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Reworking of the sediment surface by the movement and feeding activity of mobile epifauna is likely to affect rates of solute flux across the sediment–water interface, with consequences for the geochemistry of deeper sediment layers. Mobile epifauna are also important scavengers and predators that control the community of sediment infauna and affect the distribution and abundance of other surface-living species. Activity of subtidal epifauna in a sea loch was monitored by video in order to test the hypothesis that patchiness of visits by epifauna to small areas of sediment surface is generated by direct behavioural responses to surface features, such as mounds and holes built by large infauna, and as an alternative hypothesis that patchiness is generated by the properties of the search paths as measured by rates of turning and step lengths. Trajectories of gobies (Lesueurigobius friesii, Pomatoschistus minutus), shrimps (Crangon allmani), and Nephrops norvegicus were digitized in 12 half-metre square quadrats over 24 h. Analysis of frequency of visits to small areas within each quadrat quantified the patchiness of impacts of each species. Trajectories tended to be area concentrated and in many cases more than would be expected on the basis of search path properties alone. Spatial patterns of frequencies of visits showed that movements of gobies L. friesii, P. minutus and Nephrops were strongly influenced by proximity to burrow openings, while shrimps (Crangon) were not affected. Observed patchiness in P. minutus was positively correlated with simulated patchiness, suggesting that properties of search paths may change in response to food and consequently affect patchiness. Crangon allmani often followed furrows, also increasing patchiness. The effect of the characteristics of search paths on patchiness of visits was small compared to the direct response to topographical features. |
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