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on the basis of the preceding and the following month.
Macoma (Fig. 5A) reached their maximum biomass in
May and June. Scrobicularia. Mya and Arenicola (Fig.
sR. C & H i in June. ( erostoderma and Nereis (Fig. 5D
& F) in June to October, and Mytilus and Nephtys (Fig.
51; &. CD even later. The biomass remained at about the
same level during the winter half of the year in
Macoma and Scrobicularia. while it decreased during
these months in Cerastoderma, Nereis and Nephtys.
Taking all species together, the peak biomass occurred
in June and the lowest value in March. The seasonal
trend shown lor ihe total biomass (Fig. 51) closely
resembled the graph given for the Balgzand area. 90
km to the west (Beukema 1974).
There was a considerable increase in biomass
during the 3 to 4 months between late winter and mid
summer. The ratio of ihe highest to the lowest biomass
value was rather low in Scrobicularia and Macoma
(1.6 and 1.8. respectively), but considerably higher in
Nereis (2.6), Mya (2.9) and Cerastoderma (3.3). To
establish the degree to which this difference was due to
a change in ihe body weight of animals ol similar size,
we calculated the ratio between highest and lowest
body weight as given in Fig. 3 and in Fig. 8 of Zwarts
(1991). In all species, about 6XW6 of the seasonal
variation in biomass was due to a change in body
condition. Thus, only 40%. of the difference can be attributed
to growth and change in numbers. In
Scrobicularia and Mamma, however, the ratio
highest/lowest biomass was about as large as the ratio
highesi/lowesi condition, implying that, in these
species, the increases in biomass due to growth were
counterbalanced by decreases in biomass due to
mortality.
The seasonal change in body condition explains
nearly all the variation in biomass over the year in
Macoma. Scrobicularia and Mya digs. 4. 5: Fig. 8 in
Zwarts 1991). However, Cerastoderma reached the
highest biomass in July lo September, at a time when
ihe average body condition had already decreased by
This is also true for Mytilus which attained the
highest biomass in autumn, despite the body condition
peaking in June. The explanation for these exceptions
is that ihe seasonal trajectories of somatic growth and
shell growth were not identical. The shell continued to
grow at a higher rale than the flesh, apparently causing
tlesh weights at a given length to stop increasing or
FOOD SUPPLY HARVESTABLE BY WADERS
57
worm length (cm)
Fig. 6. Burying depth as a function of body size in summer and
winter in six benthic species Winter refers to the period December
I" I ehnurv in Macoma balihica. December to April in Areniiolu
marina, and December to March in the four other species. Summer
refers to the nionlhs June lo August, except in Macoma (Mav to
July) and Arenicola (May to August). Number of cases is given,
rum Zwarts & Wanink 11989) for the bivalves and Esselink &
Xvv.irts i l')S9) for Nereis diversicolor