waders and their estuarine food supplies - Vlaams Instituut voor de ...

total biomass since most of those that died belonged lo
the 0+ cohort, while most of the older age classes
survived the winter, as has been reported elsewhere
(Ha*-*cocl A. Urquharl 1964).
Mytilus (shell length I to 54 mm) was rarely found
betore 1983. Subsequently, there weie three spatfalls
w ith 45, 400 and 2000 spat m 2 in August 1983, 1984
and 1985. respectively. Most did not survive the
autumn and only a few percent of the 1983 and 1984
cohort was still alive the following year, so the biomass
curve 1lig. 41-) refers only to two year classes.
The biomass oi Nereis (1 to 22 cm) varied betw ecu
I and 14 g m -' ilig. 41). The biomass was high in the
autumn following a successful settlement of juveniles.
as occurred in ihe summers of 1979, 1980. 1981 and
1986 when Mill 300 to 350 worms m 2 were found to
be present in August.
There was recruitment oi Arenicola onlj in 1978
when 30 juveniles m • were found. The biomass curve
(Fig. 4G) refers to members of this cohort being 2 to 7
cm long in 1978 and 5 to 10 cm in 1974. The species
was found only incidentally in later years.
Nephtys (1 to 12 cm) occurred at densities of 0 to
40 worms m 2 . equivalent to 0 lo 2 g in ' in biomass
terms (Fig. 4H). The biomass reached, as in Nereis, a
high value some months after a considerable settlement
had occurred in the summers of 1982 and 1983.
As in other Wadden Sea areas (Beukema et al. 19931,
the species was absent or very rare alter severe winters.
Four bivalve species, Macoma. Scrobicularia, Mya
and Cerastoderma, contributed ihe lion's share lo the
toial biomass of the macrozoobenthos (Fig. 41). The
highest biomass was about 70 g AFDW m 2 for each of
these species, which is several times as much as the
average biomass per species calculated over all
sampling dates. Cerastoderma and Scrobicularia were
absent in several years, and Mya also occurred at only
very low densities in some years. The variability in
biomass oi Macoma was less extreme, as was also the
case for Nereis.
The values given for the total biomass included not
only the eight species shown, but also the contribution
from four other species. Nereis virens (0.1 I g in •'.
averaged over all sampling dates) was rare, except in
1980 when its biomass was over 1 g DO* 2 . Carcinus
(0.12 g nr 2 ) occurred in late summer at a density of 5
to 15 specimens m 2 . Corophium was only found in
FOOD SUPPLY HARVESTABLE BY WADERS
55
1977. with 270 individuals m -' and a biomass s-H
0.2 g nv 2 . Lamce conchilega was very rare and onlv
Obsei ved before the severe w inter of 1978/79.
Two worm species. Heteromastits fiuformii and
Scoloplos armiger, were not sampled and the Mud
Snail Hydrobia ulvae was sampled only from 1981
onwards. During those 5 years. Hydrobia occurred at
high densities of io to 20 thousand snails tn : . But as
mosl of the snails were juveniles, the total biomass was
not high, vary ing beiw ecu 2 and 5 'J. nv-. Together with
HeteromastUS and Scoloplos ihe values for Hydrobia
have not Iieen included in Fig. 41. On average, the
resulting underestimation must have been about 5 to
10 gm \
It should be noted that all biomass values were
obtained with a I-mm mesh screen. Hence bivalves • I
mm long were rarely found, and many somewhat
larger specimens must also frequently have been
missed. As a consequence, spatlall was never noticed
before August. A 0.5 mm sieve, mounted below a I
min sieve, showed that the proportion of Nereis
passing through the I mm sieve, but retained below on
the finer sieve, increased Irom 22 r 5 mm were completely
sampled: 90.66.51 and 28% of the si/e classes
2. 3, 4 and 5 mm. respectively, passed through the
I-mm sieve and were retained on the 0.5 mm sieve.
However, we suspect that Corophium was the only
species for which a 0.5 mm sieve, instead of a 1 mm
sieve, would have resulted in much higher biomass
esiimates.
The total biomass in the study site (Fig. 41) was
about four limes higher than the average biomass of
the macrobenthos on the tidal Hats of the Dutch
Wadden Sea (Beukema 1976). The species composilion
in the samples was also different. Scrobicularia is
rare in the Dutch Wadden Sea. but common along the
Frisian mainland coast, while ihe reverse is ihe case in
Arenicola. The intertidal substrate along the Frisian
Coast consists of mud and muddy sand, whereas 85%
ol tidal Hals of the Wadden Sea are sandy with a clay
contenl of < 39E (de Glopper 1967. Zwarts 1988b). The
different species composition in our study area
compared with the Wadden Sea as a whole, can be
understood as the distribution of macrohenthic species