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Shellfish Water Quality

There are six designated shellfish waters located in or close to the study area. The location of these shellfish waters can be seen in the Surface Waters (Shellfish) (Classification) Regulations 1997 map.

Shellfish Waters

NB: Any land above the Mean High Water Mark is excluded from the designation e.g., islands that fall within these areas .

The Shellfish Waters Directive (79/923/EEC) is designed to protect the aquatic habitat of the bivalve and gastropod molluscan species of shellfish. Species covered include oysters, mussels, cockles, scallops and clams but not shellfish crustaceans such as crabs, crayfish and lobsters. The Shellfish Waters Directive is implemented in the UK under the Shellfish Waters (Shellfish) (Classifications) Regulations 1997.

Shellfish waters are monitored for various parameters based on water quality standards established by the Directive. These parameters include suspended solids, salinity, dissolved oxygen (DO), organo-halogenated substances (e.g. PCBs, organochlorine pesticides), metals and guideline values for coliforms in shellfish flesh. For each substance, the Directive specifies the minimum number of samples to be taken, the water quality standards to be met and the percentage of samples that must meet these standards. The standards are either a numeric limit or a descriptive standard (see the table below). The water quality standards have been met if the following percentage of the samples analysed do not exceed the limit values:

  • 100% for metals and organo-halogen compounds;
  • 95% for salinity and dissolved oxygen (DO);
  • 75% for other substances; and
  • No evidence of harm to the shellfish from organo-halogenated compounds.

 

Parameter

Units

Standard

Suspended solids

mg/l

A discharge affecting shellfish waters must not cause the suspended solid content of the waters to exceed by more than 30% the content of waters not affected.

Salinity

Parts per thousand (i.e. g/l)

?40 parts per thousand
A discharge affecting shellfish waters must not cause the salinity to exceed by more than 10% the salinity of the waters not affected.

Dissolved oxygen

% saturation

Average of individual values >70% and an individual measurement may not indicate a value lower than 60% unless there are no harmful consequences for the development of shellfish colonies.

Organo-halogenated substances

-

The concentration of each substance in the shellfish waters or in the shellfish flesh must not reach or exceed a level, which has harmful effects on the shellfish and their larvae.

Metals (Ag, As, Cd, Cr, Cu, Hg, Ni, Pb, Zn)

mg/l

The concentration of each substance in the shellfish waters or in the shellfish flesh must not reach or exceed a level, which has harmful effects on the shellfish and their larvae. The synergic effects of these metals must be taken into consideration.

 

This table summarises the Environment Agency’s shellfish water quality monitoring data for the designated shellfish waters for the period 1999 to 2001. Samples are taken at designated points within each of the shellfish water selected to be representative of the quality of the whole area of shellfish water.

Parameter

Units

Southend

Outer Thames

Foulness

Sheppey

Suspended solids

mg.l-1

31

27

No data

33

Salinity

g.l-1

28

31

No data

31

Dissolved oxygen

% saturation

94

103

103

100

Organo-halogenated substances

µg.l-1

All reported as"<" values. All values <0.005 µg.l-1 (or less)

All reported as"<" values. All values <0.005 µg.l-1 (or less)

All reported as"<" values. All values <0.005 µg.l-1 (or less)

Many reported as"<" values. All values <10 µg.l-1 (or less). Variations include total values of DDT, HCH and 'drins, for which no data has been recorded since December 2000

Metals (dissolved)
As
Cd
Cr
Cu
Hg
Ni
Pb
Zn

µg.l-1

 

1.92
0.06
0.26
4.17
0.03
1.63
0.33
9.78

 

1.48
0.12
0.52
3.20
0.02
1.11
0.19
5.12

 

1.41
0.18
0.70
1.55
0.02
0.93
1.2
9.87

 

1.7
<0.25
0.55
2.23
<0.01
<3
<2.5
7.90

In 2013  the directive will be repealed by the EC WFD, in order to stabilise or improve the level of protection offered to shellfish waters (which are classifed by the WFD as protected areas)

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