Annual Macrofauna Production in A Hard-Bottom Reef Community

Abstract

This paper is the third and last in a series employing cohort growth-analysis techniques to describe the secondary production of the major component species in the macrobenthic communities of the Bristol Channel. Previous studies were made in soft-bottom areas of a Venus community (Warwick, George & Davies, 1978) and an Abra community (Warwick & George, 1980). The hard-bottom areas of the Channel support what might classically be denned as a Modiolus community (sensu Thorson 1957; see Warwick & Davies, 1977), but within these communities Modiolus itself is inconspicuous because of its small size. This community is found in its purest form on the rock-floors of the central region of the Channel, but gravel accumulates in areas of slower current seaward of this region and may often have admixtures of finer sediment accommodating a mixed-bottom subgroup of the Modiolus community, containing additional species more typical of soft bottoms. The intention of this study was to examine a site representative of the community in its purest form. Quantitative grab sampling in the rock-floor areas was naturally impossible, but fortunately areas exist in which extensive reefs of the tubeworm Sabellaria spinulosa Leuckart are found, and the structure of these reefs is sufficiently brittle that quantitative grab samples can be collected. Such sites have a more diverse fauna than the open rock areas, the interstices of the reef providing a refuge for a large number of smaller species.