SUPPLEMENTARY METHODS
(a) Selecting the appropriate size class for depth analysis
M. balthica collected in the field were brought to the laboratory, where shell length was measured to
nearest mm. As burrowing depth relates strongly to shell length (Zaklan & Ydenberg 1997; Zwarts &
Wanink 1989; Zwarts & Wanink 1993), we only selected the 10-mm class (9.5-10.5 mm) to analyze
the proportion of individuals burrowed shallowly (i.e. found in the top layer). In the beginning of our
study period (the late 1990s), it was this length class that lived at a burrowing depth of approximately 4
cm. Therefore, we expected the largest changes in the proportion of individuals living in the top to
occur in this class. An additional reason to focus on this intermediate length class is that knots are
unable to ingest M. balthica > 16 mm long (Zwarts & Blomert 1992; M. balthica can grow up to 22-25
mm long; Gilbert 1973; Kamermans et al. 1999). However, to see how general our results were, we
also performed the analyses for a broader size class with an average length of 10 mm (namely 7.5-12.5
mm). Prior to the analysis, the proportion in the top layer was arcsine-square-root transformed per
station for each year (Krebs 1999). The final field season, 2007, was excluded from the analysis, since,
at the time of writing, individual shell lengths had not yet been determined.
(b) Determining M. balthica’s body mass
Body mass is expressed as the ash-free dry mass (AFDM) of an individual’s fleshy parts. AFDM was
determined by removing the flesh from the shell, subsequently drying it for 3 d in a ventilated oven at
55-60ºC to constant dry mass (to nearest 0.1 mg), then incinerating it for 5 h at 550ºC, where after the
ash mass got determined (to nearest 0.1 mg) and subtracted from the dry mass. We analyzed the
average (untransformed) AFDM per 10-mm individual per station for each year (both low-tide and
high-tide samples). As for depth, we also analyzed the broader 7.5-12.5 mm class. Also here, the values
for 2007 were not yet available at the time of writing.
(c) Analysing population change
Population growth rates r were analysed by calculating the change in density D (m-2; all size classes)
between year t and subsequent year t + 1 per sampling station:
 D  56 

r  log e  t 1

 Dt  56 
1
Note that we added the value of 56 as the log of 0 does not exist. Because our cores were 1/56 m 2,
adding a value of 1 per core leads to adding a value of 56 per m2. Subsequently, the average population
growth rate was calculated by averaging r across stations. Note that, in order to normalise the data,
densities were also log-transformed when analysing the overall decline from 1997 to 2007.
Gilbert, M. A. 1973 Growth rate, longevity and maximum size of Macoma balthica (L.). Biol. Bull.
145, 119-126.
Kamermans, P., van der Veer, H. W., Witte, J. IJ. & Adriaans, E. J. 1999 Morphological differences in
Macoma balthica (Bivalvia, Tellinacea) from a Dutch and three southeastern United States
estuaries. J. Sea Res. 41, 213-224.
Krebs, C. J. 1999 Ecological methodology. Menlo Park, CA: Benjamin/Cummings.
Zaklan, S. D. & Ydenberg, R. 1997 The body size burial depth relationship in the infaunal clam Mya
arenaria. J. Exp. Mar. Biol. Ecol. 215, 1-17.
Zwarts, L. & Blomert, A.-M. 1992 Why knots Calidris canutus take medium-sized Macoma balthica
when six prey species are available. Mar. Ecol. Prog. Ser. 83, 113-128.
Zwarts, L. & Wanink, J. 1989 Siphon size and burying depth in deposit- and suspension-feeding
benthic bivalves. Mar. Biol. 100, 227-240.
Zwarts, L. & Wanink, J. H. 1993 How the food supply harvestable by waders in the Wadden Sea
depends on the variation in energy density, body weight, biomass, burying depth and
behaviour of tidal-flat invertebrates. J. Sea Res. 31, 441-476.
2