How can POPs Enter and Accumulate in Fish and Other Living Marine Organisms?

POPs in the marine environment:

• The air water film interface
• Open sea sediment deposition and cycling of POPs
• Sediment burial and anoxic environments
• Food web interactions
• Biomagnification or Lipid Control?

POPs in the marine environment

Fig (OSPAR Commission, 2000).

The Air Water Film Interface

Chlorinated hydrocarbons are to a different extent insoluble in water with a saturation concentration of no more than 1 ppb but, they are soluble in fats and adsorb strongly onto particles. The surface layer of the sea is a film with a maximum thickness of 1 mm, which is known to contain fatty acids. Because of their lipid solubility, organochlorines may therefore accumulate in it. The organochlorine enrichment of the surface film may be of considerable importance to surface living organisms or to birds, such as petres, that skim fat off the surface of the sea.

Open Sea Sediment Deposition and Cycling of POPs

Planktonic organisms play a very important role in open sea sedimentation. In fact, in this environment, plankton is the main source of sediments and sediment contamination. When these organisms die, they sink trough the water column from euphotic zones, (where the light is still present), to deep sea environments where they form sediments. Cycling of POPs can therefore occur in these areas. When the particles sink through the water column, a degradation of the organic matrix occurs and consequently, POPs are released into the surrounding water. This process is particularly important close to the water sediment interface, where bacterial activity is more intense and mineralisation occurs at a higher speed.

Sediment Burial and Anoxic Environments

Particles, with POPs adsorbed onto them, can be buried by sediments and thus create submerged pools of contaminants. The contaminants can then be released under particular conditions. It has, for example, been shown that under anoxic conditions, sediments containing organic carbon could release contaminants into the surrounding water. This process is likely to occur also with organochlorines, making the pollutants available for the bentic fauna. Furthermore during anoxia (when oxygen is depleted), gases may be generated which can scrap pollutants from particles or cause physical mixing. A severely contaminated sediment will, inevitably, become a source of pollution (Skei et al., 2000).

Food Web Interactions

Planktonic organisms are the first link for pollutant transfer in the pelagic system. Traditionally, primary producers, (all those organisms that are able to synthesise organic matter capturing the energy of the sunlight) such as phytoplankton have been considered as the initial step for transport of POPs into food webs. Recent studies, however, point out that the capacity of uptake of bacteria is an important route for POPs transportation via the microbial food chain. The microbial food chain is the link between microorganisms in the sea.

Microorganisms in pelagic system are mainly constituted by viruses, bacteria, flagellates ciliates, phytoplankton, and microzooplankton. Because of the high abundance of bacteria, their small size and relatively fast turnover times (hours to days), bacteria cells represent the largest biological surface area in natural waters. This, in addition to other factors, make them especially important as an adsorptive matrix for POPs. Bacteria therefore have the potential to take up a larger proportion of POPs from the water than phytoplankton. (Larsson et al. , 2000). Fig. Amibio.

Biomagnification or Lipid Control?

Water living organism can assume POPs in the following two ways:

• directly from the surrounding environment. Scientists speak about lipid control when this route of transfer to the food chain is the predominant one. This because of the the higher POPs content registered in species with fat molecules more likely to store organochlorine structures.
• Indirectly from contaminated food. In this case scientists speak about biomagnification since the highest POPs concentrations are found in fish, birds and marine mammals, i.e. at the highest steps of the food chain.

Direct uptake and storage of POPs from water is defined as the bioconcentration capacity of a living organism. The bioconcentration process occurs at different steps of the pelagic food web trough mechanisms that are peculiar for each level of the food chain. For example in phytoplankton, POPs diffuse inside the cells trough a gradient of concentration after finding a more or less favourable set of lipomolecules in the external membrane. In fish instead water passes through gills and selectively diffuse into the fatty tissues of the fish. (Baird, 1997).

The bioconcentration factor, (BCF) is the measure of the intensity of bioconcentration in a organism. Every living organism has is own bioconcentration factor. BCF represents the ratio between the concentration of a chemical in an organism relative to the concentration of the same chemical in the surrounding water (BCF=Corganism/Cwater). The BFC is used when the only uptake mechanism of the organism is through diffusion.

The bioconcentration factor of a chemical can be predicted, to within about a factor of ten, from a simple laboratory experiment: the chemical is allowed to equilibrate between the liquid layers in a two phase system made up of water and 1-octanol, which has been found experimentally to be an adequate surrogate for the fatty portions of the fish. The partition coefficient (Kow) for a substance is defined as the ratio of the concentration in octanol and water and it is often reported at a logarithmic scale.

Indirect assumption or biomagnification results from a sequence of bioaccumulation steps along the food chain. E.g. fish can also bioaccumulate organic chemicals from the food they eat and from their intake of particulate in water and in sediments onto which the chemicals have adsorbed. In many such cases the contaminants are not metabolised by the fish, the substance simply accumulates in the fatty tissue of the fish where its concentration increases with time.

In practice, chemicals are biomagnified when experimental data shows an increase of the concentration of the POPs with age of the fish together with a pronounced increase of concentrations through its food web. Conversely, a lipid controlled assumption occurs when a constant concentration of POPs is found in fish fat and no clear increase of POPs is seen in its food web. For example, in the Baltic Sea there is evidence of a lipid controlled mechanism of POPs in fish such as salmons. (Skei et al., 2000).

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