Chemical monitoring in the Dutch Wadden Sea by means of benthic invertebrates and fish
0 Ministry of Transport and Pubfic Works, Rijkswaterstaat, Tidal Waters Division; P. O. Box 207, 9750 A E Haren , The Netherlands
1 9 Biologische Anstalt Helgoland , Hamburg
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. * Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1-4 November 1988)
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
I N T R O D U C T I O N
In formulating a p r o g r a m m e for chemical monitoring, it is essential to consider at
least the questions Why?, What?, Where? and How? It should be m a d e clear why
monitoring is n e e d e d , or in other words which purpose(s) should be s e r v e d by monitoring
(cf. Meijers, 1986)
. A monitoring p r o g r a m m e can be set up to c h e c k on set standards with
respect to the d e g r e e of contamination of the ecosystem. T h e results will t h e n show
w h e t h e r those standards are e x c e e d e d or not. If standard values are e x c e e d e d ,
authorities can take action. 'A monitoring p r o g r a m m e can also serve the p u r p o s e of
d e t e c t i n g trends, e.g. a decrease or increase of chosen p a r a m e t e r s with time.
What is to be monitored has to be defined carefully, not only with respect to chemical
compounds, but also with respect to the ecosystem c o m p a r t m e n t (water-dissolved,
waterparticulate, sediment, organisms) in which the chemical c o m p o u n d s are to be monitored.
If organisms are chosen, one or more species are to b e selected r e l e v a n t to the previously
defined purposes of monitoring. D e p e n d e n t on the organism(s) and c h e m i c a l
compound(s) selected, a decision has to be m a d e w h e t h e r the w h o l e organism or only a
specified organ or tissue is to be used for the chemical analysis.
W h e r e monitoring is to be p e r f o r m e d is of course largely d e t e r m i n e d by the purposes
defined. Monitoring in order to c h e c k on set standards may be appropriate in k n o w n "hot
spots". Monitoring for trends is m e a n i n g f u l in areas to w h i c h it can be f o r e s e e n that
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
. 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 organo
chlorine 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 y
HCH (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)
)jg. g-1 fat
1 . 4 - - |
0 . 8
0 . 6
0 . 2
[ - - I
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
0 . 4
0 . 4
0 . 4
0 . 8
0 . 4
1 2 3 4 5 6 7
Fig. 2. Concentrations of 6 PCB congeners in pooled samples of Nereis diversicolor in the Ems
Estuary in 1984. See Figure la for locations
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
2 . 8
2 . 4
2 . 0
0 . 8
0 . 4
collected of Mytilus edufis, Arenicola marina a n d Macoma balthica which will be
a n a l y s e d for trace metals a n d org'anohalogens.
Carrying out surveys b y collecting organisms from the field m a y be referred to as the
"passive" method. The "active" method of exposing organisms at different locations for
some time to a m b i e n t pollutant concentrations can also be u s e d (see below). A n example
of this active method, applying Mytilus edulis to survey zinc c o n t a m i n a t i o n in Dutch
coastal waters, is given b y De
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
. No data are available on
c a d m i u m discharges in the Eros Estuary.
= 0 . ,
F i g u r e 7 shows the results of t r e n d m o n i t o r i n g for m e r c u r y p o l l u t i o n in the Ems
Estuary u s i n g Mytilus edulis (Pries et al., 1984). For this m o n i t o r i n g p r o g r a m m e two
different m e t h o d s w e r e used. At first, "passive" monitoring was a p p l i e d , u s i n g mussels
from local populations. Later, the "active" m e t h o d was used, in w h i c h m u s s e l s from a
r e l a t i v e l y u n p o l l u t e d r e f e r e n c e location w e r e t r a n s p l a n t e d to the monitor locations a n d
1 2 3 4 5 6 7 8
exposed for some time
(cf. De Kock & Kuiper, 1981; De Kock, 1983)
. The strong decrease
of mercury concentrations in mussels coincides with a decrease of Dutch mercury
discharges into the estuary (cf. Essink, 1988), which indicates the positive effect of the
g o v e r n m e n t - i m p o s e d pollution a b a t e m e n t scheme.
Similar results have b e e n obtained in a monitoring p r o g r a m m e u s i n g viviparous
b l e n n y (Zoarces viviparus), a non-migratory teleostean fish
(Essink, 1980, 1985, 1988)
significant decrease in contamination could be d e m o n s t r a t e d for the w e s t e r n Dutch
W a d d e n Sea as well as for the Eros Estuary (Fig. 8), following a n effective reduction of the major mercury discharges into Dutch coastal waters (cf. Essink, 1988). In Figure 8, a baseline level of 70 ~tg.kg -1 wet weight is p r e s e n t e d derived from Scottish data (Essink, 1985, 1988).
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
Cd ug,g - 1
T82 85 86I
i82 85 8SI
]Jg. g -1
infauna, or to stationary species such as Zoarces viviparus. The use of eggs of
shoreb r e e d i n g bird species also meets this prerequisite as is shown by
Chemical monitoring in organisms c a n n o t be considered equal to biological effects
monitoring; both types of monitoring are rather complementary. Chemical monitoring in
organisms does provide information on the a c c u m u l a t i o n of c o n t a m i n a n t s in these
organisms. This information may be interpreted in terms of biological effects. However,
such a n interpretation can be based only on sound k n o w l e d g e of the kinetics and
toxicology of the c o n t a m i n a n t s and species c o n c e r n e d
(cf. A d e m a et al., 1972; Boon,
1985; Boon & Duinker, 1986; Everaarts, 1986; Reijnders, 1986; De Wolf et al., 1972)
Concentrations of c o n t a m i n a n t s in intertidal invertebrates, although b e i n g sampled
at the same location, may vary largely b e t w e e n species (Figs 1, 3, 5 a n d 6). This implies
that a species which is quite appropriate for monitoring certain c o n t a m i n a n t c o m p o u n d s
will not necessarily b e appropriate for other pollutants. So the proper choice of species in
a chemical monitoring p r o g r a m m e is highly d e p e n d e n t on the purpose set with respect to
the r a n g e of c o n t a m i n a n t s to b e monitored
(cf. Zauke et al., 1987, 1988)
Monitoring of local populations is not always possible b e c a u s e of the (temporary)
a b s e n c e of the species that has to be monitored. "Active" monitoring m a y b e applied to
overcome this problem. There is, however, at least one d r a w b a c k to be m e n t i o n e d . What
h a p p e n s when, in an active monitoring programme, Mytilus edulis is t r a n s p l a n t e d to a
location w h e r e growth conditions are very poor, e.g. the Dollard
(cf. Essink & Bos, 1985)
Will the a c c u m u l a t i o n process of the c o n t a m i n a n t s be similar to that u n d e r good growth
conditions? How m u s t we interpret the concentrations d e t e r m i n e d at the e n d of the
exposition period? A n experimental approach is n e e d e d to elucidate these problems.
In estuaries where the distribution pattern of most species is strongly d e t e r m i n e d b y
the salinity gradient, there is the problem of species selection for monitoring. Only
e u r y h a h n e species, such as Nereis diversicolor, can be used to monitor locations
distrib u t e d over the far greater part of the salinity r a n g e in estuaries
(Essink et al., 1986)
According to Pries et al. (1984), the mussel Mytilus edulis can b e u s e d well i n the salinity
r a n g e 15-35 %0 S.
As m e n t i o n e d above, monitoring of pollutant concentrations in organisms does not
provide direct information on the pollution effects in the organisms concerned. W h e n
authorities responsible for the m a n a g e m e n t of estuaries a n d coastal waters, as well as
scientists observing elevated c o n t a m i n a n t levels in organisms, w a n t to l e a r n w h a t the
effects are, additional experiments
(bioassays; Meijers, 1986)
are necessary. To
investigate the effect of elevated m e r c u r y contents in the Ems Estuary as c o m p a r e d with the
w e s t e r n Dutch W a d d e n Sea in 1974-78, the survival of fry of Zoarces viviparus was u s e d
as p a r a m e t e r in a laboratory bio-assay. Although the mercury contents in Z. viviparus
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.
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