Organic carbon or organic matter enters the marine environment from allochthonous (external) and autochthonous (internal) sources.

Natural allochthonous sources include river-borne phytoplankton and organic detritus, and marginal vegetation. These natural sources are supplemented considerably by anthropogenic point sources including sewage effluent, some industrial effluents and cage fish farm installations. The activities of Man in river catchments can increase the export of organic detritus to rivers, estuaries and coastal waters.

Natural autochthonous sources include phytoplankton, marginal submerged vegetation, faunal faeces and pseudofaeces. The increase in loads of nutrients (nitrogen and phosphorus) to the marine environment as a result of Man's activities stimulates the production of autochthonous organic matter, principally in the form of phytoplankton and submerged marginal vegetation (macroalgae). For example, organic matter is derived from decaying seaweed (seasonal scouring of macroalgal, notably kelp communities, can result in large amounts of decaying seaweed being washed onto the shore - in some years more than 40,000 tonnes per year of stranded seaweed is removed from the beach at St Helier, Jersey (Parr et al 1996)). Deposits of organic matter in the marine environment can be redistributed by storm events.

Organic matter occurs in natural waters in dissolved (dissolved organic matter DOM) and particulate (particulate organic matter POM) forms. DOM is the organic matter passing through a 0.5 mm filter and POM is that retained by the filter.

The concentration of organic matter does not appear to be routinely measured in UK tidal waters, but the concentration is likely to vary enormously on both a temporal and spatial basis, depending on river flow, resuspension of sediment during windy/stormy weather. However, total suspended solids and ash-free suspended solids are monitored (subtracting the latter from former provides a measure of the particulate organic matter (POM) of water samples). While inorganic material comprises the majority of total suspended solids in English and Welsh coastal waters, along the Wash and North Norfolk coast, organic material is responsible for a greater proportion of total suspended solids (Parr et al 1998).

Total organic carbon (TOC) is frequently measured in sediments, particularly in monitoring programmes of sediment quality in relation to the disposal of organic wastes.

The fate and behaviour of organic matter in the marine environment is complex and is usefully reviewed for estuaries in Kennish (1986).

Dissolved organic matter (DOM) comprises substances from biological origins (e.g. polypeptides and polysaccharides) and from geological origins (e.g. humic substances). Much of the biological DOM is metabolised by heterotrophic bacteria, while most of the geological DOM is resistant to microbial breakdown. Some other animals can use biological DOM but are largely outcompeted by bacteria and the proportion they use is not nutritionally important for them.

Particulate organic matter (POM) is also metabolised by heterotrophic bacteria but is also used directly by a wide range of estuarine invertebrates both in the water column and in the sediment. POM can be suspended in the water column or be deposited onto sediments, depending on the size and density of particle and the current velocity.

Organic carbon is readily assimilated into the tissues of marine organisms, can be transformed from POM to DOM and is lost to the atmosphere as carbon dioxide by respiration.

The addition of large amounts of organic matter from anthropogenic sources can exceed the capacity of parts of the marine environment to process it, resulting in accumulation usually in the sediments.

The effects of non-toxic substances, such as organic carbon, on the marine environment can be sub-divided into direct effects (those organisms directly affected by changes in the concentrations of organic carbon) and secondary effects (those arising in the ecosystem as a result of the changes in the organisms directly affected).

The addition of organic matter (as DOM or POM) to the marine environment has the direct effect of stimulating heterotrophic bacterial production in the water column and in the sediments.

The addition of POM, in particular, stimulates the production of invertebrate detritivores in the water column. The response of benthic invertebrate communities to increasing inputs of organic material has been characterised by Pearson and Rosenberg (1978). There are two distinct phases in the response often referred to as organic enrichment and organic pollution.

Organic enrichment encourages the productivity of suspension and deposit feeding detritivores and allows other species to colonise the affected area to take advantage of the enhanced food supply. The benthic invertebrate community response is characterised by increasing numbers of species, total number of individuals and total biomass. This type of impact is typical of many modern sewage effluent discharges in the marine environment (e.g. O'Reilly et al 1998).

Organic pollution occurs when the rate of input of organic matter exceeds the capacity of the environment to process it. Commonly, there is an accumulation of organic matter on the sediment surface that smothers organisms, depletes the oxygen concentrations in the sediment and sometimes the overlying water which in turn changes the sediment geochemistry and increases the exposure of organisms to toxic substances associated with organic matter. The benthic invertebrate community response is characterised by decreasing numbers of species, total number of individuals and total biomass and dominance by a few pollution tolerant annelids. This type of impact is not common other than in localised areas in the estuaries and coastal waters of the UK but has recently been observed in relation to cage fish farm installations.

Eleftheriou et al (1982) showed that an organic loading rate of 2.1 g C.m^-2.d^-1 in a sea loch resulted in an enriched sediment fauna, while a loading rate of 4.1 g C.m^-2.d^-1 on marine mesocosms degraded the sediment community. Mesocosm studies at the University of Rhodes (Kelly and Nixon 1984, Frithsen et al 1987, Oviatt et al 1987 and Maughan and Oviatt 1993) showed that, while organic loading rates of 0.1 g C.m^-2.d^-1 had little effect on benthic community status, loading rates of 0.1-1.0 g C.m^-2.d^-1 produced an enriched community and rates in excess of 1.5 g C.m^-2.d^-1 degraded the benthos community.

Edwards and Jack (1994) suggested that the loading rates at which detrimental effects were reported varied so much between the mesocosm and sea loch studies because all of the organic matter applied in the mesocosms was deposited in the experimental system while in the sea loch study, a large proportion of the organic matter applied was probably lost from the immediate area being studied.

The model BenOss predicts the pattern of sedimentation and biological impact of organic matter around marine outfalls, relying on sedimentation coefficients for a range of particle sizes while taking account of dispersion by currents and turbulence. The model employs critical resuspension speed of 9.0 cm.s^-1 and a critical deposition speed of 4.5 cm.s^-1 (Cromey et al 1996). The model also predicts the effects of excess organic carbon from outfalls on the benthic faunal population.

Increasing amounts of both DOM and POM can contribute towards increased turbidity in the water column.

Stimulation of microbial degradation in the water column and sediments results in an increased oxygen demand and decrease in available oxygen for other organisms. Microbial degradation of organic matter also results in the mineralisation of nutrients (N and P) providing additional sources of these nutrients for phytoplankton and macroalgae. The effects of eutrophication and organic enrichment are difficult, if not impossible, to distinguish. However, the distinction is only important when considering control mechanisms on the supply of nutrients (N and P) and organic matter.

The effects of increased oxygen demand in water and sediments are considered in more detail in the section on dissolved oxygen. In the water column, reduced oxygen concentrations can result in dissolved oxygen sags (DO sags) in estuaries which can act as a barrier to migratory fish and damage estuarine fish communities. The combined effects of organic pollution by sewage, including the input of organic carbon and consequent depletion of dissolved oxygen in the water column, resulted in the eradication of the fish populations in the Thames estuary. Judicious management of water quality, including the artificial aeration of the water column, has allowed the fish populations to recover almost completely. Such a dramatic effect on the fish community must have had an equally dramatic effect on piscivorous birds using the estuary.

Organic enrichment of sediments in the marine environment can stimulate the production of benthic invertebrate communities which, while different in composition from pre-enrichment conditions, can provide significant food supplies for fish and birds. It has been suggested that some of the bird populations at European marine sites in estuaries are sustained by benthic invertebrate production resulting from anthropogenic sources of organic carbon, such as sewage (e.g. Pearce 1998). Concerns have been raised that current pollution control policy aimed at reducing the organic carbon content of sewage discharges might adversely affect these bird populations.

Organic pollution of the sediments tends to result in a reduction in biodiversity of benthic invertebrate communities and the dominance by relatively few taxa, usually annelid worms. While these taxa are of some importance to fish and birds as food items, the reduction in the variety of prey will result in a decrease in the variety of fish and bird predators.

The contribution of POM to turbidity in the water column can contribute to reduced production of phytoplankton, macroalgae and other plants.

Potential effects include:

• stimulation of heterotrophic bacterial production in the water column and sediments;
• mineralisation of nutrients (N and P) from organic matter and contribution towards the effects of eutrophication;
• organic enrichment of the water column and sediment by stimulation of detritivore production, resulting in a change in community composition in favour of these animals and, possibly, their predators, including sea birds;
• organic pollution of the water column and sediment which can reduce the biodiversity of animal and plant communities with adverse consequences for fish and bird populations;
• increased oxygen demand in the water column that can have sub-lethal and lethal effects on fish populations, resulting in reductions in biodiversity with possible consequences for fish eating birds;
• contribution to turbidity in the water column with potential to suppress production of phytoplankton, macroalgae and other submerged aquatic plants.

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