Organic Enrichment

The effects of organic enrichment on sedimentary systems and their benthos is well documented and shows a consistent sequence of response - the Pearson-Rosenberg model (Pearson & Rosenberg, 1978). The organic matter may be as particulates or dissolved, including nutrient enrichment and can be derived from many sources: sewage, either discharged as domestic or industrial effluent to intertidal and inshore areas, or as sludge dumped to subtidal areas, food and waste from aquaculture, pulp and paper-mill effluents, and degraded petroleum hydrocarbons. The major causes of such changes are point-sources in which the assimilative capacity of the receiving waters is insufficient to degrade the organic matter.

In essence, high organic inputs, coupled with poor oxygenation leading to conditions of slow degradation will produce anaerobic chemical conditions in the sediments. In turn, this increases microbial activity and reduces the redox potential of the sediments (Fenchel & Reidl, 1970). Ultimately this increases the production of toxins such as hydrogen sulphide and methane. The changed status to anaerobiosis will limit the sedimentary macroinfauna in anoxic/reducing muds to species which can form burrows or have other mechanisms to obtain their oxygen from the overlying water.

The changes to the primary benthic community parameters of species richness, biomass and abundance as the result of organic enrichment are described by SAB curves (Gray, 1982). Moderate enrichment provides food to increase the abundance and a mixing of organisms with different responses increases diversity (Elliott, 1994). With greater enrichment, the diversity declines and the community becomes increasingly dominated by a few pollution-tolerant, opportunistic species such as the polychaetes Capitella capitata, in sand flats, and Manayunkia aesturina in mudflats. In grossly polluted environments, the anoxic sediment is defaunated and may be covered by sulphur-reducing bacteria such as Beggiatoa spp. Such a change will affect the palatability of the prey and thus impair functioning of marine areas. This sequence has been observed on intertidal mudflats (e.g. McLusky, 1982) as the result of organic petrochemical effluents, sandflats and sandbanks (e.g. Majeed, 1987) as the result of hydrocarbon pollution.

Any nutrient stimulation of marine areas may be regarded as hypernutrification which, if not controlled, produces symptoms of eutrophication, defined as the adverse effects of organic enrichment (Scott et al, 1997). Such a symptom on intertidal sand and mudflats is an increased coverage by opportunistic green macroalgae, such as Enteromorpha, which will create anoxic conditions in the sediment below the mats, reduce the diversity and abundance of infauna and interfere with bird feeding (Simpson, 1997).

Changes in the species composition and density of benthic diatoms of an intertidal brackish mudflat diatom populations is also evident after organic enrichment (Peletier, 1996). This may be the result of the reduced densities of the macrofaunal diatom grazers Nereis diversicolor and Corophium volutator. As a consequence of this change in the microphytobenthos, because of the role of their role in mucous production, there may be a reduction in the stability of intertidal mudflats (see Chapter II).

Of particular concern in certain parts of the marine environment is the increase in the inshore aquaculture of salmonids and its high potential for increasing organic enrichment of sedimentary areas. This industry produces a point-source input of waste, excess food and excretory material (e.g. Gowen & Bradbury, 1987; Brown et al, 1987). However, although small and marginal intertidal mudflats in sealochs may be affected, it is unlikely that subtidal sandbanks, typical of more open coastlines, will be degraded. The present management practices, in selecting sites with good water exchange, will reduce the possibility of organic enrichment by enhancing the dispersal of solid waste.

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