Scale-dependent patterns of variability in species assemblages of the rocky intertidal at Helgoland (German Bight, North Sea)

Abstract

A growing body of literature shows that benthic communities are hierarchically structured on spatial and temporal scales. In two study locations at Helgoland (North Sea), the northern and the western locations, we: (1) investigated the variation in abundance of specific algae and invertebrates at two spatial scales; and (2) evaluated the relationship between elevation and specific species at these scales. We were also interested in using this information about the spatial pattern of individual algae and invertebrates as well as the patterns of elevation to help develop a monitoring programme of the rocky intertidal. We examined the variation of individual algae and invertebrates by means of a hierarchical nested design. Data were taken from five replicates per plot, with plots located in transects (two transects per location).At the northern location, the highest variability in cover of most algae and invertebrates occurred at the scale separated by about 50 m (scale: transect). This was a direct result of differences between the high- and the low-shore. Most species at high-shore showed a relatively low frequency of occurrence in contrast to a highest frequency of occurence (~100%) and maximal values of cover at low-shore. However, neither a linear nor a non-linear relationship between elevation and the specific species occurred. At the western location, the highest variability in most macroalgae and invertebrates investigated was among replicates (10s of centimetres apart). No relationship between elevation and individual species occurred at this location. Macroalgae at both locations were more consistent over time than invertebrate species. Our results suggest that the relevant processes shaping the individual macroalgae and invertebrates at the Helgoland rocky intertidal vary between locations and the specific species.The potential causes of variation in macroalgal and invertebrate species at different spatial scales are discussed and suggestions for a future monitoring programme are given. Temporal inconsistency in the spatial patterns, and the fact that some individual algae and invertebrates comprising the benthic assemblages vary at different scales, speak in favour of a multiple-scale sampling approach for monitoring change in the intertidal communities at Helgoland.