Chemical monitoring in the Dutch Wadden Sea by means of benthic invertebrates and fish
9 Biologische Anstalt Helgoland, Hamburg
0 Ministry of Transport and Pubfic Works, Rijkswaterstaat, Tidal Waters Division; P. O. Box 207, 9750 A E Haren , The Netherlands
In monitoring, it is of utmost importance to carefuUy define the purpose, the sampling strategy, as well as the analytical chemical and statistical requirements. Surveys are appropriate for describing the geographical variation in environmental contaminant levels, Repeated surveys and recurrent data collection at permanent locations provide means of detecting temporal trends. Results are presented here of surveys on pollution by trace metals, polychlorinated biphenyls and organochlorine pesticides in the Eros Estuary and Dutch Wadden Sea using Mytilus edulis, Mya arenaria, Arenicola marina, Nereis diversicolor and Crangon crangon as test organisms. Trends towards decreasing pollution by mercury are illustrated by monitoring data on Mytilus edulis and Zoarces viviparus. It is stressed that the results of chemical monitoring in organisms may be interpreted only in terms of the biological effects on the basis of relevant toxicological knowledge and/or additional bio-assays.
C h e m i c a l; m o n i t o r i n g; in t h e D u t c h W a d d e n; S e a b y m e a n s; of b e n t h i c i n v e r t e b r a t e s a n d fish*
K a r e l E s s i n k
pollution will disperse, as well as in areas with valuable natural resources. Reference
areas, i.e. areas with minimal pollution impact, should be included in a monitoring
programme to increase the possibility of discriminating between naturally fluctuating
and pollution-induced parameter values. These discriminations are essential to
authorities responsible for the management of estuarine and coastal water bodies (cf.
Beukema & Essink, 1986).
The question of how monitoring is to be performed pertains to the sampling strategy
(network of samphng stations, sampling frequency, sample size and number) (Cuff &
Coleman, 1979; Pearce & Despres-Patanjo, 1988) and analytical methods, including a
quahty-assurance programme (Cofino, 1989). The entire set-up of the monitoring
programme should enable a sound statistical treatment of the d a t a (Anonymous, 1982;
Phillips & Segar, 1986; Segar & Stamman, 1986).
In this paper, data will be presented on surveys of contaminant concentrations (trace
metals and organochlorines) in benthic invertebrates in the Eros Estuary, Dutch Wadden
Sea and coastal waters of western Europe. Results of trend monitoring of mercury
pollution in the Dutch Wadden Sea will be illustrated by data on the mussel Mytilus
edulis and the viviparous blenny Zoarces viviparus.
In 1984, a small-scale survey was carried out in the Ems Estuary (Essink et al., 1986).
The purpose of this survey was to identify pollution sources of PCBs and some
organochlorine pesticides. Three benthic invertebrates, the bivalve Mya arenaria and the
polychaetes Arenicola marina and Nereis diversicolor were used. M. arenaria and A.
marina were sampled at three locations; N. diversicolor was sampled at seven locations
on the intertidal flats of the estuary.
Figure I shows that elevated concentrations of hexachlorobenzene (HCB) were
found at location 4, indicating a major source of HCB near Delfzijl. Similar results were
obtained for the PCB congeners PCB-101, PCB-138, PCB-153, PCB-180 and, less clear,
for PCB-28 and PCB-52 (Fig.2), and hexachlorobutadiene (HCBu) (Essink et al., 1986).
Near Delfzijl,industrial waste discharge was responsible for the observed distribution of
pollution by HCB and HCBuin the Ems Estuary. As to the PCBs, there is no knowledge of
discharge of PCB-containing wastewater. However, in Delfzijlfresh water is sluiced out
from a large drainage area, which is supposed to contain PCBs. For the first four
PCBcongeners mentioned, the concentrations in N. diversicolor are higher at locations 1 to 3
(outer part of the estuary) than at locations 5 to 7 (Fig.2), indicating that for these
contaminants the coastal waters of the North Sea are an additional source. Apparently,
the congeners PCB-28 and PCB-52 have a much more diffuse origin.
Quite a different distribution pattern of concentrations was found for the pesticide
yHCH (hndane) (Fig.3). The higher concentrations found at location 1 are attributed to
local discharges of surplus fresh water from Dutch agricultural areas as well as to the
relatively high y-HCH concentrations in coastal waters due to discharges by the rivers
Weser and Elbe (Gaul & Ziebarth, 1983).
Surveys extending over a larger area, viz. the Dutch Wadden Sea and Ems Estuary,
have been carried out for organochlorines and trace metals (Dulnker et al., 1983; Kramer
et al., 1985). Figure 4 shows that for HCB and y-HCH the Ems Estuary (locations 4 and 5)
Fig. 1, Map of the Eros Estuary with sampling locations (1-7) for intertidal benthic invertebrates (a)
and concentrations of hexachlorobenzene (HCB) in pooled samples of Arenicola marina, Nereis
diversicolor and Mya arenaria in 1984 (b)
a n d the westernmost part of the W a d d e n Sea (location 1) are more c o n t a m i n a t e d t h a n the
central part (locations 2 a n d 3). A similar c o n t a m i n a t i o n p a t t e r n was found for c a d m i u m in
1982 (Pig. 5). So, in the area surveyed two c o n t a m i n a t e d s u b a r e a s are present, i n d i c a t i n g
two important sources of pollution. This also holds for pollution by mercury as assessed by
a survey of intertidal mussels Mytilus edulis in 1971-73 (De Kock & Kuiper, 1981).
Large-scale surveys as carried out b y De Wolf (1975) along the West E u r o p e a n coasts
b e t w e e n Arcachon (Prance) a n d Cape S k a g e n (Denmark), a n d b e t w e e n L a n d ' s E n d a n d
E d i n b u r g h (Great Britain) e n a b l e us to consider local pollution in a w i d e r g e o g r a p h i c a l
and e v e n international context. In 1971/72, m e r c u r y contents in intertidal mussels
Mytilus edufis from the Ems Estuary w e r e a m o n g the highest found in the entire survey.
In S e p t e m b e r 1988, another large-scale survey was carried out by the Tidal Waters
Division including the Danish, G e r m a n and Dutch W a d d e n Sea as well as t h e r e m a i n i n g
Dutch coastal zone as far as the Western Scheldt estuary. In this survey, s a m p l e s w e r e
I Arenieola mar.
Fig. 3. Concentrations of y-HCH in pooled samples of Arenicola marina, Nereis diversicolor and Mya
arenaria in the Ems Estuary in 1984. See Figure la for locations
Repeated surveys, carried out in the Dutch W a d d e n Sea in 1982, 1985 a n d 1986, offer
the possibility of detecting trends in trace metal concentrations of certain i n v e r t e b r a t e
species. In Figure 6, data are p r e s e n t e d on concentrations of copper a n d c a d m i u m in the
species Arenicola marina a n d Crangon crangon. The concentrations are m e a n values of
the concentrations found in three different l e n g t h groups of animals. A trend for copper is
not p r e s e n t at a n y of the locations sampled. For cadmium, however, a t r e n d of d e c r e a s i n g
concentrations is observed. The concentrations found in 1985 a n d 1986 are considerably
lower t h a n in 1982 at most of the locations. The higher concentrations o b s e r v e d at
location 8 in 1986 as compared with 1985 are not statistically significant. The decrease of
c a d m i u m contamination in the w e s t e r n part of the Dutch W a d d e n Sea can be e x p l a i n e d
by the strong reduction of the discharges via the river Rhine: from ca 100 tonnes.yr-1 in
1980/81 to ca 10 tonnes.yr-1 in !984/85 (Folkertsma, 1987). No data are available on
c a d m i u m discharges in the Eros Estuary.
Fig. 4. M a p of the Dutch Wadden Sea and Eros Estuary (a) and relative concentrations of HCB (e)
and y - H C H (1) in Arenico]a maz~na at five locations in 1979 (b). Concentrations found at location 2
are set at 1. (After DuJnker eta]., 1983)
Monitoring of contaminants in the biotic c o m p a r t m e n t of the ecosystem, e.g. the
water column, as compared to the abiotic compartment, has some advantages.
Contami n a n t concentrations in an organism are usually well above the detection limit of
analytical methods. Furthermore, these concentrations represent a n i n t e g r a t i o n of
temporal variations in the occurrence of the c o n t a m i n a n t s in the aquatic e n v i r o n m e n t . An
essential prerequisite, however, for a p p l y i n g biota in monitoring p r o g r a m m e s is a
stationary mode of life of the species concerned. This prerequisite restricts m e a n i n g f u l
chemical monitoring in biota to sedentary organisms, such as fucoids a n d macrobenthic
CU ug.g -1
Cd JJg.g -1
Fig. 7. Decrease of mercury concentrations in Mytilus edulis in the Ems Estuary. A: mean and range
of local intertidal population. B: mean and range of transplanted mussels. (After Pries et al., 1984)
Fig. 8. Decrease of mercury concentrations in 180 mm Zoarces viviparus from western Dutch
Wadden Sea and Ems Estuary. Concentrations and 95 % confidence intervals estimated from
regressions of mercury concentration on length. (After Essink, 1980, 1985, 1988). Baseline level
founded on data from Scotland (see text)
f r o m E m s E s t u a r y a n d w e s t e r n D u t c h W a d d e n S e a n o l o n g e r d i f f e r e d s i g n i f i c a n t l y i n 1981
(Fig. 8), a s i g n i f i c a n t d i f f e r e n c e in fry s u r v i v a l w a s f o u n d . T h e m e a n n u m b e r of d a y s a f t e r
w h i c h 50 % of t h e fry of a f e m a l e w a s still a l i v e ( s u r v i v a l t i m e : STso) w a s g r e a t e r i n fry
b o r n of f e m a l e s f r o m t h e w e s t e r n D u t c h W a d d e n S e a (Table 1). S u r v i v a l w a s b e t t e r i n t h e
a r e a t h a t h a d e x p e r i e n c e d l e s s e r m e r c u r y p o l l u t i o n . In S w e d e n , fry of e e l p o u t h a v e b e e n
s u c c e s f u l l y u s e d i n b i o - a s s a y s t u d i e s b y J a c o b s s o n e t al. (1986).
A c k n o w l e d g e m e n t s . Thanks are due to Dr. J . M . Everaarts (Texel) for permission to use his
unpublished data on trace metals in intertidal invertebrate species. R. Jungcurt prepared the figures.
L I T E R A T U R E C I T E D
Fig. 5. Cadmium concentrations in Arenicola man'na (pooled samples of specimens weighing 3-5 g) and Mytilus edulis (mean of pooled samples of 3--4, and 5-6 cm shell length) at 10 locations in the Dutch Wadden Sea and Ems Estuary in 1982. (After Kramer et al ., 1985 )
Fig. 6. Concentrations of copper and cadmium m Arenicola marina (Aren. mar.) and Crangon crangon (Cra . cra. ) at 5 locations in the Dutch Wadden Sea and Eros Estuary in 1982 , 1985 and 1986 . ( From J. M. Everaarts , unpubl.)
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