Sedimentological, ecological and hydrodynamic effects of a tube-building polychaete (Lanice conchilega Pallas)

Abstract

A tidal channel habitat dominated numerically by the tube-building polychaete Lanice conchilega is described from Tentsmuir Beach in N.E. Fife, Scotland. This habitat was named the Lanice tidal channel sub-environment and was characterized by sedimentological features and macrofauna associated with aggregations of Lanice. These aggregations formed a habitat structurally and faunistically more heterogeneous than surrounding mobile sand habitats. The aggregations occurred in discrete areas protected by a linear sand bar from onshore swell but were subjected to relatively strong tidal currents. Eight divisions of the sub-environment were described from surface features and used to compare sediment and macrofauna distribution in relation to environmental features. The dense tube beds provided a stable substratum for settlement and growth of algal species. Mounds and stable flats were produced during the spring by sediment deposition combined with rapid growth of algal thalli and tube extension. The structures were quickly eroded after the death of the algae although some areas maintained high tube densities. The average sediment of Tentsmuir was a fine sand with a moderately well-sorted, negative-skewed, mesokurtic grain size distribution. This average distribution included a coarse tail, primarily composed of carbonate fragments. This coarse tail was concentrated in the areas numerically dominated by Lanice conchilega. Two faunal assemblages were recognized: a shallow-sand association not strongly associated with Lanice density (Nephtys caeca, Tellina tenuis, Ophelia limacina and Donax vittatus), and a Lanice conchilega association (Lanice conchilega, Eumida sanguinea, Anaitides maculata, Harmothoe lunulata). The effect of the projecting tubes of Lanice on particle flux in the benthic boundary layer was studied in laboratory flume experiments. Experiments were performed with dye and denatured egg albumen particles to determine flow patterns and net particle movement in individual tube wakes. Projecting tubes produced characteristic vortex patterns which may increase resuspension of sediment particles. The characteristic vortex patterns were not strongly dependent on obstacle shape or flow regime and resulted in pronounced upward motion of fluid from the bed. Particle velocities within the wake were found to be lower than ambient channel flow velocity. It is suggested that Lanice can utilize this flow disturbance to feed on resuspended food particles.