organism will pass through a one mm mesh will also depend upon whether it is alive or dead at the time of sorting.

Large invertebrate benthic organisms, which may be known as macrobenthos, macrozoobenthos, or macroinvertebrates (NABS 2008), include such marine examples as sea stars , oysters , clams , sea cucumbers, brittle stars , and sea anemones , and such freshwater examples as snails , crayfish , and the larva of many aquatic insects ( dragonflies , mayflies , caddisflies). A microscope is required for viewing the microbenthos, which include diatoms , bacteria , and ciliates (NHPTV 2008). The term meiobenthos was first coined in 1942, by M.F. Mare, but organisms that would fit into the meiofauna category have been studied since the eighteenth century. They may include some sea spiders, sea worms, and water bears. The benthic fauna is very diverse. Smith (1996) notes that faunal composition and diversity increases with depth until the mid or lower bathyal region, then decreases toward the abyssal plain.

Bottom organisms tend to have four feeding strategies (Smith 1996): filtering suspended material from the water (for example, coelenterates), collecting material that has settled on the sediment surface (ex., sea cucumbers), deposit feeding (for example, polychaetes), and predatory (for example, brittle stars). Because light does not penetrate very deep ocean water, the energy source for the benthic ecosystem is often organic matter from higher up in the water column that drifts down to the depths. This dead and decaying matter sustains the benthic food chain ; most organisms in the deep water benthic zones are scavengers or detritivores. References Census of Antarctic Marine Life (CAML). 2008. Benthos. Census of Antarctic Marine Life. Retrieved April 18, 2008. Coastal Services Center (CSC), National Oceanic and Atmospheric Administration (NOAA). 2008. B What is benthic habitat? National Oceanic and Atmospheric Administration. Retrieved April 18, 2008. Mare, M. F. 1942.

A study of a marine benthic community with special reference to the micro-organisms. Journal of the Marine Biological Association of the United Kingdom 25: 517-554. About the Benthic Zone: the Sea Floor The Benthic Zone: The Sea Floor written by: Terrie Schultz•edited by: Laurie Patsalides•updated: 11/11/2010 The benthic zone, or sea floor, extends from the water's edge to the deepest ocean trenches. Learn about this varied habitat and the organisms that live there. slide 1 of 4 The Benthic Zone The benthic zone begins at the edge of the sea at the intertidal zone , extends out along the continental shelf, and continues down the continental slope to the abyssal plain , thousands of meters below sea level. It includes the entire seabed, rising along the mid-ocean ridges where new sea floor is continually being made, and dives down into the deep ocean trenches. The habitats of the benthic zone vary widely depending on how far below the surface the bottom lies. Just beyond the intertidal zone, the shallow water receives ample sunlight and oxygen.

Continuing down along the continental slope, the environment becomes increasingly dark and cold, and the pressure increases. A remarkable variety of organisms live in these different habitats, each with specialized adaptations reflecting the conditions of their environment. slide 2 of 4 Benthos: Inhabitants of the Benthic Zone The organisms that live in the benthic zone are collectively called "benthos." They include crustaceans, mollusks, worms, fish, and any other type of organism that makes its home on or in the seabed. The majority of benthos are scavengers or detritus feeders that eat decomposing organic matter. Much of the food supply in the benthic zone is in the form of "marine snow," small particles of decaying organic matter that slowly descend through the water column and accumulate on the ocean floor. Benthos in the intertidal zone include sea stars, barnacles, mussels, sea anemones, sea urchins, snails, and other creatures that attach themselves to the rocky substrate or burrow in the sand. Benthos in deeper zones include fish, crustaceans and tube worms.

slide 3 of 4 Benthic Fish Benthic fish, also known as groundfish, are those that stay on or very near the sea bottom, whether in shallow or deep water. Unlike fish that spend their time swimming, benthic fish are very dense and have negative buoyancy, allowing them to effortlessly lie on the bottom or bury themselves. Other adaptations commonly seen in benthic fish include the lack of a swim bladder and a flattened body shape. They are predominantly bottom feeders that eat detritus, or ambush predators that lie in wait for their prey to come within striking distance. There are many different types of benthic fish. Flatfish, including flounder, halibut, plaice, sole and turbot, lie on the the sea floor or bury themselves in the sand. Asymmetrical physical adaptations include having both eyes on one side of their head, and different pigmentation on each side of their bodies; the side facing down is usually pale, while the side facing up is camouflaged. Some species of flatfish are predators, feeding on smaller fish, while others eat mainly invertebrates.

Rays and skates are also flat, but are bilaterally symmetrical, with their eyes on top of their head. These cartilaginous fish are predators that bury themselves and wait for prey, feeding largely on crustaceans, clams, oysters and snails. Rattails are benthic fish that have a large head and mouth, and a body that rapidly tapers down to a long, narrow tail. They live in the deep sea, and are scavengers, feeding chiefly on invertebrates. Some species of rattails are brotulas, chimaeras and grenadiers. slide 4 of 4 Benthic zone (habitat) Benthic zone (habitat) Benthic zone (habitat) Overview Benthic habitats cover about 70% of the earth surface. Of the marine species, 98% live on or in the ocean floor. The benthic zone maintains a substantial part of the world’s biodiversity. However not all of the benthic habitats are equally susceptibl...

Risk of Species Invasion The generalized model of an ‘invader-friendly’ benthic habitat could be briefly defined by the following features: habitat lacks certain species which should to be present under normal conditions (there are ’vacant niches’); Don't need the entire report? Generate a print friendly version containing only the sections you need. Benthic sample with zebra mussel druses from the soft bottom of the Curonian lagoon. Copyright Anastasija Zaiko Benthic sampleBenthic sample with zebra mussel druses from the soft bottom of the Curonian lagoon.Anastasija Zaiko Identity Benthic zone (habitat) International Common Names English: abyssal zone; bathyal zone; benthal zone; benthonic zone; deep sea; hadal zone; ocean bottom; ocean floor; sea bottom; sea floor Russian: bental' Top of page Benthic habitats cover about 70% of the earth surface. Of the marine species, 98% live on or in the ocean floor. The benthic zone maintains a substantial part of the world’s biodiversity. However not all of the benthic habitats are equally susceptible to alien species invasion.

The so-called ‘hot spots’ for the introduction of benthic invasive species are first of all the nearshore or estuarine zones, since these areas are extensively exploited and disturbed by humans. The majority of established benthic invasives tend to occur in the coastal inlets, lagoons and gulfs. Almost no invasions are known to have occurred in the mid-ocean benthic habitats. Most likely this is due to the very specific physical, geochemical and biological characteristics of those environments. Context Top of page The term benthic refers to anything associated with or occurring on the bottom of a body of water. The animals and plants that live on or in the bottom are known as the benthos. Benthic habitats can best be defined as bottom environments with distinct physical, geochemical and biological characteristics. Benthic habitats vary widely depending upon their location and depth, and they are often characterized by dominant structural features and biological communities. The benthic environment can be subdivided into two larger units, which will then be subdivided on the basis of various criteria.

These are the subneritic province, which extends from the spring high-tide shoreline to a depth of 200 m (approximately the continental shelf), and the suboceanic province, which includes all the benthic environment below the 200 m depth (Thurman, 1991). There are two regions that are commonly classified as a part of the littoral zone, but also may be defined as benthic habitats. A transitional region above the high-tide line is the supralittoral zone, commonly called the spray zone. It is covered with water only during periods of extremely high tides. The intertidal zone, classified also as the littoral zone, is from low tide to 200 m. The subneritic is called the sublittoral (inner or outer) or shallow subtidal zone. It includes the sea bottom to a depth of 200 m or the shelf break, the seaward edge of the continental shelf (Thurman, 1991). The subneritic region is also classified as the nearshore or estuarine zone. The suboceanic province is divided into bathyal, abyssal and hadal zones.

The bathyal zone lies along the slopes of continents and on seamounts and underwater rises. It extends from the edge of the shelf to the beginning of the abyss (200 to 2000 m) (Zezina, 1997). The deepest benthic habitats include the abyssal zone from 2000 to 6000 m depth and the hadal zone below 6000 m (Kennish, 2001). Because the hadal, abyssal, and bathyal zones all occur in relatively deep ocean waters, benthic habitats in these zones have been much less commonly observed and mapped. Estuarine and nearshore benthic habitats can be highly diverse, including shallow submerged mudflats, rippled sandflats, rocky hard-bottom habitats, seagrass beds, kelp forests, shellfish beds, and coral reefs. Description Top of page Summary of common/typical species Of all marine species, 98% live on or in the ocean floor. These organisms are called benthos, or bottom dwellers. There are those called infauna that live buried in the sand, shells or mud.

Those that live attached to rocks or move over the surface of the ocean bottom are the epifauna. Benthos such as some shrimp and demersal flounder that live in the bottom but move with relative ease through the water above the ocean floor are the nektobenthos (Thurman, 1991). Benthic organisms are different from those elsewhere in the water column. Many are adapted to live on the substrate (bottom). In their habitats they can be considered as dominant creatures. Many organisms adapted to deep-water pressure cannot survive in the upper parts of the water column as the pressure difference can be very significant. Because light does not penetrate very deep ocean-water, the energy source for the benthic ecosystem is often organic matter from higher up in the water column which drifts down to the depths. This dead and decaying matter sustains the benthic food chain; most organisms in the benthic zone are scavengers or detritovores. Main ecological processes Benthic habitats are important for a variety of reasons. Estuarine and nearshore benthic habitats support a wide diversity of marine life by providing spawning, nursery, refuge, and foraging grounds for fisheries species.

They function in nutrient cycling, and contribute to the removal of contaminants from the water column. Benthic organisms are also important members of the lower food web, consuming organic matter and phytoplankton and serving as food sources for higher-level consumers. Many benthic habitats (for example, coral reefs, eelgrass beds, and kelp forests) have three-dimensional structures that serve as shelter and provide storm protection by buffering wave action along coastlines. Benthic habitats can play an important role in maintaining water quality. Many benthic organisms, including filter feeders like hard clams, bay scallops, and mussels, obtain their food by taking in seawater. As the water flows through their bodies, sediments, organic matter, and pollutants are filtered out and ingested. This role makes these benthic communities good indicators of health in estuarine ecosystems. Benthic habitats play a critical role in the breakdown of organic matter, through the actions of scavengers, deposit-feeders, and bacteria. Succession and change Most of the benthic habitats are in deep, pressured areas of the ocean.

Because of the high pressure and seclusion neither tidal changes nor human interference has had much of an effect on these areas, and the habitats have not changed much over the years. Many benthic organisms have retained their historic evolutionary characteristics; some organisms have significantly changed size. Human mediated history Most of the human mediated changes in benthic habitats occur in response to major disturbances of the substrate. These include natural conditions, such as reworking of sediments and till, turbidity flow and icescouring, and cultural factors, such as industrial pollution, dredging and deposition of mud. Benthic plants and animals will respond by re-colonizing the substrate. Submerged structures, such as shipwrecks, also provide substrate for the colonization of species. Commercial fishing is one of the most important human impacts on the benthic environment. One such impact is through disturbance to benthic habitats as fishing gear (trawls and dredges) are dragged across the seafloor. As the result, the bottom landscape, structure and characteristics of sediments and, finally, benthic communities are strongly affected and sometimes destroyed (Thrush et al., 1998).

Distribution Top of page Benthic habitats cover about 70% of the Earth (Leontyev, 1982). The abyssal zone accounts for 75% of the benthic habitat area of the oceans, and the bathyal and subtidal zones 16% and 8% of the area, respectively. Finally, the hadal zone accounts only for 1 % of the total benthic area (Kennish, 2001). The benthic sub-habitats are not evenly distributed over the world. The subneritic (continental shelf) zone is extended along the continental margins and its width may vary from 0 to 1500 km (average about 80 km). In some regions the continental shelf is extremely wide or narrow: e.g. in the Arctic ocean, from the southern coast of the Hudson Bay towards the Baffin Bay, it is almost 2000 km wide. While along some African coasts, the width of the continental shelf does not exceed 2 km (Duxbury, 1971).

The bathyal zone is a substantial part of the ocean, being larger than the shallow shelf zone, including the sublittoral. However this zone does not form a continuous belt. In some places it is interrupted by steep continental rises or deep-sea trenches on the edge of the continental slope. The largest area of the ocean floor is occupied by the abyssal zone. It includes the abyssal hills and abyssal plains. Abyssal hills or mid-ocean ridges are present in all the oceans. They cover an area of about 55 million square kilometers of the ocean floor. Abyssal plains are separated from each other by deep-sea trenches or seamount ridges. These landscape forms are poorly pronounced in the Pacific Ocean, where the most deep-sea troughs and longest island chains are situated. The hadal zone is a restricted environment, found only in the trenches along the margins of continents (Thurman, 1991). The generalized model of an ‘invader-friendly’ benthic habitat could be briefly defined by the following features: habitat lacks certain species which should to be present under normal conditions (there are ’vacant niches’); habitat is disturbed due to natural or anthropogenic factors, e.g.

big storm, bottom dredging (every new disturbance event may promote a new surge of invasions); ecosystem properties are altered due to previous introductions, creating unstable conditions (successfully established habitat engineering species should be considered as a powerful facilitative factor for further invasions); habitat is subject to increased amounts of utilized resources, such as from eutrophication (the abrupt increase of nutrient loads should be considered as an invasibility stimulating factor) (Zaiko et al., 2007). The so-called ‘hot spots’ for the introduction of benthic invasive species are first of all the nearshore or estuarine zones, since these areas are extensively exploited and disturbed by humans. The main pathways of alien species invasion that are related to human trade are shipping, aquaculture, fisheries, leisure activities, etc. (Minchin and Gollasch, 2002). The majority of established benthic invasives tend to occur in the coastal inlets, lagoons and gulfs.

In a rare deep-sea example of a successful human-mediated invasion, the North Pacific king crab Paralithodes camtschatica was intentionally transported and successfully released by Russian fisheries biologists into the Barents Sea in the 1960s. Large males are now common in depths to at least 300 m (Kuzmin et al., 1996). Whether accidental means of transport, such as deep-sea trawling operations, could effectively move benthic species to new regions appears to have been largely uninvestigated, although this concept has been explored, in part, for deep-water sponges off the coast of South Africa and Namibia (Carlton, 2002). Among the unintentionally introduced benthic invasive species, those having planktonic larval stage or/and high ecological plasticity are considered as the most successful ones, since such organisms have much more opportunities to be transferred to another even more remote ecosystem and to establish viable populations in a short time. Among the invasive benthic invertebrates, the suspension-feeders are the most widespread type, followed by predators, deposit-feeders and grazers (Olenin and Daunys, 2003).

Impact Top of page Environmental impacts The impact of benthic invasive species on native biodiversity and ecosystem processes may assert itself as competition for food and/or space, habitat change, food-prey for native species, predation on native species, herbivory, hybridisation with native populations, parasitism, community dominance – changes in community structure, benthic-pelagic interaction – introducing new linkages between benthic and pelagic environment, bioaccumulation – storage of toxic substances. However not all of these possible impacts are equally typical for benthic invaders. The most abundant are habitat change, benthic-pelagic coupling and community dominance. Among these, habitat change or habitat engineering should be emphasized as one of the most important, as it may be relevant to many ecosystem functions and elements. As reported by Vitousek (1990), invasive habitat engineer species have much larger effects on their new community than non-engineer species, since both biological and physical characteristics of the environment will be altered. On the other hand, the combination of invasive species and habitat modification is often considered as a presage for new invasions (Cuddington and Hastings, 2004).

Impacts on biodiversity are revealed mostly through aggressive predation and grazing mechanisms. This is particularly important for benthic ecosystems that are not able to adjust as fast to alien species, as alien species can do. Economic and social impacts It is easier to assess and evaluate the economic impact of an invasive species than its impact on ecosystem functioning. Yet these impacts are closely related to each other. The possible impacts on uses/resources include impacts on aquaculture, aquatic transport, fisheries, etc. Among the benthic invasive species, the biggest economical losses are caused by fouling organisms, e.g. Balanus improvisus [Amphibalanus improvisus], Dreissena polymorpha, Dreissena bugensis, some seaweed species. Their ability to settle down on almost every substrate together with rapid growth may cause clogging of water abstraction pipes, fouling of ship hulls and underwater harbour constructions, and deterioration of recreational areas (Eno et al., 1997). Another side of the problem is a lack of management mechanisms for an established nuisance species. Most of the techniques applied are not efficient and are rather expensive.

The predatory benthic species, like Asterias amurensis (northern Pacific seastar) eating almost anything it can find, including dead fish and fish waste, is considered a serious aquaculture pest. In Japan seastar outbreaks cost the mariculture industry millions of dollars (NIMPIS, 2002a). Habitat Management: Protection Top of page Risk assessment for marine bioinvasions provides a tool to aid managers in controlling and reducing the rate of current invasions. It can provide a mechanism to better understand the invasion problem and determine better operating procedures, high-risk routes or system failures, or can aid in the process of management strategy evaluation to determine what components of the invasion process are more susceptible to control, or more apt to fail under specific management regimes (Hewitt and Hayes 2002). The principal ways to avoid potential problems caused by benthic invasive species include: strict control of import of all live specimens adoption of regulations on a regional basis attempt eradication before species establishes (Eno and Hamer, 2002). The main prevention measures of bioinvasions occur as a barrier control or quarantine response.

Concerning the benthic invasive organisms, the control of ballast water introductions is likely the most abundant and popular preventive approach, since a majority of benthic aliens are being transported via extensive world-wide shipping and, particularly, in ballast water tanks. One reason for this arises from three different ‘habitats’ inside ballast water tanks: tank walls, ballast water, and the sediment – where benthic organisms or their larvae may be transported. Because of the diversity in ship design and improved technology (e.g. double hulls, higher economical cruising speeds), the survival rates of some species carried in ballast tanks have increased, and consequently, many introductions of non- indigenous organisms in new locations have occurred in recent years highlighting the need for effective ballast water management. This quarantine approach does not intend to provide an absolute barrier to prevent the introduction of unwanted species but rather aims for a significant reduction in risk. It has become clear that that no single treatment process is likely to universally achieve the required inactivation, kill or total removal of all unwanted organisms in ballast water. Two-stage treatment may comprise some form of mechanical removal of organisms (e.g.

filtration, cyclonic separation, sedimentation/floatation, etc.) followed by a physical or chemical treatment method (heating, cooling, UV irradiation, ultrasonic, electrical inactivation, hypochlorite, ozone, peracetic acid use, etc.) (Taylor et al., 2002). To date, international guidelines have been adopted as the IMO Assembly Resolution. The IMO has not generally promoted regionally different systems, emphasising that a universal global approach is preferred to solve the ballast water problem. The guidelines are not concerned only with the exchange of ballast water at sea but include also reference to ballast water management practices that would reduce the risk of introducing non-native benthic species, such as precautionary procedures when taking on ballast water in shallow areas; discharging ballast sediments to on-shore facilities (if available) (Taylor et al., 2002). References Benthic animals | Marine life | Ecosystem | Ministry of Fisheries and Agriculture Benthic animals Benthic animals Various benthic animals. 1. common whelk, 2. blue mussel, 3. Neptune whelk. 4. periwinkle, 5.

Yoldia hyperborea, 6. sand gaper, 7. limpet, 8. scallop, 9. chiton, 10. common starfish, 11. green sea urchin, 12. brittlestar, 13. ragworm, 14. pectinaria worm,15. scaleworm, 16. skeleton shrimp, 17. isopod, 18. barnacle, 19. benthic amphipod Illustration: Kristín Líf Valtýsdóttir The diversity of marine life is most obvious on the bottom where animals have adapted to a multitude of niches and with extremely diverse looks and behaviour. Some are buried in sediment all their life, others are attached to stones or even other organisms, some are slow and sluggish, others roving and rapid. Feeding habits are also diverse. Many filter feed, that is they actively or passively sieve food particles or small organisms from the ocean. Others eat the bottom sediments (detritus feeders), graze on kelp (grazers), eat carrion (scavengers) or hunt other animals (carnivores).

Although total biomass and biomass production is much higher in the pelagic environment, the diversity of life is much higher on the bottom. This diversity is primarily because the bottom offers many more types of habitats that organisms can adapt to than the pelagic environment. The most diverse ecosystems in the ocean are on hard bottoms. In shallow waters these are usually covered with coral reefs in the tropics, but kelp forests in the temperate and Arctic waters. Below the sunlit zone, sessile animals such as sea anemones, sponges, hydrozoans, tunicates and many other animal groups take over these habitats from the benthic algae. Cold water corals can also be found in deep and cold waters, including the waters around Iceland. Most of the ocean floor is, however, not covered with hard substrate, but is sandy or muddy. The large conspicuous, often sessile, animals are not common there, and in fact these habitats may at first look rather lifeless. This, however, is a deceptive impression as most of the benthic animals in these habitats live buried in the bottom. Many large polychaeta worms and bivalves occur there as well as a myriad of smaller species.

Many species of benthos are harvested and several of them are crucial the ecosystem. Important fish species such as haddock, catfish and most flatfishes primarily eat benthos. Many benthic animals, primarily bivalves, sieve the ocean and effectively clean it up. Others clean up the bottom by scavenging on dead organisms. Detritius feeders and other active diggers regularly move the bottom sediments around and therefore increase their oxygen content and overall productivity, much like earthworms on land. There are six main groups of benthic animals, described further below. It should be noted that many other less common groups of benthic animals exist, but most contain few and rare species. Sponges Sponges (phylum porifera) are probably the least animal-like of all animals. They are in fact the most primitive of the multicellular animals. They lack all internal organs, a neural system and sensory organs and can almost better be described as a colony of single celled animals. They do, however, have specialized cells that have different roles within the sponge. Sponges cannot move at all and can look like mats or spongy balls. Some species are erect and branch out like plants.

All sponges are filter feeders. Bathroom sponges were originally made from warm water sponges. However, this is impossible with Icelandic sponges as they contain a multitude of small needles that can badly irritate the skin. Sponges also defend themselves with chemicals and these have drawn the attention of the biotechnology industry. Sea anemones are animals, not plants. Photo: Erlendur Bogason Cnidarians These are the second most primitive animals after the sponges. The cnidarians have a very simple construction, but contain some internal organs as well as a primitive neural system. The best known cnidarians are planktonic jellyfish, but well know benthic groups are corals, sea anemones and hydrozoans. Corals and sea anemones are closely related, but differ, mainly because corals are colonies of very small individuals, while sea anemones are large solitary individuals. Corals are usually associated with the tropics, but coral reefs also occur in deep waters in colder areas of the oceans, including Icelandic waters. Cnidarians are predators that eat whatever reaches their tentacles and they can handle. Cnidarians are not harvested in the North Atlantic.

However, in the Far East some species are harvested and eaten. Bristleworms or polychaets Bristleworms are segmented worms, closely related to earthworms. However, they have quite diverse looks compared to the rather simple looking earthworms. All of them have appendages and bristles, but they vary enormously in size and shape. Some do not look like worms at all. Bristleworms are found all over the ocean bottom, and a few species are even planktonic. The feeding habits are also diverse; they are filter feeders, detritus feeders, scavengers and carnivores. Polychaets have not been harvested commercially in Iceland but have been collected for bait. Polychaets are important food for many fish species, especially flatfishes. Molluscs The three main groups of molluscs are bivalves (shells), gastropods (snails) and cephalopods (squids and octopuses). Feeding habits among these groups are quite different.

Bivalves are primarily filter or detritus feeders, gastropods are algae scrapers or predators on sessile animals, cephalopods are active predators on fishes and other cephalopods. Many species within this group can grow quite large and are an important commercial catch in many parts of the world. Significant commercial species in Icelandic waters are scallop , ocean quahog , blue mussel and whelk . Cephalopods have occasionally been harvested in Icelandic waters. Two crustaceans, a spider crab (Hyas spp.) in the upper left corner and a squat lobster (Munida spp) at the bottom. Photo: Erlendur Bogason Crustaceans Crustaceans are the most species rich group of marine animals and can be found in all marine habitats. Crustaceans in the ocean are comparable to insects on land which they are actually related to. A few crustacean species have also been able to invade land. Not a single species of insect, however, lives below the ocean surface. Benthic crustaceans are of various sizes.

Many, such as amphipods and isopods, are insect-sized and are, as insects are on land, important for the benthic ecosystem as scavengers and grazers. Shrimps are intermediate in size. Many species of shrimp can be found around Iceland, but only one is abundant enough to sustain catches; northern shrimp , commonly referred to as just “shrimp” in Iceland. The largest are the true crabs and lobsters. Several species of crab can be found in Icelandic waters, but no species is abundant enough to sustain commercial catches. However, only one species of lobster lives in Icelandic waters, Nephrops lobster , which is an important commercial species. One group of crustaceans, the barnacles, is sessile; they are found in abundance on the seashore, but also occur deeper. Echinoderms Not many echinoderm species occur in Icelandic waters compared to the above mentioned groups, but they are large and conspicuous animals, and therefore among the best known of marine invertebrates. The five groups of echinoderms have quite different habits but all have similar internal structure. Sea stars are probably the best known group.

Although innocent looking, they are in fact voracious predators on other slow moving or sessile animals. Fishermen consider them a pest when they invade scallop beds. Brittle stars or ophiuroids are closely related to sea stars but smaller and more mobile. They mostly eat detritus or are filter feeders. Sea urchins are grazers or detritus feeders. Sea cucumbers are detritus or filter feeders. The last and least known groups are the filter feeding crinoids. Sea urchins and sea cucumbers have been harvested in small quantities in Icelandic waters. References and further information Gunnarsson, K., Jónsson, G., & Pálsson, Ó. K. (1998). Sjávarnytjar við Ísland (Marine resources in Icelandic waters). Reykjavík: Mál og Menning, 280 pp. (in Icelandic) Sigvaldadóttir, E., Helgason, G. V., Guðmundsson, G., & Fauchald, K. (2000). Bioice: Benthic invertebrates of icelandic waters.

Bulletin of Marine Science 67 (1), 673-674. Hreiðar Þór Valtýsson, University of Akureyri

Question: Where is the ‘benthic zone’?
Answer: Sea Bed