Dense aggregations of tube-building polychaetes: response to small-scale disturbances.

Many species of tube-building polychaetes form dense aggregations or ‘patches’ within marine soft-bottom habitats. Although the ecological importance of such patches has been well studied, the effect of small-scale disturbances within patches has not been investigated. On the intertidal sandflat Drum Sands, Firth of Forth, Scotland, the tube-building spionid Pygospio elegans Claparède forms patches approximately 1–1.5 m2, wherein its density is significantly greater than in surrounding sediments. This study explicitly investigates whether the initial communities colonising small-scale disturbances within P. elegans patches differ in terms of community structure and colonisation mode from those of surrounding sediments.Defaunated sediments (1/125 m2) were implanted into the sediments within and outside patches during April, August and December 1997, and sampled after 3 weeks. The results indicated that some species colonised patches and non-patches in significantly different numbers, at least at certain times of the year. Following these controlled disturbances, P. elegans larvae colonised patches in significantly higher numbers compared to non-patch areas during April 1997, but showed the opposite trend in August 1997 when larval recruitment was relatively low. Adult P. elegans colonised patch sediments in higher numbers than non-patch ones during August and December. Capitella capitata (Fabricius) consistently colonised all defaunated areas in relatively high numbers while Cerastoderma edule (L.) and Macoma balthica (L.) colonised non-patch azoic sediments in significantly higher numbers than patch azoics during April 1997. Multivariate analyses, together with nonparametric analysis of similarities (ANOSIM) tests, revealed that the community composition of the disturbed sediments was significantly different between patches and non-patches during April 1997 only. These analyses also indicated that there was a strong temporal effect on community composition since the April community was markedly different from those of August and December 1997. Sediment measurements taken during December suggested that the observed faunal differences were unlikely to have been a response to differences in bulk sediment characteristics or redox potential.It is proposed that the initial recolonisation differences were likely to have been due to the near-bed hydrodynamic effects of P. elegans tubes creating local sediment stabilisation. The ecological implications of differential recruitment following small-scale disturbances within patches compared to outside patches are discussed. The ability of P. elegans patches to enhance successful conspecific colonisation, as demonstrated here, may explain the longevity of such patches. Their persistence promotes alpha diversity and small-scale patchiness, together with providing a local source of colonists for the initial recovery of large-scale disturbances