The Benthic Zone And Its Organisms Biology Essay

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Introduction

The primary objective of the essay is to use the functional attributes to analyse and understand the ecology of Marine benthic communities and how the feeding and life traits of these benthic species affects their relationship with the environment and the ecological value of this investigation.

Section 1. This will outline what a benthic organism is and the major abiotic features that affect marine benthic communities both natural and manmade, displaying how benthic communities can adapt or be altered in reaction to these changes.

Section 2. Will explore the relationship between biodiversity in benthic communities and their environment and some impacts species have on each other.

Section 3. How benthic groups influence or impact on other groups within the community and a general description of the major benthic groups feeding and attributes.

Section 1. What is the benthic zone and its organisms, and how is its environment affected by abiotic factors?

The benthic region begins at the shore line (intertidal or eulittoral zone) and extends downward along the surface of the continental shelf out to sea. The continental shelf is a gently sloping benthic region that extends away from the land mass. At the continental shelf edge, usually about 200 meters deep, the gradient greatly increases and is known as the continental slope. The continental slope drops down to the deep sea floor. The deep-sea floor is called the abyssal plain and is usually about 4000 metres deep. The ocean floor is not all flat but has submarine ridges and deep ocean trenches known as the hadal zone. A benthic organism is an organism that spends most of its life living and feeding in the benthic zone. However when breeding its young may be dispersed through the water column as eggs/larvae or have a brief pelagic stage in its life cycle and so on to facilitate dispersal and colonisation of new areas. Most of the animals that live in the benthic zone of the oceans are infaunal, which means that they live mostly in the substrate such as various polychaete worms, mussels and other species. Some organisms are Epifaunal (living above the sediment) are mostly microorganisms, shrimp, molluscs, hermit crabs and are living at the surface of the sediment.

The animals can also be split into groups according to their size,

Microfauna ( < 10μm)

Meiofauna ( > 10μm - <0.5mm or1.0mm depending on benthic biology school)

Macrofauna ( > 0.5mm or > 1.0mm)

Plants are rare except in shallow coastal waters where kelp, sea weeds, sea grass and algae can form or become symbiotic partners with various organisms such as corals or some sponges. In the open sea diatoms and photosynthetic bacteria are normally the only primary producers and as a result much of the organisms that lead a benthic existence rely on marine "snow" which is detritus and other organic material known as POM (Particulate Organic Matter) that gradually sinks in the depths, mainly because light can rarely penetrate deep enough to allow primary productivity efficiently after approximately 100 meters depending on turbidity, unless it can migrate like various diatoms. The other main source of energy is "secondary production" where predation, scavenging and the use of bacteria to digest tougher sources of energy that drift down as POM and become part of the sediment. The distribution and abundance of benthic species can be profoundly influenced by a wide variety of physical and chemical parameters (e.g. substrate composition, water temperature, depth, dissolved oxygen concentrations, pH, salinity,). Temporal and special differences in benthic species communities may also be influenced by a range of biological factors such as primary productivity, competition and acclimatisation by organisms to changing conditions as well as natural succession as colonizing organisms such as polychaete worms and sponges are replaced by more structured communities of benthic organisms. Natural seasonal and inter-annual changes in these variables can also modify recruitment success and mortalities of individual species, and consequently affect the community structure of the benthos. Such cyclical changes and often random inter-annual variation from currents and multi-seasonal cycles often make long-term changes in benthic communities caused by nature humans difficult to detect and to repair.

Benthic communities can change in response to nutrient enrichment causing eutrophication as a result of mostly human activities such as sewerage and industrial effluent being simply pumped into the sea or estuarine waters bringing fertilizer run-off from farms back out to the sea. The effects of nutrients on benthos are theorized to be mainly indirect. Nutrients may initially stimulate benthic communities because more food is supplied in the form of plant material and organic detritus leading to a short term increase in mostly microbacterial activity to use these normally scarce nutrients. However, as sediment organic matter increases, the oxygenated portion of the sediment can become limited to the sediment surface or be eliminated altogether because of its use in respiration by the now blooming bacteria and fungi, dissolved oxygen concentrations can drop to levels that are lethal for some organisms.

Under extreme conditions organic enrichment can lead to increased periods of hypoxia or even anoxia. Under such conditions, mobile organisms that are able to leave the affected area move to less affected areas and sessile species tend to die or be reduced.

Defaunated areas tend to be recolonised by a small range of opportunistic species capable tolerating low oxygen conditions through various adaptations or those better at first exploiting new space opened up as a result of the previous organisms having died or migrated, the new colonists tend to be small and very adaptable to changing conditions such as small polychaete worms, nematodes and certain molluscs. Toxins produced by harmful algae can bioaccumulate to poisonous or lethal levels in molluscs, crustaceans, polychaetes and echinoderms, and cause the loss of herbivorous and predatory species causing fish kills and infamous "red tides".

Heavy metals such as lead or mercury and other toxicants derived from a range of agricultural, industrial and domestic sources can have several lethal and sub-lethal effects on benthic organisms generally affecting reproduction and growth. Under contaminated conditions, communities tend to become simplified as they lose the diversity provided by the previous organisms as they become dominated by fewer more tolerant species able to live and reproduce under those conditions. Physical abnormalities may also occur in the progeny. For example, imposex in marine gastropods (Where the male genitals are reduced and sometimes infertile) is caused by butyl and phenol tins in certain boat paints.

Trawling and dredging disrupts the sediment and resuspends it resulting in smothering of remaining organisms in some cases and causes the dislodging of epifauna and infauna caught in the nets and normally results in the collection and mortality of the bycatch. The magnitude and persistence of dredging impacts varies between species depending on its lifespan and how quickly it can recuperate losses to habitat. The loss of sensitive species can cause a change in community structure, although such changes are often hard to detect and can be small in comparison to natural variability measured over seasons and years and so as a result little is known about the long term negative impacts as the benthic environment is difficult to study reliably.

Modification of river flow to estuaries by various forms of abstraction/regulation in the form of dams, artificial lakes and docks can modify the salinity, water temperature, pH and ionic structure of the water column which in turn can have drastic effects on lower dissolved oxygen concentrations at depth. Replacement of the existing benthic community with other benthic species may occur as a result of physiological stress or by competition and predation by species better physiologically adapted to the more diverse conditions.

Accidently introduced marine pests from ballast water or careless human activities can displace or destroy indigenous marine species directly by predation and competitive exclusion due to lack of predators or accessory introduction of disease that the new species may have a immunity to or indirectly by changing the physical and biological characteristics and structure of habitats forcing a change in the function of the ecosystem. Structural changes include the loss of supporting habitat such as seagrass meadows and sandy soft bottom areas. Functional changes may include altered nutrient cycling up to and including changes in denitrification rates and denitrification efficiencies as well as phosphate levels. The effects of exotic species on benthic communities can be long-term and are often irreversible.

Low pH runoff from acid sulphate soils and farms can cause mass mortalities of crustaceans and shellfish only to be replaced by more tolerant organisms such as small polychaete worms, nematodes and clams.

Section .2 The relationship between biodiversity in

benthic communities and their environment .

There are only a few principles that are accepted in assessing benthic biodiversity and how it affects and develops their environment which I've simplified as the following as the following:

Diversity decreases with increasing latitude and with increasing depth as less and less energy from primary productivity becomes available (With some rare exceptions such as hydrothermal vents and brine seeps)

Diversity increases with habitat size and productivity of said habitat allowing habitat complexity to form as niches are opened up for organisms to adapt to.

Diversity decreases in "stressed" areas such as estuaries and poorly oxygenated waters or areas where eutrophication or excessive disturbance occurs.

The following are a simple list of bio-modification organism's cause to the environment they live in and some of the effects it can cause.

-Biostabilization: Is where various organisms stabilise the sediment they live in or on by various methods such as how spionid polychaete tubes allow compacting of sediment by acting as anchors and disrupting the flow of water around them leading to reduced amounts of sediment resuspension allowing other organisms to live in a much more stable zone, giving rise to a succession of increased habitat size and complexity. Certain Macrofauna such as sea and eel grass form stems and rhizomes that bind and solidify the sediment substrate allow for a more advanced habitat to develop with new species arriving to take advantage of the cover and energy supplied by the Macrofauna as well as the predators that will arrive to keep the herbivores in check and assist in regulating the habitat. Even microorganisms can help stabilise the sediment by producing mucopolysaccharides which stick and bind the sediment together which will also act as a food source for direct deposit feeders.

-Bioerosion: Is where an organism such as Polydora a burrowing polychaete worm or certain bivalves actively bores or digs into a hard substrate can lead to increased niches as organisms such as various fish species, lobsters and moray eels now have places to use as shelter allowing for more habitat complexity to develop.

-Biodeposition: Is a result of feeding by suspension feeders and the production of faeces/pseudofeaces leading to more sedimentation of organically derived material from the water column to the sediment bed. This can allow the formation of various mussel beds and stabilise the sediment as well as opening up niches for sediment eaters and organisms that feed on mussels such as crabs and echinoderms.

-Bioturbation: Is mostly the result of disturbance, egestion and actual physical turnover of the sediment bed, this normally results from the burrowing of organisms such as the Dublin bay Nephrops extensive developing of tunnels for shelter against predators which indirectly provides homes for other organisms such as small crabs and shrimp and a habitat for the red band fish (Cepola rebenscens) or head down feeders such as Arenicola marina which defecates onto the surface sediment. This increase in surface layer material, irrigation and increase in Redox Potential Discontinuity layer (which is essentially how far oxygen is able penetrate the sediment before anaerobic conditions appear) allows for new colonisation and niches to be opened up in the habitat as well as some Meiofauna that live in specific worm tubes and holes and as such are dependent on the disturbance caused by the worms existence. Bioturbation can also cause resuspension of sediments which can prevent other organisms gaining a foot hold in the area as bivalves tend to be smothered and larvae of many organisms cannot get into the sediment before being removed and consequently die as a result.

Section 3.

The Polychaeta or polychaetes are a class of annelid worms, and are mostly marine based. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin leading them to be sometimes called bristle worms. Among the more commonly seen species include the lugworm (Arenicola marina) and the sandworm or clam worm Nereis. Polychaetes are segmented worms and so segment bears a pair of paddle-like and highly vascularised parapodia, which can used for movement and in many species, act as the worm's primary respiratory surfaces, those adapted to burrowing usually lack gills, breathing only through their body surface. Most other species however have external gills generally associated with the parapodia.

Bundles of bristles, called setae, project from the parapodia. However, polychaetes vary widely from this generalised pattern, and can display a range of different body forms depending on habitat and diet. The most generalised polychaetes are those that crawl along the bottom and others have adapted to many different ecological niches in the benthic environment, including burrowing, pelagic life, tube-dwelling or boring, commensalism, and parasitism, requiring various modifications to their body structure. The head, or prostomium, is relatively well developed, compared with other annelids. The mouth of polychaetes varies in form depending on their diet, since the group includes predators, scavengers and parasites (normally Errantia), herbivores, filter feeders (normally Sedentaria). Normally however, it possesses a pair of jaws and a pharynx that can be rapidly everted, allowing the worm to grab food and pull it into the mouth or in some worms move and harvest algae in its tunnels. In some species, the pharynx is modified into a lengthy proboscis. The digestive tract is a simple tube to remove any organic matter from whatever it filters or directly eats.

The head normally includes two to four pair of eyes, although there are some blind species. These eyes are normally only photosensitive spots but some predatory species have developed more advanced eyes . The outer surface of the body wall consists of a simple columnar epithelium covered by a thin cuticle. Underneath this, are a thin layer of connective tissue, a layer of circular muscle, a layer of longitudinal muscle, and a peritoneum surrounding the body cavity. Additional oblique muscles move the parapodia. In most species, the body cavity is divided into separate compartments by sheets of peritoneum between each segment,.

The head also includes a pair of antennae, tentacle-like palps, and a pair of pits lined with cilia, known as "nuchal organs". These latter appear to be chemoreceptors, and help the worm to seek out food and detect predator hormone traces. Most burrow or build tubes in the sediment helping stabilise and allow succession of a more diverse habitat if there is sufficient food available, and some live in commensalism. A few are parasitic. The mobile forms (Errantia) tend to have well-developed sense organs and jaws to catch and feed on prey, while the stationary forms (Sedentaria) lack them but may have specialized gills or tentacles used for respiration and deposit or filter feeding such as Arenicola marina .

Polychaetes have a varying number of protonephridia or metanephridia for excreting waste, which in some cases can be relatively complex in structure. Polychaetes as a class are robust and widespread, with species that live in the coldest ocean temperatures of the abyssal plain, to forms which tolerate the extreme high temperatures near hydrothermal vents. Polychaetes occur throughout the Earth's oceans at all depths and will continue to do so.

Most polychaetes have separate sexes, rather than being hermaphroditic. The most primitive species have a pair of gonads in every segment, but, in most species, there has been some degree of specialisation. The gonads shed immature gametes directly into the body cavity, where they complete their development. Once mature, the gametes are shed into the surrounding water through ducts or openings that vary between species or in some cases by the complete rupture of the body wall (and subsequent death of the adult). A few species copulate, but most fertilise the eggs externally.

The fertilised eggs typically hatch into trochophore larvae, which float among the plankton, and eventually metamorphose into the adult form by adding segments. A few species have no larval form, with the egg hatching into a form resembling the adult, and in many that do have larvae, the trochophore never feeds, surviving off the yolk that remains from the egg.

Some of the polychaetes exhibit remarkable reproductive strategies. Some species in the genus Eunicie reproduce by a process called epitoky. For much of the year, these worms look like any other burrow-dwelling polychaete, but as the breeding season approaches the worm undergoes a remarkable transformation as new, specialized segments begin to grow from its rear end until the worm can be clearly divided into two halves. The front half, the atoke, is asexual. The new rear half is responsible for breeding and is known as the epitoke.

Each of the epitoke segments is packed with eggs and sperm and features a single eyespot on its surface. The beginning of the last lunar quarter is the cue for these animals to breed and the epitokes break free from the atokes and float to the surface. The eye spots sense when the epitoke reaches the surface and the segments from millions of worms burst, releasing their eggs and sperm into the water.

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