Dense aggregations of Pygospio elegans (Claparède): effect on macrofaunal community structure and sediments

Abstract

Epibenthic biogenic structures such as polychaete tubes are conspicuous features of many marine soft-bottom habitats. This paper compares the benthic macrofauna in patches with high and low densities of the tube-dweller Pygospio elegans on intertidal sandflats in eastern Scotland (UK). The main aim of this study was to determine potential differences in the macrofaunal community structure, the size distribution of individual species and sediment properties. Multivariate data analyses revealed that the macrofaunal community composition (excluding P. elegans) within patches was always significantly different from outside patches, mainly due to variability in the abundances of Cerastoderma edule and Corophium volutator. In addition to P. elegans, 5 taxa were sufficiently abundant for univariate analyses, 4 of these ( Capitella capitata, C. edule, Macoma balthica and C. volutator) being significantly more abundant within P. elegans patches than in surrounding, non-patch sediments. The size distribution of P. elegans was significantly different between patches (bimodal distribution) and non-patches (skewed distribution). Similarly, there was a greater proportion of larger C. capitata individuals within patches compared to non-patch sediments. Sediment organic content and silt/clay fraction were always significantly higher in patch sediments while redox profiles showed no differences except at the end of the study period when the top 2 cm within patches were more positive and more negative at 4 cm. These results imply that even relatively small (1–1.5 m 2) P. elegans patches can have large effects on the spatial variability of macrofaunal community structure on intertidal sandflats. Towards the end of the study there were marked visual changes in the P. elegans patches, such as wave-ripple marks on the surface, which signified their demise. This coincided with dramatic changes in the invertebrate community structure within patches. Along with the decline in P. elegans numbers, dramatic increases in the densities of the 2 bivalve species C. edule and M. balthica occurred, while remaining comparatively stable outside patches. This suggested that the conditions within P. elegans patches were particularly favourable to successful bivalve spat settlement. We postulate that P. elegans patches have limited longevity and propose that enhanced bivalve competition within them leads to rapid decreases in P. elegans numbers.