For quantifying epibenthic predators, once per experiment at high tide four sam- ples were taken with a drop trap (Pihl and Rosenberg 1984) just outside the plot at each site. The trap was made of aluminum, had 0.5 m2 sampling area and was 70 cm high. To avoid scaring the predators away, it was operated hanging from a 8 m long pole (made of two windsurf masts) by two persons. Sampling took place as soon as the water had receded enough that it would not swash over the sides after dropping the trap (i.e. below 70 cm). The content of the trap was intensively fished with a net with a stable rectangular frame and 1 mm mesh, following Polte et al. (2005). Drop trap sampling was not possible at Balgzand in June, through a combination of weather conditions and logistic constraints. Crustacean samples were stored frozen and Crangon crangon length from scaphocerite to telson measured with 1 mm accuracy.
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Bivalve sampling and sample processing
At the end of the exclosure experiment, bivalves were sampled with a sharpened Perspex tube of 20 cm2 area to a depth of 3 cm. Because of high abundances of M. balthica, in May at Oddewatt a corer of 12.5 cm2 area was used. Nine cores were pooled per experimental unit. Samples were sieved through a 500 µm mesh. Bivalve samples were frozen at -20°C, or (bivalve samples without calcein at Sylt) stored in 4% formalin in seawater, buffered with borax to prevent decalcifi- cation (Sturm et al. 2006). The material of the bivalve samples was sorted in por- tions systematically from Petri dishes. Samples from Sylt were first repeatedly brought in suspension and decanted from the coarse sediment into a 250 µm sieve. Calcein-treated bivalves were measured under a stereomicroscope equipped with a Mercury UV-lamp and appropriate emission and excication fil- ters. The length of the fluorescent growth ring was measured as the longest span from front to back, and ring height was measured perpendicular to that through the umbo (top of the shells at the hinge). The calcein ring measurements were up to 10 µm accurate at the highest usable magnification. For larger measurement values, which had to be done at lower magnifications, accuracy was about 2%. The ring was often not complete and the height was more frequently measurable than the length. Height and length of the complete growth rings were highly cor- related (R2 = 0.99). When ring length was not measurable but height was, the missing length was filled in using the correlation. The daily instantaneous growth rate µ was calculated per individual as
µ = log ( L10 / L0 ) / t (3.1)
where L10 is the shell length at the end of the 10-day experiment, L0 is the initial length measured by fluorescent ring, and t = 10 days is the duration of the experiment.
To be retained by a 500 μm mesh C. edule have to be 0.7 mm long and M. balthica 0.75 mm. Especially for M. balthica at Sylt in May, we missed the smallest individuals (Fig 3.2 c). When growth rates differed between exclosure treatments, individual counts were corrected for the sieve selection, using the measured growth rates. For the data involving growth ring size, we also corrected for a sieve artifact. Individuals with a small initial size were caught on the sieve only when they had a high growth rate (missing lower left corner in Fig. 3.3). That means average growth rates would be overestimated where the initial size is small. Average growth ring size would be overestimated where growth is slow, because slow growing individuals with a small initial size are missed by the
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sieve. Individuals with growth rings smaller than 0.7 mm were excluded from the analysis of growth rates and growth ring sizes. On average the mean growth rate and median growth ring size are based on about ten individuals per sample, sometimes only one or two; the maximum was 68 marked individuals in one sample. A low n within a sample will lead to higher variation between samples.
Fig. 3.2 Macoma balthica and Cerastoderma edule. Size-frequency distributions of shells
in calcein treated experimental units with (black) and without (white, stacked) fluorescent mark, and in migration nets (grey). For the migrants, which had been caught at two occa- sions during the 10-day experiment, the theoretical final length at the end of the ten days was estimated using growth rates obtained from the marking experiment. Note the vary- ing scales for frequencies and for sizes.
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Assessment of immigration
In the early benthic phase, M. balthica and C. edule can perform secondary migrations. A difference in bivalve size or abundance between exclosures and uncaged samples could be caused by (size-specific) migration. In the samples treated with calcein, not all individuals had a fluorescent mark. These could have settled into the plots after initiation of the experiments. To evaluate this, migrat- ing bivalves were sampled over two 24 h periods during each experiment. A migration net consisted of a nylon bag of 500 µm mesh size, glued to a PVC ring of 20 cm diameter, which was mounted on a pole 10 cm above the sediment (similar to plankton net in Armonies 1994). The opening could rotate freely pointing towards water current directions. Four migration nets were installed at each site just outside the sampling plot. The aim was to compare the sizes of the bivalves caught in the water column with the ones from the benthic samples of the experiments. As the sampling of migrating bivalves took place at two times during the experimental periods, for comparison their hypothetical size at the end of experiments was calculated using the obtained growth rates from the marked animals. The size distributions of the animals caught in the nets mostly resemble the size distributions of the animals without a fluorescent mark in the calcein- treated samples (Fig. 3.2). It was concluded that individuals without a mark had entered the sampling units later. For the subsequent analyses of abundances, they were excluded from the data. As immigrants could not be identified in undyed samples, analyses of exclosure effects used dyed samples only.