The relationship between observational scale and explained variance in benthic communities.

This study addresses the impact of spatial scale on explaining variance in benthic communities. In particular, the analysis estimated the fraction of community variation that occurred at a spatial scale smaller than the sampling interval (i.e., the geographic distance between samples). This estimate is important because it sets a limit on the amount of community variation that can be explained based on the spatial configuration of a study area and sampling design. Six benthic data sets were examined that consisted of faunal abundances, common environmental variables (water depth, grain size, and surficial percent cover), and sonar backscatter treated as a habitat proxy (categorical acoustic provinces). Redundancy analysis was coupled with spatial variograms generated by multiscale ordination to quantify the explained and residual variance at different spatial scales and within and between acoustic provinces. The amount of community variation below the sampling interval of the surveys (< 100 m) was estimated to be 36-59% of the total. Once adjusted for this small-scale variation, > 71% of the remaining variance was explained by the environmental and province variables. Furthermore, these variables effectively explained the spatial structure present in the infaunal community. Overall, no scale problems remained to compromise inferences, and unexplained infaunal community variation had no apparent spatial structure within the observational scale of the surveys (> 100 m), although small-scale gradients (< 100 m) below the observational scale may be present.

The availability of microorganisms attached to sediment particles as food for Hydrobia ventrosa Montagu (Gastropoda: Prosobranchia).

The deposit-feeding prosobranch Hydrobia ventrosa Montagu feeds most rapidly upon sediment particles that pass through a 10 µm sieve. Ingestion rate decreases with particles 80-125 µm, then increases with larger particles, which are fed upon by scraping fine material from their surfaces. Hydrobia is capable of digesting diatoms and bacteria from sediment particles, but with generally lower efficiencies than reported when fed pure cultures. Digestion of microorganisms appears to be constrained by ability of the snail to detach cells from sediment particles; only those cells detached from sediment seem to be available for digestion. In contrast, the amphipod Corophium volutator is capable of utilizing most of the diatoms not digested by Hydrobia. For a given sediment, a constant number of microorganisms appear to be safe from digestion by H. ventrosa, and bacteria and microalgae over this amount constitutes the available food.

The role of sediment type in growth and fecundity of mud snails (Hydrobiidae).

We test the hypothesis that body size and population density of the deposit-feeding gastropod, Hydrobia truncata, are greater in muddy than in sandy habitats as a result of faster growth on fine- compared to coarse-grained sediments. We refute this hypothesis using a combination of field measurements and laboratory experiments. Three out of three populations tested had higher maximal growth rates and two of three populations approached their asymptotic size more quickly on sand than on silt-clay fractions of natural sediment. Growth decreased with increasing snail density and was as high or higher on sand as on silt-clay at all densities. Two populations were more fecund on sand than on silt-clay, and fecundity of the third population was not affected by sediment type. We show that the smaller body sizes observed in snails from the sandiest habitat result from late recruitment of these snails, relative to the other populations.