Succession of macrofauna on macroalgal wrack of an exposed sandy beach: effects of patch size and site

Abstract

In this study, we used experimental manipulation of algal wrack to test hypotheses about influences on macrofaunal assemblages inhabiting the upper shore level of different sites along an exposed sandy beach. First, we hypothesized that decomposition of algal wrack depends on wrack patch size and site. With respect to macrofauna, we tested the hypotheses that (1) abundance of colonising individuals and species vary with wrack patch size, (2) succession (i.e. sequence of colonisation and species replacement) depends on time, and (3) as a result, macrofaunal assemblages associated with wrack patches vary with the patch size and time. We also predicted that responses could be different across sites because of their slightly different environmental conditions. The decomposition of wrack patches was similar in all sites and was dependent on wrack patch size. It was strongly influenced by time-specific environmental and/or biological factors. The pattern of colonisation, i.e. total number of species and individuals, varied among wrack patch sizes. Small patches had fewer species and individuals than medium and large patches. Nevertheless, pattern of colonisation varied among species, across sites and through time. Colonisation of wrack patches was rapid (i.e. within 3 days) for most species. There was some evidence to support the hypothesis that macrofaunal assemblages change in response to patch size and time.