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Geconcerteerde Akties Oceanografie
Technisch Rapport Benthos 81/06
This paper not to be cited without prior reference to the author
Harpacticoid Copepod community Structure ~n two North Sea Estuaries
in Relation to Pollution.
D. C. Heip, R. Herman & M. Vaeremans
Marine Biology Section, Zoology Institute,State University of Ghent, Belgium.
Harpacticoid Copepod COlIllJ1unityStructure in two North Sea
Estuaries in Relation to Pollution
D. Van DalIllJ1e,C. Heip, R. Herman & M. Vaeremans
Marine Biology Section, Zoology Institute,State University of Ghent, Belgium
KEYWORDS:
Copepoda; meiobenthos; community structure; estuaries; inter-
tidal; pollution; heavy metals; North Sea.
ABSTRACT
The harpacticoid copepod assemblages of two estuaries in
the Netherlands, the Westerschelde and the Eems-Do LLard , are
compared. Both estuaries have similar physical characteristics
but the Westerschelde is much more polluted than the Eems-
Dollard. Harpacticoid copepod assemblages in both estuaries
are similar in terms of species composition. However, there
is a striking difference in quantitative characteristics :
the highest annual averages of density, biomass and diversity
in the Westerschelde are below the lowest annual averages in
the Eems-Dollard estuary. Because both estuaries are exten-
sively monitored, data concerning many possible causes of this
difference could be compared. It is argued that it is the
high load of heavy metals in the Westerschelde which is re-
sponsable for the observed decline. Harpacticoid copepods
seem to be a very suitable tool in ecological monitoring of
estuarine systems and can provide a good indication of gene-
ral 'health' of rivers and coastal zones.
Introduction
The monitoring of pollution in the sea has its principal
aim in public health considerations but concern for the ad-
verse effects of pollution on living marine resources is also
an important motive; the possibility of looking for effects
in marine organisms and communities more directly - in the
field - is clearly attractive but has not received appropria-
te attention (McIntyre & Pearce, 1980). In routine monitor-
ing biological parameters are usually discarded, mostly becau-
se the inherent variability of biological material and exist-
ing environmental 'noise' inhibit a clear interpretation of
the response of biological parameters to changes in the en-
vironment. In research, the main effort is still on the che-
mical analysis of the water and biological methods are, in
many instances, used only additionally or not at all.
Moreover, much of the existing research effort in the bio-
logical monitoring of pollution is directed towards the indi-
vidual organism because non-specif:i,ceffects of pollution can
be diagnozed by physiological, pathological and biochemical
responses which enable quantitative assessment of animal health,
with the additional advantage that an early warning can be
signalled (Bayne, 1979).
In this paper we will argue the case for what thus appears
to be the most unpopular way to look at the impact of pollu-
tion, i.e. by looking for changes in biological communities
in the field. There are several drawbacks in this approach
which we recognize : it provides no early warning as it will
show changes only when they are already important; and, these
changes will be difficult to interprete and impossible to link
to some particular cause with absolute certainty. Moreover,
ecological monitoring will only be useful if done over time
periods which are long compared to the response time of the
system.
But, there is not a single method which will provide the
environmentalist with all the answers. The effect of many
kinds of pollutants acting together is far from simple and
predictions based on simple concentration measurements as ln
routine chemical analysis are so over-simplified that they are
extremely dangerous. The availability of pollutants to organ-
isms is often not a linear function of total concentration,
e.g. copper in sea water is complexed actively by dissolved
organic matter and this complexation is pH-sensitive whereas
cadmium is complexed by chloride ions and is not significant·-
ly affected by dissolved organic matter. Also, the physiolo-
gical effect of some pollutant on an organism is governed by
many extrensic and Lnt.rins i,ccauses of variation which necessi-
tate the simultaneous measurement of a lot of environmental
factors together with intrinsic causes such as reproductive
and nutritional status (Bayne et al., 1980).
Monitoring the canges in communities in the field has at
least this advantage that these changes are directly measured.
In many existing estuaries and coastal areas an early warning
is no longer particularly needed. What we need is some set of
measures that allow us to see if things go better or worse -
even if we cannot explain the change in terms of causal rela-'
tionships.In this study we look at assemblages of harpacticoid cope-
pods from two estuaries in the Netherlands. Harpacticoid
copepods are very small crustaceans (about 0.5 to 1 rom) which
live in or upon the sediments. Their possible utility in eco-
logical monitoring has been discussed by Heip (1980) who al-
ready stressed that they should prove to be excellent 'indi-
cator-organisms', as they live associated with the sediments,
which are the ultimate sink and thus the best indicators of
pollution in most instances, and as they are small and should
therefore be more sensitive.
The two estuaries which we will compare are the Western
ScheIdt, the most southern estuary in the Netherlands, and the
Eems-Dollard, which is its most northern estuary, on the bor-
der with Germany. Although situated in the Netherlands, the
Western ScheIdt carries a huge load of many different kinds
of pollutants originating from human activities ~n Belgium
(and even France) and it is an open sewer right to the Belgian-
Dutch border. The Eems-Dollard is only occasionally polluted
by organic matter from potato-flour mills, annually in early
autumn. For both estuaries a lot of data concerning water
quality have been collected over the year by the Rijkswater-
staat (Water Control. Public Works Department) and by several
Belgian groups connected with the Management Unit of the Math-
ematical Model of the North Sea and the ScheIdt Estuary (Min-
istry of Public Health, Belgium). Apart from this difference
in the degree of pollution both estuaries are comparable and
they harbour very similar harpacticoid species.
Material and Methods
The localisation of the stations is charted in Fig. 1. TheF1l -t .,,~
,.1 term 'stationgroup', is used to design all stations from a
particular transect. The Vlissingen-stationgroup includes the
I'
I!..?
7 stations (WS 51-57) from Breskens to Vlissingen. The Ter-
neuzen-stationgroup encloses the station WS 41-,45, the
Ossenisse and Valkenisse transects comprise resp. the stations
WS 31-34 and WS 21-25. Most of the stations are intertidal
and situated on shallow sand- or mudflats between + 1.0 m and
- 3.0 m MTL. Truly subtidal samples were taken in the chan-
nels at depths of -25 to -60 m on each transect during Sep-
tember 1980. During this same period macro-algae were col.Lec;-:
ted a.n intertidal pools on sandbanks in the mouth.
In the Westerschelde estuary 21 stations divided over four
transects were sampled from September 1978 till September 1979
on the following dates: 27-29.09.1978; 11-13.12.1978; 03-05.
04.1979; 24-26.06.1979; 03-05.09.1979. Meiofauna at the Doel
transect was sampled and studied a year earlier (Holvoe~ 1978)
from May 1977 till May 1978 over 6 sampling periods on the
following dates: 05.05.1977; 17.06.1977; 01.09.1977; 18.10.
1977; 26.01.1978; 09.05.1978.
The sampling technique differs according to tidal level.
At the deeper stations a modified Reineck boxcorer (Farris and
Crezee, 1976) was used, from which four subsamples were taken.
Two replicates for meiofauna were fixed with warm formalin
(lO°C) to a final concentration of 4 %, the two other cores
for chemical and sediment analysis were frozen immediately.
In shallow water (-3.5 m) samples could be collected with a
'meiosticker', a telescoping tube (max. length 5.5 m) equiped
with a head into which a plastic core can be screwed (Govaere
and Thielemans, 1979). At the other intertidal stations the
samples were handcollected. Plastic cores with a 10.2 cm2
inner surface were used for all direct sampling and subsampling.
In the Eems Dollard estuary benthic harpacticoids weresampled from September 1976 till July 1977.
At Uithuizerwad, 9 stations on a transect of 2400 m were
sampled in autumn (21.09.1976) and winter (01.02.1977); at
Eemshaven a 150 m transect (8 stations) and at Hoogwatum a
100 m transect (4 stations) were sampled in autumn (23.09.1976)
and spring (25.03.1977). At the Reidersplaat the 4 stations
along a 400 m transect were sampled in winter (28.12.1976) and
spring (29.03.1977). The Heringplaat-stations (interdistance:
150 m) were sampled only in sunnner 1977 (27.07 •.1977) and 5
stations at the Oost-Friesche Plaat (interdistance : 500 m)
were sampled in late summe r and autumn 1976 (15.09.1976; 16.11.
1976). The location of these stationgroups is also marked in
fig. 1. All stations are situated on intertidal sand~ or mud-
flats.
All samples were handcollected with a 3.4 cm inner diameter
core except at the Oost-Friesche Plaat where a 2.4 cm inner
diameter core was used.
In the laboratory the samples were passed over 250 l.lmand
38 l.lmsieves.
For sandy sediments, the meiobenthos was elutriated from
both fractions using the trough method (Barnett, 1968; Heip,
1976).
For muddy sediments the organisms from the smaller fraction
were extracted using a density gradient centrifugation techni-
que (Bowen et al., 1972; de Jonge and Bouwman, 1977). After
staining with rose Bengal all groups of the meiobenthos were
counted, and the harpacticoids were studied systematically
(Van Damme et al.., 1981).
For estimating biomass, a Mettler ME 22/BA 25 microbalance,
accurate to 0.1 ~g, was used. The individual adult dry weight
per species was measured after drying the specimen in an oven
at 110° C for 2 h. Copepodites were assumed to have one eight
of adult weight.
As measure of species diversity the Brillouin formula wasused: H = -N1log __ .E.~__ . (Pielou, 1975).
n n .... n1 2 S
Median grainsize values were obtained according to the
technique described in the IPB handbook (Buchanan & Kain, 1971).
Sediment analysis was done on a 25 g ovendried subsampLe . For
classification of the sand fraction the Wentworth scale was
used. Stations with a mud fraction (silt-clay particles
< 62 11m) lower than 2 % are considered here as 'pure sand sta-
tions', while stations with a mud content in excess of 2 % are
here termed 'muddy sand stations'.
The amount of organic matter was estimated by loss of
weight on ignition at 550°C (Wollast, 1976). For the deter-
mination of organic carbon we used the method of El Wakeel and
Riley (1956).
The mean annual salinity per station was used to classify
the different station groups into the salinity zones of the
Venice system (Remane and Schlieper, 1971).
Chemical analysis of the water column and determination of
trace metals in water and sediment of the Verdonken Land van
Saaftinge was done by Dr. F. Vaes at the Provinciaal Instituut
voor Milieuhygiene, Oost-Vlaanderen (for heavy metals X-ray
fluorescence and atomic absorption spectrofotometry was used,
for detergents in Saaftinge the methylene blue method).
Characteristics of the two e$tuaries---_.-The main hydrodynamical and physical characteristics of the
two estuaries are summarized in table 1. Differences in these
factors and topography (the Westerschelde is elongated and
narrow, the Eems-Dollard truncated and broad) are correlated
with the distance of the :respective estuaries to the bottle-
neck of the English channel (Van Veen, 1950). Tidal volumes
and tidal amplitudes are higher in the Westerschelde and hence
turbulence is higher, which is still more pronounced because
the Westerschelde is open and directed to the prevailing winds
and not protected by an island chain as the Eems-Dollard.
Lateral extensions, traces of fermer inundations, occur in
both estuaries. The Verdronken Land van Saaftinge (the Drowned
Land of Saaftinge) results mainly from an inundation for mili-
tary purposes in 1584 (Brand, 1975). It has evolved into a
high saltmarsh (27 km2) with a Spartrina and Pueeinel.l.ia type
of vegetation, crisscrossed by an intricate system of tidal
creeks connected with the Westerschelde. During low tide these
creeks drain completely. During flood the tidal currents are
strong s~nce the tidal amplitude is about 4.5 m in that area
of the Westerschelde (Wolff and Beeftinck, 1975).
In the Eems-Dollard estuary, the Dollard is a vast interti-
dal mudflat (100 km2) interlaced with creeks. Vegetation is
confined to the edges. It was formed by repeated inundations
between the 13th and the 16th century (De Smet & Wiggers, 1960).
In the Westerschelde, pure sands with a median grai,n size
around 200 l.lmand a mud content never in excess of 2 % are
found on the sandflats situated near the tidal channels, where
tidal currents and wave turbulence are strongest. The sediment
is well oxigenized up to the maximal depth sampled (.:!:. 20 em)
and is displaced during floodperiods in a 'sawteeth'-movement
around the banks (Peters and Sterling, 1976). The organic
matter content averages between 1.5·-2.5 % and the organic car-
bon content is around 0.05 % (table 2).
Pure sand stations occur also in the Eems-Dollard in areas
with similar hydrodynamical conditions, but the mean grain sl--
ze here is smaller (110 to 125 11m) and the organic carbon corr-
tent higher (0.11-0.26 %). Muddy sands occur over the whole
Westerschelde estuary, along the edges, where currents and wave
turbulence are minimal, allowing fine particles to settle. The
grain size is on average around 150 11m and the mud percentage
is well above 3.0 % throughout the year. Stations with very
soft sediments (black muds) occur at Doel (WS 14 : avo 53 %
mud) and at the Vlissingengroup (WS 51 : avo 21 % mud).
The organic matter content of muddy sands lies aroung 5.0
to 7.0 % and organic carbon averages between 0.30 and 1.60 %,
except at Doel where it is higher. At this station group, all
stations, even in turbulent waters, contain more than 3 % mud
during at least part of the year, due to their location in the
flocculation zone (see further) (table 2). The top sediment
(5 em) of the saltmarsh of Saaftinge consists also of black
muds and the sands below are anoxic.
The muddy sands of the Eems-Dollard have a mud content of
4 to 14 % and an organic carbon content of 0.80 to 2.70 %.
Ttans£ott and accumulation of pollutants
Westerschelde estuary and Saaftinge saltmarsh
About 1.52 x 106 ton/yr of suspended solids are discharged
into the estuary by the river Schelde, highly polluted by or-
ganic compounds (N = 2.0,600 ton/yr; P = 125,000 ton/yr; O.M. =250,000 ton/yr) and heavy metals (Zn = 2000 ton/yr; eu = 49.5
ton/yr; Pb = 450 ton/yr; Hg = 1.52 ton/yr).
In a zone with a salinity between 1 0/00 and 5 0/00, most
of the suspended matter flocculates and is sedimented. The
corresponding zone of highest mud accumulation and turbidity
is located between Antwerpen and Doel (where residual bottom
currents are zero, Peters and Sterling, 1976). The seaward
limit of the flocculation zone may shift as far as Hansweert
during periods of increased river discharge (250 mS/s) (Peters,
1975). About 1.2 x 106 ton/yr (60 %) of solids of continental
origin are trapped in this area together with 0.8 x 106 toni
yr (40 %) of solids, mainly coarser clastics, of marine origin.
Most of the heavy metal load is coprecipitated and fixed in
the sediment as insoluble sulphides (Wollast, 1976; Wollast
and Peters, 1978). The amounts of these pollutants in the se-
diments and in the suspended matter are therefore very high
(table 3).r-:5 .::"')
Degradation of the organic matter may release again com-
plexed or adsorbed heavy metals. Most of the organic matter
1S removed from the water by sedimentation in biodegradation
in the flocculation zone. Only 20 % reaches the seaward part
of the estuary (Wollast, 1976). As a result of the activity
of heterotrofic bacteria, the waters in the zone Antwerpen-
Bath are frequently anoxic, especially in summer and toxics
such as H2S and ammonia are released (Billen et aI., 1976).
Nitrogen and phosphorus are also precipitated through the ac-
tivity of auto- and heterotrofic organisms but relatively larg-
ge amounts still persist in the seaward part of the estuary
(Hansweert-Vlissingen) and even at the mouth ammonia is still
present. The longitudinal profiles of oxygen, ammonia, total
phosphates and organic carbon are listed in table 4.
About 2.2 x 105 ton/yr of solids of continental origin are
deposited in the seaward part of the estuary. The concentra-
tions of heavy metals, especially copper and to a lesser de-
gree zinc, in the suspended matter decrease as they dissolve
or become desorbed, e.g. dissolved eu increased from about 10
~g/l at Doel to 20 ~g/l at Vlissingen; dissolved Pb increased
from about 7 ~g/l at Antwerp to 40 ~g/l at Doe L, then decrea-:
sed again seawards to 20 ~g/l (Wollast, 1976). Dissolved Hg
remained constant around 0,1 ~g/l (Anonymous, 1978-1979).
Iron and magnesium on the other hand precipitate in this
zone as hydrous oxides (Wollast and Peters, 1978). Since in
this form they are known scavengers of heavy metals in sol-
ution (Duinker and Nolthing, 1976, 1977) this could partially
explain the still very high amounts of heavy metals in the
sediments of the zone Hansweert-Vlissingen. Hoenig (1978)
also discovered the existence of a relatively small but im-
portant accumulation area in front of the harbour of Terneuzen
and noted periodical high peaks of dissolved and particulate
heavy metals in this zone. The suspended matter, deposited in
the Terneuzen area (2 x 104 ton/yr (Wollast, 1976))originates
from the Gent-Terneuzen channel, which runs through an indus-
trial complex (a.o. chemical and petrochemical, woodpulp and
sugarbeet industry, metallurgy) and which is regularly flushed
by highly polluted Schelde waters in order to halt the land-
inward progress of the salinity front (Bakker and Heerebaut,
1971). Although the input of solids from the channel is minor,
the relative concentration of heavy metals in the particulate
matter is extremely high (table 3).
Apart from the ones already mentioned, the concentration
of the following toxic compounds have been measured in the
surface waters of the Westerschelde during the period of our
survey (Anonymous, 1978-1979).
Pesticides and PCB's occur over the whole Westerschelde ~n
extremely low concentrations; only lindane is sporadically
found in the watercolumn (0.01 jlg/l). The concentrations of
aldrin, telod:dn, dieldrin, endin, DDE, hexachlorobenzene and
PCB's are similar or even lower in mussels and shrimps from the
mouth (Vlissingen) of the Westerschelde estuary when compared
to populations of other localities along the Dutch coast and
specifically the Eems-Dollard estuary (see further). Fluoride
is found ~n a concentration of about 1.5 mg F/l over the whole
estuary, this value is 2-3 times higher than in all other Dutch
waters. Phenol does not reach the seaward part in high con-
centrations; at the mouth the concentration is sporadically in
excess of 1.0 ug /L. In this region concentrations of oil are
usually around 0.1 jlg/grwith a maximum of 1.2 jlg oil/gr.
Polynuclear aromatic hydrocarbons were present at Doel in con-
centrations of 0.3-0.7 jlg/l. Synthetic anionactive detergents
are practically continually present in a concentration of
0.04-0.05 mg maxonol oT/l over the whole estuary (at Doel,
Nihoul and Boelen (1978) found values between 0.07-1.39, ex-
pressed in mg/l). During a Belgian survey (1971-1975) cyanide
was not detectable in most samples collected near Doel; in on-
lyone sample a concentration of 11 ~g/l was recorded (Nihoul
and Boelen, 1978).
Sediments, a.o. the toxic muds before the harbour of Ter-
neuzen, are continually dredged in the Westerschelde and (in
the Dutch part) the dredge spoil is again dumped 1n the estu-
ary in floodchannels, a.o. at Everingen (fig. 1) so that
coulds of muds are transported streamupward with the flood
currents (this is done on purpose in order to counter or halt
relief changes in tidal channels which could be hazardous for
the naval traffic). The amount of dredged sediment in 1972 was
9 x 106 m3/yr (Theuns, 1975).
In september 1980 a water and sediment sample was taken at
the mouth of the main creek in the Saaftinge saltmarsh. Nu-
trients in the water column are low (table 4) and comparable
to the concentrations found at the mouth of the Westerschelde.
Concentrations of heavy metals, especially copper, are very
low in the sediments in comparison to the Westerschelde (table
3). Ammonia however is relatively high (1.6 mg N/l), fluoride
(0.69 mg F/l) about half of the concentration found in the
estuary, while the amount of detergents was higher (3.2 mg/l).
The main cause for the lower concentrations of most toxic
compounds in the saltmarsh is probably the very small degree
of exchange between the tidal watermass that moves in and out
of the creeks and the Westerschelde waters (De Pauw, 1975).
However, there is probably also a mechanism of selective trans--
port of suspended matter at work.
The Eems-Dollard estuary
The amount of suspended solids carried by the river Eems
is estimated at 65,000 ton/yr (Hinrich, 1974). The turbidity
maximum is limited to a narrow area between Ditsum and Emden
(Postma, 1960). All the particulate matter originating from
the river is transported by tidal currents to the eastern
(German) part of the Dollard where is settles. Freshwater is
also carried into the Dollard from the Westerwoldse Aa river.
This river is organically polluted by potatoflour mills. The
input of organic matter varies from 1200 ton in summer to
25,000 ton in late autumn, this is about 0.5-10 x 103 ton C/yr
(Schroder et al. 1976). Concentrations of nutrients and or-
ganic matter in this estuary therefore reach maxima in the
south-eastern part of the Dollard (Oost-Friesche plaat), where
annual averages of oxygen, nutrients and organic matter are
similar to the values occurring near the seaward limit of the
flocculation zone in the Westerschelde (table 4). During the
peak activity of the potatoflour mills, maximal concentrations
of these parameters are even above the ones occurring at Doel.
Nutrients N, P and Si are never limiting for the primary pro-
ducers and during autumn the water becomes anoxic and high
amounts of ammonia and H2S are observed (Bouwman & Kop, 1979).
The concentrations of oxygen and nutrients in the rest of the
Eems-Dollard are similar to the ones occurring in the seaward
part of the Westerschelde.
The Dollard sediments are predominantly (as is the rest of
the estuary) of marine origin (~ 90 %), and the apport of
solids from the North Sea is estimated at I x 106 ton/yr
(De Smet & Wiggers, 1960).
The concentrations of heavy metals are very low in com-
parison with the Westerschelde, both in sediments and in sus-
pended matter (table 3). However, comparison is rendered dif-
ficult because Salomons and Mook (I977) and Essink (1980) ex-
press the concentrations at 50 % sedimentparticles smaller
than 16 um ('because of the linear' correlations between heavy
metal concentrations and the amounts of particles < 16 ]lm l.n
sediments, making it possible to characterize the content of a
specific metal of a whole group of co-genetic sediments by a
single value' (De Groot, 1973».
The Westerschelde values are expressed as concentrations in
mg/kg dry weight of the total sediment, which rarely contains
more than 20 % mud. If the Westerschelde values would be cal-
culated at 50 % particles < 16 ]lm (but this would be a danger-
ous extrapollation) the differences in heavy metal concentra-
tions between both estuaries would be even more pronounced.
From 1960 till 1975 the Eems-Dollard estuary received sev-'
eral tons of mercury per year, mainly from a chloro-alkaline
plant and a pesticide factory. The concentration of dissolved
Hg varied from 0.31 ]lg/l in 1975 to 0.10 ]lg/l in 1978, while
the mercury content of the particulate matter averaged between
1.76-1.77 ]lg/g (Essink, 1980). The average concentration in
the seaward part of the Westerschelde is in comparison 506.6
]lg/g (min. : 324 ]lg/g) according to Hoenig (1978), but Ano-
nymous (1978-1979) cites similar values for the Westerschelde
as those occurring in the Eems-Dollard, so Hoenig's values are
probably erroneous.
Salomons and Mook (1977) found no evidence of mobilisation
of trace metals from the sediment in the Eems-Dollard estuary.
Koeman (1971) found pesticide concentrations at Delfzijlof 0.009 ppm wet weight of aldrin, 0.002 ppm telodrin, 0.065
ppm dieldrin, 0.020 ppm endrin and 0.015 ppm DDE in muscle
tissue of MytitU8 eduZis. Concentrations at the mouth of the
Westerschelde were at least 2-3 times lower except for endrin
which was equal (0.017 ppm). Hexachlorobenzene was present
usually below measurable concentrations « 0.01 jlg/l) in the
surface waters (Fonds and Greve, 1973) and with a concentra-
tion of 0.05-0.99 ppm wet weight in shrimps (Crangon crangon),
Le. about 2-3 times higher than found in these organisms at
the mouth of the Westerschelde (Hagel and Tuinstra, 1974).
Concentrations of PCB's on the other hand are similar in
Eems-Dollard shrimps (0.05-0.14 ppm wet weight) and at the
mouth of the Westerschelde (0.07-0.18 ppm wet weight) (Ten
Berge and Hillebrand, 1974; Essink, 1976).
We possess no figures on concentrations of phenols, fluori'-
des and oil in the surface waters of the Eems-Dollard. Accord-
ing to De Groot (1973) the main mobilizing compounds of heavy
metals in the Eems are phenoles originating from peat layers.
The maximal amount of anionactive detergents measured in the
Eems river was 0.3 mg/I.
The Harpacticoid fauna
Density and biomass
In the Westerschelde the annual average density over all
pure sand stations is highest at the Vlissingen station group
at the mouth (26.4 ind./10 cm2), peaks down at the Terneuzen
stations (0.64 ind./10 cm2), increases again at Ossenisse
(12.0 ind./10 cm2) and decreases to very low levels at Valke-
!'/,I
h (I",~"
nisse and zero at Doel. Biomass shows a similar trend with
6.6 ~g dwt/IO cm2 as the highest average. Maximum density
noted was 128 ind./IO cm2 (Vlissingen station group), maximum
biomass 33 ~g dwt/IO cm2 (Ossenisse station group) (table 5).
In muddy sands a similar trend 1n density and biomass is
observed with respective averages of 10.36 ind./IO cm2 at the
Vlissingen stations, 2 •.30at Terneuzen, 9.60 at Ossenisse,
0.77 at Valkenisse and 0.10 ind./IO cm2 at Doel (table 5, fig.
2). Maximum density recorded was 68 ind./IO cm2, maximum bio-
mass 146.5 ~g dwt/IO cm2• In 66 % of all samples (pure sands
and muddy sands) no harpacticoids were found. Per station
group this percentage is resp. 22 % at Vlissingen, 50 % at
Terneuzen, 60 % at Ossenisse, 82 % at Valkenisse and 94 % at
Doel.
In the Saaftinge saltmarsh the annual average density and
biomass were respectively 69.7 ind./IO cm2 and 236 ~g dwt/IO
cm2 (table 5, fig. 2).
A survey in September 1980 of the subtidal sediments (muddy
sands and pure sands) along the Westerschelde yielded maxima
of 13 ind./IO cm2 at the mouth. No harpacticoids were found
in the subtidal region upstream from Terneuzen. Investigation
of harpacticoids of macroalgae in tidal pools at the mouth
during the same period yielded very low densities, approxi.-'
matively 0.10 ind./IO cm2, although other meiofauna groups such
as nematodes, turbellarians and gastrotrichs were numerous.
The average annual density in the Eems-Dollard is highest
at the station group at the mouth with no significant differ-
ence between pure sandy sediments (87.7 ind./IO cm2 at Eems-
haven) and muddy sands (89.9 ind./IO cm2 at Uithuizerwad).Upstream in the Eems estuary, densi~y is halved (46.1 ind./
10 cm2 at Hoogwatum). Similar densities were found in the
outer part of the Dollard with higher values in the muddy
sand stations (44.5 ind./IO cm2 at Reidersplaat) than in the
pure sand stations (36.0 ind./IO cm2 at Heringsplaat) (arti-
fact due to low number of samples ?). At the most polluted
stationgroup (Oost-Friesche Plaat), the lowest average of
17.3 ind./IO cm2 was recorded (table 5, fig. 2). Biomass
shows a similar trend, declining from 147.9 l1g dwt/IO cm2 at
the mouth (Eemshaven) to 29.2 l1gdwt/IO cm2 at the Oost-Frie-
sche Plaat. Maximum density and biomass noted in this estuary
were respectively 478 ind./IO cm2 and 602.911g dwt/IO cm2 (Uit-
huizerwad (table 5).
Density is zero in 22 % of the samples at the Oost-Friesche
Plaat. At all other station groups Harpacticoids occur in all
samples.
Community structure and diversity
A total of 23 species was found on the intertidal flats of
the Westerschelde estuary (table 6). They can be divided in
two distinct communities : a mesobenthic community, consisting
of small interstitially living grazers and an epibenthic com-
munity consisting of large burrowing and epibenthic detritus-
feeders. There is no overlap or intermediate community in the
Westerschelde, because the two habitats are incompatible for
the communities involved : in the pure coarser sands the ab-
sence of detritus or epibenthic diatoms and the high turbulen-
ce exclude the presence of epibenthic copepods. In the low
energy zone the interstitial habitat is unavailable because
of the fine texture of the sands « 200 ym) and a too high
mud content in excess of the limit of 7 % (according to Ward,
1975) or 2-3 % (according to Van Damme and Heip, 1977 and
Govaere et al., 1980).
The occurrence of a relatively well structured interstitial
community is restricted to 2 very turbulent stations at the
mouth (Vlissingen station group). Here a total of '7species
was found with as dominant forms Paral-eptiaetacue eepinul.atue
(33.8 %), KUopsy Tlue eonetx-ictue (29.2 %) and Parameeochra
simiUs (26.5 %). In the eu-polyhaline zone at Terneuzen only
1 species was recorded: Euaneul.apygmaea (3 specimens) al-
though the sediment is suitable for interstitial life at 3
stations (15 samples). In the poLy-: mesohaline zone, 3 spe-
cies were found with P. espinuZatus extremely dominant (93 %).In the mesohaline zone (Valkenisse) only a few specimens of
P. espinuZatus were found, while at Doel the pure sand habitat
does not exist (table 6).
Mean annual diversity is highest at the mouth (Vlissingen)
with H = 1.08, falls to zero at Terneuzen and rises again to
H = 0.11 at the Ossenisse station group and is zero at Valke-
nisse (table 5). Maximum diversity (H = 1.67) was noted at
the Vlissingen station group during autumn. Diversity was ze-
ro in 80 % of the pure sand samples.
The best structured epibenthic community again occurs at
Vlissingen with a total of 13 species, decreasing to 8 at Ter-
neuzen, 7 at Ossenisse and 3 at Valkenisse and Doel.
Tachidius discipes is dominant in the eu-polYhaline (35 %)and poly-mesohaline zone (66 %) and StenheUa paZustris in
OJ.0'1,
the meso-(81 %) and meso-oligohaline zones (57 %) where T.
diseipes did not occur (table 6). T. discipes., S. palustris
and Asellopsis intermedia represent 80-90 % of the whole epi-
benthic fauna in all salinity zones.
Mean annual diversity of this community declines from H =0.37 at Vlissingen to H = 0.24 at Terneuzen, H = 0.16 at
Ossenisse, H = 0.04 at Valkenisse and H = 0.01 at Doel (table
5, fig. 2). Maximal diversity of H 1.24 was recorded during
winter at the Vlissingen transect. Diversity was zero in 84 %
of the muddy sand samples.
In the subtidal area of the eu-polyhaline zone a totally
different epibenthic community occurs, consisting of Pseudo-
nuchocamptue proorimue, MicroarthY'idion littorale., AJrrphiascoi-
des debi.l.ie and Robertiquxmeua ep ; Only some specimens of T.
discipes were found on the macroalgae.
At the saltmarsh of Saaftinge, a total of 5 species was
recorded with as dominant one Nannopus palustris (63.7 %).The Saaftinge epibenthic community is totally different from
the Westerschelde estuary in species composition and structure,
since, except for Stenhelia paZustris, the other species do
not occur or are very rare in estuarine samples. Mean annual
diversity was H = 1.14 (max H = 1.39 in summer; min H = 0.81
in winter) (table 5, fig. 2).
In the Eems-Dollard estuary a total of 24 species was re-
corded (tabie 7). They all belong to the epibenthic community
and are found on the pure as well as on the muddy sands. Their
presence on the first is probably made possible by the large
amounts of epibenthic diatoms which cover the surface as a
film during low tide. An interstitial harpacticoid community
is absent in pure sands due to the small grain size « 200 ~m).
In the eu-polyhaline zone (Uithuizerwad, Eemshaven) a total
of 21 species was recorded with no outspoken dominance. The
3 most abundant species are Haieotrinoeoma ourtrioorne (19.0 %),
Harpatrioue fZexus (16.2 %) and Tachidiue dieeipee (15.1 %).
In the poly-mesohaline zone (Hoogwatum) the total was 17
species with as dominant forms Microarthridion l.ititorale
(57.4 %), T. di.eeipee (11.8 %) and Hs ourbiooime (10.4 %). In
the mesohaline zone (Reiderplaat, Heringplaat) at the mouth of
the Dollard, the number of species decreased to 11 with domi-
nance of H.curticorne (42.6 %),Paronychocamptus nanus (16.9 %)and M. UttoraZe (10.7 %). In the meso-oligohaline zone (Oost-
Friesche Plaat), l.nthe organically polluted south-eastern cor-
ner of the Dollard, the total number of species was reduced to
6 and only 2 species, StenheUa paZustris (49.1 %) and Nanno-
pus paluetx-ie (39.9 %), were still relatively abundant. Both
are typical for intertidal black muds (Lang 1948; Noodt, 1957)
and Nannopus paZustris can survive prolonged periods (96 h) in
anoxic conditions (Coull et al ,, 1979). However, during the
period of maximal sewage output of the potatoflour mills, only
StenheUa paZustris occurred.
Highest annual average diversities occur at the mouth of the
estuary with H = 2.20 in pure sands (Eemshaven) and H = 1.47 in
muddy sands (Uithuizerwad). Diversity does not decrease sig-
nificantly upstream since values of H = 1.42 (Hoogwatum) and
H = 1.44 (Reiderplaat) were recorded. The relatively low
value of H = 0.93 at Heringplaat, the other station group at
the mouth of the Dollard, maybe an artifact due to the small
number of samples. The lowest annual average was noted at the
Oos t.-Eri.e sche Plaat : H = 0.41 (table 5, fig. 2). Maximum
diversity of H = 2.68 was recorded in March at Eemshaven.
Diversity is zero in 1 % of all Eems-Dollard samples, which
correspond to the 22 % of the Oost-FI'iesche Plaat samples and
one of the Uithuizerwad samples (with a low density).
Discussion
In both estuaries, mean annual density and biomass decline
from the station group closest to the mouth to the one most
inland (fig. 2). While in the Eems-Dollard this decrease is
gradual, this is not the case in the Westerschelde where an
inverse peak occurs at the Terneuzen station group. Whereas
there is some resemblance in pattern between the two estuaries,
absolute values of all community parameters are completely
different. The highest annual averages of density and biomass
in the Westerschelde are lower than the lowest annual averages
occurring even at the Oost-Friesche Plaat group in the Eems-
Dollard (fig. 2). On the other hand, density recorded in the
Saaftinge saltmarsh approaches the highest ones found in the
Eems-Dollard estuary, while the biomass recorded in this salt-
marsh exceeds all values recorded from this latter estuary.
Sunnner averages recorded in the adjoining estuaries Ooster-
schelde and Grevelingen are about 4-7 times higher than the
peak summer densities (23 ind./IO cm2) noted in the Wester-
scheLde • Surkyn (1977) found averages of 119 ind./10 cm2 on
intertidal mudflats of the Oosterschelde (n = 7) and 200 ind./
10 cm2 on permanently flooded mudflats in Lake Grevelingen
(n = 7).
Values recorded in literature are also substantially high-
er, both for organically polluted as for unpolluted environ-
ments. Arlt (1975) records densities of 88 ind./IO cm2 in
front of a sewage outlet of a small town, 165 ind./IO cm2 .30
meters furthe r off and 96 indo /10 cm2 at an unpolluted station
on muddy sand and sandy sediments of the oligohaline Greifs
Walder Bodden. Warwick et al. (1979) record an average of
279 ind./IO cm2 on an intertidal mudflat of the Lynher estu-
ary and Coull et al. (1979) and Fleeger (1980) give a range
of average monthly densities of resp. 73-262 ind./IO cm2 and
75-620 ind./IO cm2 for S.-Carolina salt marshes. In a long
term study (9 years; 205 samples) Heip (1980) found a har-
pacticoid density of on average 211 ind./IO cm2 in a shallow
brackish water pond.
Although Westerschelde and Eems-Dollard are closely rela-
ted with regard to species composition, since 65 % of the
epibenthic and burrowing species of the Westerschelde occur
also in the Eems-Dollard, the qualitative parameters reflect
a impoverishment of the fauna in the first estuary. In the
first place there is the overwhelming dominance of the same 3
species AselZopsisintermedia., Taehidius diseipes and Stenhe-
lia palustris along the whole salinity gradient in the Wes-
terschelde, whereas dominance shifts to different combinations
of species in the Eems-Dollard at each station group, and the
above mentioned species remain unimportant except at the Oost-
Friesche Plaat. Also interesting to note is the presence of
Mie:'f'oarthridion littorale in all intertidal station groups of
the Eems-Dollard and its confinement to the eu-polyhaline sub-
littoral in the Westerschelde, although the species must be
extremely plastic physiologically (Coull et aT., 1979). M.
littorale is dominant and often the only species occurring in
the polluted subtidal muds before the Belgian coast (Van Damme
and Heip, 1977; Govaere et al.; , 1980).
In summer samples of an intertidal mudflat of the Ooster-
schelde an average diversity of H = 1.8 was recorded; dominant
were Aeel.lopei.e intermedia (37.4 %), HarpaetricuefZexus (36.1%)
and Axenoeetel.La ep; (15 %). In a subtidal mudflat in the
G:revelingen the average diversity was H :::;1.9 and the dominant
species we.re Canuel-la epp: (C. fureige.ra-C. perpl.eea : 56.3 %),
Ameira pamnila (20.8 %) and Harpactricue flexus (7.1 %) (Surkyn,
1977) •
Fleeger (I980) records monthly mean averages of H :::;1.44'-
1.69 for two subtidal stations and H:::;1.50 for an inte:rtidal
site at North Inlet, South Carolina. The highest annual aver-
age diversity for the epibenthic community recorded at the
Westerschelde (H :::;0.59) is beneath the lowest average found
at the Dollard and is about 3-4 times lower when compared to
the saltmarsh of Saaftinge and adjacent estuaries or with the
monthly averages recorded by Fleeger (1980). Heip (1980) re-
corded in his long term study a mean annual diversity of H :::;
I .06.
The very low average values of all pa:rameters in the Wester-
schelde are in the first place due to the complete absence of
Harpacticoids in most samples (65 %). In the remaining ones
density remains also low as does dive:rsity, since the average
number of species per sample is usually also very low (H :::;0
in 80 % of all samples). This impoverishment is apparently
not confined to the intertidal flats since epiphytic and sub-
tidal epibenthic populations also a:re present in very small
numbers only and confined to the seaward part of the estuary.
As to the planktonic copepods, their longitudinal profile
of annual average standing stock shows similar characteristics
as observed for all parameters studied here (De Pauw, 1975).
According to this author, the quantitatively most important
16
species Eurytemora affinis occurs in the highest concentrations
in the area before the Saaftinge saltmarsh. Here, the per-
centage of egg carrying females and nauplii was also highest,
while the relative number of males increased with increasing
distance from this area. De Pauw (1975) considers the area
just in front of and in the saltmarsh as the distribution and
reproduction centre of this planktonic copepod.
Compared to the related epibenthic isoconnnunities occurring
in other estuarine systems and in the Eems-Dollard estuary,
the poverty of the Westerschelde fauna is striking. Yet a
large numbe r of abiotic factors similar in both estuaries.
There are 3 basic differences : (i) The Westerschelde estuary
is more turbulent, (ii) The amount of solids of continental
origin that is sedimented is much higher and (iU) Water and
sediments are more polluted. These differences may explain
the impoverishment of the copepod fauna of the Westerschelde.
1. The possible influence of turbulence, dredge spoil dumping
and increasing load of fine grained solids :
The Eems-Dollard and the saltmarsh of Saaftinge are protect-
ed from extreme physical conditions, while the Westerschelde
is not. Giere (1968) and De Pauw (1975) consider the degree
of exposure to high turbulence and current velocity more im-
portant for the production of the planktonic fauna than the
nutrient supply. However, the high turbulence in the Wester-
schelde is beneficial for diversification of meiobenthic fauna,
since areas of coarse grained sands, where interstitial life
can develop, are maintained on the sandflats between the tidal
channels. Moreover, at the mouth were turbulence and stream
velocities are highest, sometimes removing the upper 20 cm of
sand during storms, a well developed interstitial fauna exists.
Thus, higher turbu lence alone can not be the limiting factor
at the mouth.
However, upstream the areas of coar-se grained sands are
confined and can be considered as islands. The continuous
dumping of dredge spoil and the increasing amounts of solids
brought in by the Schelde could affect the interstitial fauna
drastically When the interstitial pores would be even brief-
ly clogged. As interstitial copepods have no free swimming
larvae, recolonization of the 'islands' would be difficult.
This could theoretically explain the near-absence of inter-
stitial fauna in such area's as the sandflats at the Terneuzen
station group. However, occasional clogging was not observed
during our campaigns and the sand remained very pure. And, of
course, clogging can not explain the poverty of the epibenthic
communities which live on intertidal mudflats in low energy
zones where continuous sedimentation of finer clastics is a
normal process and where occasional physical disturbances ap-
pear to have no or little effect on the meiofauna (Sherman and
Coull, 1980).
2. The possible influence of high amounts of organic matter and
nutrients :
The drastic decrease of the copepod fauna at the Oost-Frie-
sche Plaat (Eems-Dollard) during late autumn clearly coincides
with the maximum output of organic material from the potato
flour mills. Organic pollution can hence be considered detri-
mental at the concentrations recorded.
Since at the Doel transect extreme conditions, as occurring
at the Oost-Friesche Plaat during a limited period, persist
almost throughout the year, this type of pollution can explain
the poverty of the benthic copepod fauna in such situations.
At the remaining station groups of the Westerschelde, the
nutrient load and oxygen depletion are much lower than at the
Oost-Friesche Plaat and the poverty of the epibenthic popula-
tions on the mudflats, especially in the seaward part of the
Westerschelde estuary, can therefore not be explained by this
type of pollution.
3. The possible influence of heavy metal pollution:
Concentrations of heavy metals in sediments, particulate
matter and water of the Westerschelde are higher than in the
Eems-Dollard and ~n the salt marsh of Saaftinge. Very high
concentrations occur in the flocculation zone and, in the sea-
ward part, in front of the harbour of Terneuzen, where an ~n-
verse peak occurs in all parameters studied. The absence of
interstitial life ~n pure sands at the Terneuzenstation group
may be due to a periodical sedimentation mixing of even small
amounts (not enough to fill up the interstiti.al pores) of par-
ticulate matter from the Gent-Terneuzen channe.l,with its ex-
treme high load of heavy metals. By breakdown of organic
matter in the sediment organo-metallic complexes are formed
which increase significantly the amounts of heavy metals in
the interstitial waters of the oxigenized layer (Bryan, 1976).
Little is known about the effect of heavy metal pollution
on copepods. They may take up trace metals, already concen-
trated in detritus, diatoms and bacteria, via the digestive
tract or in solution via the body surface (absorption) (Peil:es,
1976). Adsorption of heavy metals to the exoskeleton of co-
pepods (a.o. Euterpina acutifrons) has also been demonstrated
(Martin, 1970).
Bioassays on the effects of heavy metals on copepods are
scarce and limited to epiphytic and planktonic forms. Corner
and Sparrow (1956) determined the lethal concentration of a
number of mercury and copper compounds, - generally considered
as being the most toxic e,lements together with silver (Bryan,
1971) - for Acartia cZausi and Hoppenheit and Sperling (1977)
determined the lethal concentration of Cadmium for Trisbe
hoZothuriae. Reeve et ale (1976) cite 24 h L.C.50-values for
mercury of 3 ppb for nauplii of Acartia tonsa. All these le-
thal concent rat i.ons are on order of magnitude higher than con-
centrations found in the Westerschelde. However, drastical
sublethal effects, especially on feeding and egg production,
have been observed at concentrations lower or similar to those
occurring in the Westerschelde. Reeve et ale (1976) found a
clear downward trend in these activities for Acartia tonsa at
concentrations of 10 to 20 ppb Cu and almost no egg production
at 50 ppb Cu (24 h L.C.50 for A. tonsa was 104-311 ppb Cu).
For CaZanus pZumchrus Reeve et ale (1977) found a 6-fold re-
duction of egg production at a concentration of 5 ug Cull and
no production at all at 10 ug Cull (24 h LoC .50 for C. plumchrue
was 2778 ppb Cu).
The mean annual concentration of dissolved copper in the
Westerschelde lies around 10 ppb at Doel increasing to 20 ppb
towards the seaward part (Wollast, 1976). Concentrations of
dissolved mercury are similar in the Westerschelde (Anonymous,
1978-1979) and in the Eems-Dollard (Essink, 1980) (0.50-0.01
11g Hg/l), but as Corner and Sparrow (1956) found that copper
l.ncreases the permeability of Acartia cZausi to mercury
poisons, the toxicity of mercury compounds may be different
in the two estuaries.
4. Possible influence of chronic oil pollution
According to Mironov (1969) a concentration of 0.001 ml/l
of crude oil is sufficient to shorten_life span of Acartia
cZausi while Ustach (1979) found that the water soluble frac-
tion of 200 111Louisiana crude oil per liter seawater and 1/2
and 3/4 dilutions thereoff halved egg production in Nitocra
affinis. Ott et al. (1978) found a significant reduction of
broodsize, life span and number of naupli for Eurytemora affi.-
nis after chronic exposure to naphtalene and methylated de-
rivates at concentrations of around 10 ppb. At the seaward
part of the Westerschelde the concentration of unsoluble oil
(which is however not toxic) in the surface waters was around
1 11g/g in fall and winter and below the detectable level in
spring and sunnner during our survey (Anonymous 1978-1979).
The concentrations of polycyclic aromatic hydrocarbons at Doel
are already at least one order of magnitude lower than the
value cited by Ott et al. (1978). In the seaward part of the
estuary the concentration is still much lower. No major oil
spill occurred in the Westerschelde in the years previous to
our investigation.
5. Possible influence of other known to toxicants :
Of the remaining toxicants, only anionactive synthetic
detergents and fluoride are continually present in the Wester-
schelde. The anionactive detergents, the only ones still in
use in Belgium and the Netherlands, are the least toxic, but
according to Bellan (1976), all tensioactive substances are
dangerous, even in very low concentrations. Arnoux and Bellan-
Santini (1972) found alterations in composition of a medi ter-
ranean Cystoseira etx-icta community starting at concentrations
of 20 to 50 ~g maxonol OT/I.
While the concentration of detergents in the Westerschelde
could have an effect on qualitative aspects of the communities,
it is extremely unlikely that it would affect the quantitative
parameters, since all concentrations for which short term ef-
fects are cited are of the order of 1-100 ppm, while for chro-
nic exposure effects a concentration of 0.1 ppm seems necess-
ary (Duursma and Marchand, 1974. Using 2 and 4 mg/l of dom-
estic detergent and 0.8 mg/l LAS, Fava and Crotti (1979) found
that the mean number of nauplii of the copepod Tisbe hoZothu-
riae either decreased or increased according to the number of
adults present. No increase of mortality in the adults was
noted; however, an earlier study (Fava and Dalla Venezia, 1976)
recorded 30 % increase in cumulative mortality after 6 days in
the 4 mg/l concentration. It must also be pointed out that
detergents are present in the saltmarsh of Saaftinge in similar
concentrations as in the Westerschelde and that Arlt (1975)
found high densities of Harpacticoids in front of an urban
sewage outlet.
Fluoride is considered a pollutant because marine organisms
can store it in large quantities (Peres, 1976; Perkins, 1976)
and it is hazardous to man. There exists no literature on
toxic concentrations for marine organisms.
Other pollutants are only intermittently present in the
Westerschelde. Pesticides, the most dangerous and persistent,
are either absent or occur in very low concentrations (lin-
dane: 0.01 ]lg/l) conform to the permissive level suggested
by Perkins (1976).
Sporadically concentrations of 1.0 to 5.0 ]lg/l of phenoles
occur at the mouth of the Westerschelde. Welch (1980) re-
commends an allowable ma.ximum concentration of 0.1 mg/l for
freshwater and aberrant behaviour in marine organisms (molluscs)
is only noted above 10 ppm levels (Perkins, 1976).
6. Possible influence of other biota
Small diatoms, probably an important foodsource for epi-
benthic copepods, were counted from three Westerschelde and
two Saaftinge samples on one occasion. They were present in
similar quantities and food in the form of diatoms or organic
matter may be excluded as a limiting factor. Meiobenthic pre-
dators such as Protohydra, do not occur in the Westerschelde,
but are abundant in the saltmarsh. There exist however im-
portant populations of the shrimp Crangon erangon and preda-
tory polychaetes in the estuary (Vermeulen, 1980), which can
increase the stress put upon the epibenthic copepod population.
CONCLUSIONS
Benthic harpacticoid copepods are a successfull group in un-
stable, estuarine environments and are as a rule quantitative-
ly, the best represented meiobenthic taxon after the nematodes.
Their extreme impoverishment in the Westerschelde estuary is
clearly abnormal and indicates severe stress. Through com-
parison of the chemical and physical characteristics of the
Westerschelde with other estuaries and an evaluation of the
scarce data from bioassays, it appears that chronic pollution
by heavy metals especially copper, which are present through-
out the year in sufficient quantities to produce important
sublethal effects, is the most probable cause for the decline.
In the Westerschelde, ecological monitoring clearly de-
monstrates the reduced quality of the environment in a way
that could be achieved only with difficulty by other monitor-
ing methods. For instance, neither oxygen nor ammonium, or
the amount of nutrients and organic matter are abnormal in the
seaward parts of the estuary and the sandy sediments themselves
appear pure to the eye. Neither does the remaining studied
fauna indicate a diminished quality. Nematodes are well re-
presented in the estuary, with mean annual densities in excess
of 1000 per 10 cm2 except at Doel and in some of the stations
1n front of Terneuzen (Van Damme et al., 1981). Wolff (1973)
found that diversity of macrofauna in the Westerschelde was
similar to that of other estuaries in the Delta region. Re-
sults from a more recent survey failed to demonstrate a gene-
ral decline in number or diversity of the macrofauna, except
locally (again at Doel and at some Terneuzen stations) (Ver-
meulen, 1980). For planktonic species in the Westerschelde,
the influence of pollution is difficult to assess (De Pauw,1975; Bakker & De Pauw, 1975), especially in the seaward part
where stocks are regularly renewed with the flood currents.
In the coastal zone of the North Sea, partial impoverish-
ment of the macrofauna could only be determined in the area of
severest pollution, where almost no harpacticoids were found,
while the region with a distinctly impoverished harpacticoid
fauna is much larger (Govaere et al., 1980).
It thus appears that harpacticoids are more sensitive to
pollution than many other benthic taxa. This has ecological
consequences for littoral and estuarine fish species which are
known predators of harpacticoids, especially the younger sta-
ges. Moreover, copepods could be used as an early warning in-'
dicator on an ecological level. They are easily recognized as
a group, even by technicians, and detailed determination to the
species level is not necessary. Especially for benthic species,
the number of samples needed to evaluate annual averages of
community parameters such as density and biomass is small. It
thus appears that a routine procedure involving sampling of
harpacticoids may be a simple and cheap monitoring technique.
Further studies on other estuaries and additional information
in the form of bioassays on sublethal toxicity effects are
necessary to corroborate these findings.
Acknowledgments
This research was made possible through grants from the Belgian
Ministry of Science Policy, Services of the Prime Minister and the
help from Rijkswaterstaat (Water control, Public Works Department,
The Netherlands). We acknowledge therefore II'.C. Bakker and
Ir. J. Gosse from Rijkswaterstaat advisor department at Flushing
for the logistic support to make this study possible. We thank
especially the crews of the R.V. WELSINGE and the R.V. WIJTVLIET
for their enthusiastic field assistance in the ~vesterschelde. For
technical assistance we could acknowledge A. Braeckman, M. De Keere,
W. Gijselinck, A. Van Bost and D. Van Gansbeke. We also thank
Dr. G. Billen, D. Claeys, Dr. N. De Pauw, II'.J.A.W. de Wit,
Dr. K. Essink, M. Holvoet, II'.H. Koopmans, Dr. F. Vaes, K. Wil-
lems and Prof. R. Wollast for discussion and information and
C. Lostrie for typing several manuscripts.
References
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Fig. 1. Localisation of sampling stations.
Fig. 2. Longitudinal profile of average annual density N
(ind./IO cm2); average annual biomass B (1Jgdwt/IO cm2
) and
average annual diversity H (bits/ind) of the epibenthic har-
pacticoids in the Westerschelde and Eems-Dollard estuaries.
THE ~... 52N
NETHERLANDS'~f~ER"'A]'"1. .••••.• / .:
l, \ 'ANTWERPEN -, , f..
BELGIUM .: I')6'SRUSSEL ( .. :..., __
4,Ot 60e
N· 86 WESTERSCHELDE ESTUARY "-w 236,2• 67,92020
10 10
6\
\\\\\\\
\\\\\\
\." "....,_.. _-_..-- -0- -- ." .~'::
I ...,.--------r-==-r.:---I Ossenisse Valkenisse \ Doel
Terneuzen SaltmarshSaaftingen
o
Q
-,------Vtissingen
N. 86100 150
EEMS - DOLLARD ESTUARY
4 60
20 30
<:> "'---7~I Heringplaat I
Reiderplaat OostfrieschePlaat
iUithuizerwad
Eemshaven
iHoogwatum
(_~ ~.60E
NORTH SEA
52N
uo1.5
10
·05
H'o
25
20
·15
10
05
,.".'c;
Table 1 : Main hydrodynamical and physical characteristics of
the Westerschelde and Eems-Dollard estuaries.
Table 2 : Average values of median grainsize (Md llm),mud
percentage (fraction < 63 ].lm)and percentage organic matter
(O.M.) and organic carbon (D.C.) at each stationgroup, divided
in 'pure sand' stations (% mud < 2 % over the whole sampling
period) and 'muddy sand' stations (% mud> 2 % during the who-
le sampling period).
Table 3 : Trace metal concentrations in sediment and suspended
matter (1) Hoenig, 1976, (heavy metal concentration in sedi-
ment expressed in ].lg/gdwt of total sample); (2) Vaes, pers.
connn., (idem); (3) Salomons and Mook, 19'77, (heavy metals in
sediment expressed in ~lg/g of a representative sediment con-
taining 50 % particles < 16 um) ; (4) Data communicated by Lr,
J.A.W. de Wit, Rijkswaterstaat RIZA, Lelystad, The Netherlands.
Table 4 : Annual averages (from fortnightly samples) of sali-'
nity, oxygen, annnonia, total fosfates and total organic carbon
in surface waters of the Westerschelde estuary (Anonymous,1978-
1979) and Eems-Dollard estuary (Anonymous, 1976-1977). Total
organic carbon averages for the second estuary were calculated
from date from Laane (1980). The Saaftinge saltmarsh samples
were collected in July 1980.
Table 5 : Mean annual density N (ind./l0 cm2), biomass B (].lg
dwt/l0 cm2) and diversity H (bits/ind) of benthic harpacticoids
of the Westerschelde estuary, Saaftinge saltmarsh and Eems-
Dollard estuary, per stationgroup and according to sediment
composition (pure sands = p.s.; muddy sands = m.s.) at each
stationgroup. (st = number of stations; n = number of samples).
Table 6 : Species list of benthic harpacticoids of the Wester-
schelde estuary and Saaftinge saltmarsh, according to the sa-
linity zones (E.P. = Eu-polyhaline; P.M. poly-mesohaline;
M = mesohaline, M.O. = meso-oligohaline). Individual biomass
B. (in Vg dwt) per species, mean density N (ind./IO cm2);1
dominance in % and absolute frequency (st = number of stations
n = number of samples).
Table 7 : Species list of benthic harpacticoids of the Eems-
Dollard estuary, subdivided according to the salinity zones.
Type of estuary
Average flood volumeAverage freshwater volumeRange of normal tidal
current velocityVertical tidal amplitudeTidal waveLength of sea armWidth at landlocked mouth
Channel depth at low tide
WESTERSCHELDE EEMS-DOLLARD
coastal plain estuary withslight partial stratificationand vertical mixing (1)
1300xl06 m3
89 m3/sec(2)
(3)
(4)
idem(5)
410xl06 m3 (5)83 m3/sec (5)
1 .0-1 .5 m/ s (5)
2-3 m (5)idem (5)
33 km(+) (5)
9 km (6)
av:4-8 mmax. 16 m
"----------------'----------- ._--.-_._------''-------_._-
0.7-1.5m/s
4-5 m (3)semilunar M2, 12h25 min (3)
80 km (3)5 km (2)
av: 10-18 m max. 63 m
(l) De Pauw and Peters, 1973 ; (2) Theuns, 1975 ; (3) Peters and Ster-ling, 1976 ; (4) Valcke et al., 1966 ; (5) Dorrestein, 1960 ; (6) VanStraaten, 1960.(+) this is the length of the landlocked sea arm part of the estuary.
Md um % Mud % OM % OC---------
Westerschelde estuaryDoel ms (8) 190 9. 1 5.2Valkenisse ps (2) 183 8.5 1.2 0.04
ms (3) 147 8.8 7.7 1.62Ossenisse ps (3) 215 0.4 1.4 0.06
ms (1) 155 2.9 4.4 0.29Terneuzen ps (3) 210 0.5 2.4 0.05
ms (2) 143 5.2 5.4 0.34Vlissingen ps (2) 230 0.7 2.7 0.05
ms (5) 164 12.9 6.3 0.37Saaftinge ms (4) 104 16.2 1.3 0.18
Eems-Dollard estuaryOost-Friesche Plaat ms (5) 132 12.7 2.74Heringsplaat ps (5 ) 122 1.7 0.26Reiderplaat ms (4) 86 14.4 1.03Hoogwatum InS (4) 93 6.2 0.87Eemshaven ps (8) 114 1.1 O. 11Uithuizerwad ms (9) 112 4.7
-_ ...-
Westerschelde (1)
Flocculation zone(Antwerpen-Hansweert)
Sediment
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Znppm
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ppmn
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Gent-Terneuzen Channel (1)
Suspension
Saaftinge Saltmarsh (2)
Sediment
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Dollard
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153
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91
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type st n N B H'-- -- - ------
Westerschelde-estuaryDoel ms 8 64 0.10± 0.05 0.23± 0.10 0.008±0.008
Valkenisse ps 2 10 0.90± 0.71 0.17± 0.17 0ms 3 13 0.77± 0.54 2.34± 2.21 0.04 ±0.04x 0.83± 0.42 1.40± 1.25 0.02 ±0.02
Ossenisse ps 3 15 12.0 ± 8.54 3.17± 2.21 O. 11 ±0.07ms 5 9.6 ± 7.31 15.34±15.10 0.16 ±0.16x 10.85± 6.58 6.21± 3.97 0.12 ±0.07
Terneuzen ps 3 15 O.64± 0.29 1.66± 0.80 0ms 2 10 2.30± 1.10 3.87± 1.81 0.24±0.13x 1.33± 0.52 2.58± 0.89 0.10 ±0.06
Vlissingen ps 2 10 26.40±10.84 6.67± 2.84 1.08 ±0.21ms 5 25 10.36± 3.25 20.17± 6.81 0.37 ±0.09x 14.94± 3.97 16.66± 5.13 0.57 ±O. 10
Saltmarsh 4 '7 69.71±24. 68 236.16±83.51 1.14 ±0.09Saaftinge ms
Eems-Dollard estuary~
Oost-Friesche Plaat ms 5 22 17.38± 4.15 29.23± 7.09 0.41 ±0.09Heringplaat ps 4 4 36.07±21.57 33.56±21.75 0.93 ±0.20Reiderplaat ms 4 17 44.57± 6.37 55.99± 9.41 1.44 ±0.11Hoogwatum ms 3 7 46.10±20.03 72.30±32. 84 1.42±0.13Eemshaven ps 5 17 87.79± 7.55 147.95± 13.12 2.20 ±0.07Uithuizerwad L ms 9 14 89.98±34.52 127.60±45.85 1.47 ±0.20
---'---
Westerschelde estuary
CanuelZa peppZexaHa'lectinosoma sarsiPseudobradya beduinaPseudobradya quoddiensisArenoseteZZa germanicaHaetriqerel la sp •Euterpina.acutifronsTachidius discipesHappacticus fZexusHarpactsicue ZittoraZisStenheZiapaZust.risRobertgurneya sp.NitocratypicaParamesochra simiZisKZiopsyZZus constrictusEvansuZa pygmaeaLeptastacus ZaticaudatusParaZeptastacus espinuZatusArenocaris bi:fidaHuntemannia sp.Paronychocamptus curticaudatusAseZZopsis intermediaPZathycheZipus ZittoraZisNumber of individuals/IO cm2
Total number of species
B.i,
3.90 0.068.40 0.051.500.101.50 0.060.63 0.010.63 -1.800.06I. 90 I. 741.80 0.011.800.033.19 0.880.80 0.250.20 -0.20 1.150.20 1.280.25 0.400.23 0.010.25 1.500.23 0.062.80 0.012.60 0.401.00 1.223.56 0.01
9.2921
Salt marsh of Saaftingest=4 n=7
AZteutha depressa 8.00Stenhe l.ia pal.uetirie 3. 19Nannopus paZustris 3.40Paronychocamptus nanus 0.60PZathycheZipus ZittoraZis 3.56
Number of individuals/IO cm2
Total number of species
EPst=12 n=69 s
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