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

However, for obvious reasons, we made no depth
measurements when mudflats were actually frozen.
The data for the December to February period revealed
no relation between sea water temperature in the range
from -1 to 6 "C and the burying depth of Macoma.
Scrobicularia and Nereis. In contrast, Esselink &
Zwarts (1989) concluded thai in winter Nereis lived in
deeper burrows at lower temperatures. However, their
winter period referred to the longer period of 15
November to 30 March, during which there was a
simultaneous decrease of temperature and burrow
depih from November until February and an increase
for both in March (Fig. 7): hence, seasonal and
temperature effects were confounded. Nonetheless, it
remains likely that Nereis do deepen their burrows as
the winter temperature drops. First, the digging of
deeper burrows by worms after a sudden fall in
temperature was observed by Esselink & Zwarts
11989). Second, Nereis were found in remarkably deep
burrows shortly after a cold spell by Linkc (1939) and
Beukema (1979). Although not confirmed in this
study, a direct response in burrow depth by Nereis
remains a distinct possibility.
There were large differences in ihe burying depths
between the eight winter and seven summer periods
studied i fig. 9). Macoma lived at depths ol only 3 to 4
cm in the winters of 1982/83 and 1984/85. but at
depths ol 6 to 7 cm in the winters of 1979/80. 1980/81
and 1986/87. The summer depth of Macoma also
varied between years by between 1 and 2 cm.
Scrobicularia reached a winter depth of 12 to 14 cm in
the first three winters when the strong year class of
1976 was still abundant, but not subsequently as the
population collapsed. Again. Nereis lived at greater
depths in the first three winters than in the last five.
Figure 9 confirms that these variations in depth were
not associated with sea waler temperature. For
example, ihe two severe winters of 1984/85 and
1985/86 did not bring about an increase in burying
depth of any of the three species.
We conclude that the seasonal variation in burying
depth oi Mamma and Scrobicularia was not regulated
by changes in temperature. This was also true for
Nereis, although this species is thought to burrow
deeper when temperature decreases sharply. In separate
papers, we will show that the seasonal variation
in depth of Macoma and Scrobicularia can be at-
FOOD SUPPLY HARVESTABLE BY WADERS
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J F M A M J J A S O N D
Iii;. 7. Seasonal variation in A. burying depth of Cerastoderma
edule (10-35 mm; n = 1802). Macoma hulilu, a t10-20 mm: n =
121241. Scrobicularia plana 130-50 mm: n = 14305) and
arenaria (30-50 mm: n = 1396). B. burrow depth of Nereis
diversicolor (7-14 cm: rt = 1946) avA Arenicola marina IA-\ 1 cm; n
= 1611 and C. sea waler icnipcutures and daylight period between
civil twilights. The depth measurements were corrected tor ihe
increase of depth with size (f-ig. 6) to a Cerastoderma of 20 mm.
Macoma of 15 mm. Scrobicularia olI" 35 mm. MyOOt 10 mm. Nereis
of 10 cm and Arenicola of 8 sin The depth measurements are
averaged for seven ' to 1986) and sea water temperature
for 15 years (1972 io 1986).