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

Methods
The study sites were situated on a tidal flat in the
eastern part of the Dutch Wadden Sea. along the
mainland coast of Ibc province Friesland (53 25'N.
6°04'E), and have been described before by Zwarts
(1991) and Zwarts el al. (1992). The siles were situated
just below mean sea level. Macrozoobenthos was
sampled monthly in site N. while depth measurements
were usually made in the nearby site D (Fig. 1 of
Zwarts et al. 1992). The substrate in both sites was
soft, averaging 5 to (fik claj I fraction < 2 uni).
Seventy-three or 292 sediment cores (15 cm 0. 40
cm deep) were taken in site N almost every month
from 1980 to 1986. and more infrequently between
1977 and 1979. The cores were sieved through a l-mm
mesh screen. The animals were taken to the laboratory
to measure their length, dry weight and ash-free dry
weight (AFDW) according to methods given by
Zwarts (1991). The length of Ragworms, Nereis
diversicolor, was defined as the maximum length of a
worm creeping along a ruler in sea water (Esselink &
Zwarts 1989). The length of broken worms was
esiimated from the relation between width of the tenth
segment and the length of intact worms (Esselink &
Zwarts 1989). The length of Lugwonns Arenicola
marina referred only to the body without tail,
measured as the worm suspended for some seconds by
the head in a pair of forceps.
The depth measurements were collected at low
tide, once or twice a month over the seven-year period
1980 to 1986. We used a corer (0 15 cm) thai was
pushed 40 cm into the mud. The extracted core was
laid down on a table and broken open. The burying
depth of the bivalves was defined as the distance
between the mud surface and the upper edge of the
shell. The burrow depth of Nereis and Arenicola
equalled the distance between the surface and the
deepest point of their U- or J-shaped burrow. The
methods are described more fully elsewhere (Zwarts
1986, Esselink & Zwarts 1989. Zwarts & Wanink
1989). The collected animals were taken to the
laboratory to determine length and AFDW of each
individual.
The energy density of ihe well-dried flesh was
measured with a Parr-1665 adiabatic calorimeter. All
determinations were done in duplicate or triplicate for
FOOD SUPPLY HARVESTABLE BY WADERS
.9
each sample. The energy density is given per g AFDW;
ash content was determined by furnace ashing ai 550
°C. A correction was made for the endothcrmic
reaction during the combustion of the Shore Crab
Carcintis maenas (Fame 1966). since half of its dry
weight consisted of CaO •
Sea water temperature was measured daily by
Rijkswaterstaat at 8 a.m. at the nearby station of
Holwerd. SPSS (Norusis 1988) was used for all
.statistical analyses.
Results
Seasonal variation in energy density
Zoobenthos biomass is usually measured in terms ot
ash-free dry weight (AFDW). Predator consumption is
often expressed the same way. the implicit assumption
being that prey weight reflects fcxxl value and that the
energy density does not differ between prey species or
seasons. Enough data were available in three species to
check for any seasonal variation in the energy density
of llesh. No significant difference was found in ihe
tellinid bivalve Macoma bultbiax according lo a oneway
analysis of variance (R 2 = 0.04. p = 0.79, n = 60).
Energy density, however, varied seasonally in another
tellinid bivalve. Scrobicularia plana, and in the Soilshell
Clam, or Gaper. Mya arenaria (Fig. 1). Both
species reached lowest values in March and highest in
May or June. This trend was evident within each year
of sampling, even though the energy density of
Scrobicularia also varied between the years (Zwarts &
Wanink 1991). These seasonal differences were
significant i see lig. I). but they amounied to not more
than 2 kJ. or 10%.
Although previous studies had found no seasonal
variation in the energy density of all three species
(Macoma. Gilbert 1973. Beukema & de Bruin 1977.
Chambers & Milne 1979: Scmhicularia: Hughes
1970b and Mya: Edwards & Huebner 1977. Winther &
Gray 1985). a seasonal variation in energy density
might be expected. Starvation in w inter and spawning
in summer lead to changes in the biochemical
composition of the body (e.g. Ansell & Trevallion
1967. Beukema & de Bruin 1977. Pieiers.-/ al. 1980.
Pekkarinen 1983. Dare & Edwards 1975. de Vooys
1975. Mayes & Howie 1985). Gametes alone may add