A.R.D. Stebbing . R.I. Willows
microlayer, as are the littoral fauna and flora with each tidal excursion. While it is
not known to what extent the indigenous biota are affected, micro layer samples
taken offshore in the German Bight were found to be toxic when tested with bioassay techniques, and TBT concentrations were found to exceed the UK EQS (Hardy
and Cleary 1992).
Different mechanisms are involved in the accumulation of contaminants at the
frontal systems that are the boundaries between water masses. Work by Tanabe
et al. (1991) has shown that persistent organochlorines (PCBs, DDT, HCH isomers)
occur at elevated concentrations in frontal regions, due to their affinity for lipids
and particles concentrated by fronts. This has been demonstrated for water samples
taken at the sea surface, but it is also true for the benthic sediments beneath frontal regions, due to the deposition of particles to which organochlorines become
adsorbed. The possible accumulation of contaminants at fronts in the North Sea
does not appear to have been investigated.
There is evidence that the thermocline and pycnocline may provide the kind of
density discontinuity that results in the accumulation of contaminants. Further
reconcentration of sea surface contamination can occur due to Langmuir circulation in open water, or axial estuarine convergences on the flood tide causing slicks
(R. G. Uncles, pers. comm.). Suspended sediments and their associated contaminants may be concentrated in estuaries at turbidity maxima due to tidal pumping
(Uncles et al.1988), or by gyres drawing suspended particles to their centres due
to centripetal forces and depositing them (Stebbing et al. 1984). Accumulation at
all such interfaces is of particular importance as these are often sites of intense
biological activity. This is because the same processes that accumulate biogenic
material, (organic matter and nutrients) on which organisms feed, also concentrate
chemical contaminants. The localized coincidence of contaminants and biota is
likely to result in exposure and cause toxic effects.
8.3.3
For Individual Contaminants
Early thinking on the "dispersion capacity" of the environment (Holdgate 1979) was
formulated in terms of the relationship between concentrations of an individual
contaminant and the capacity to disperse it (Fig. 8.2). The key statutory instrument
by which water quality is managed is the environmental quality standard (EQS).
EQSs are required under both EC (Dangerous Substances Directive, 76/464/EEC)
and UK (Environmental Protection Act 1990) legislation. An EQS is defined as the
concentration of a substance which should not be exceeded in the receiving water
in order to protect the use of the water; they provide the standards by which the
regulatory authorities operate to monitor the aquatic environment and assess the
possible biological effects of contaminants.
Critically, EQSs provide the one statistic that links contaminant concentration
in the environment to its potential biological impact, so it is important to consider
briefly how these aim to protect biological water quality by controlling chemical
inputs (Zabell, pers. comm.). First, the acute and chronic toxicity data are reviewed.
After considering available data on a contaminant (chemical/physical properties,
behaviour pathways and fate, analytical method,<;, environmental concentrations),
microlayer, as are the littoral fauna and flora with each tidal excursion. While it is
not known to what extent the indigenous biota are affected, micro layer samples
taken offshore in the German Bight were found to be toxic when tested with bioassay techniques, and TBT concentrations were found to exceed the UK EQS (Hardy
and Cleary 1992).
Different mechanisms are involved in the accumulation of contaminants at the
frontal systems that are the boundaries between water masses. Work by Tanabe
et al. (1991) has shown that persistent organochlorines (PCBs, DDT, HCH isomers)
occur at elevated concentrations in frontal regions, due to their affinity for lipids
and particles concentrated by fronts. This has been demonstrated for water samples
taken at the sea surface, but it is also true for the benthic sediments beneath frontal regions, due to the deposition of particles to which organochlorines become
adsorbed. The possible accumulation of contaminants at fronts in the North Sea
does not appear to have been investigated.
There is evidence that the thermocline and pycnocline may provide the kind of
density discontinuity that results in the accumulation of contaminants. Further
reconcentration of sea surface contamination can occur due to Langmuir circulation in open water, or axial estuarine convergences on the flood tide causing slicks
(R. G. Uncles, pers. comm.). Suspended sediments and their associated contaminants may be concentrated in estuaries at turbidity maxima due to tidal pumping
(Uncles et al.1988), or by gyres drawing suspended particles to their centres due
to centripetal forces and depositing them (Stebbing et al. 1984). Accumulation at
all such interfaces is of particular importance as these are often sites of intense
biological activity. This is because the same processes that accumulate biogenic
material, (organic matter and nutrients) on which organisms feed, also concentrate
chemical contaminants. The localized coincidence of contaminants and biota is
likely to result in exposure and cause toxic effects.
8.3.3
For Individual Contaminants
Early thinking on the "dispersion capacity" of the environment (Holdgate 1979) was
formulated in terms of the relationship between concentrations of an individual
contaminant and the capacity to disperse it (Fig. 8.2). The key statutory instrument
by which water quality is managed is the environmental quality standard (EQS).
EQSs are required under both EC (Dangerous Substances Directive, 76/464/EEC)
and UK (Environmental Protection Act 1990) legislation. An EQS is defined as the
concentration of a substance which should not be exceeded in the receiving water
in order to protect the use of the water; they provide the standards by which the
regulatory authorities operate to monitor the aquatic environment and assess the
possible biological effects of contaminants.
Critically, EQSs provide the one statistic that links contaminant concentration
in the environment to its potential biological impact, so it is important to consider
briefly how these aim to protect biological water quality by controlling chemical
inputs (Zabell, pers. comm.). First, the acute and chronic toxicity data are reviewed.
After considering available data on a contaminant (chemical/physical properties,
behaviour pathways and fate, analytical method,<;, environmental concentrations),