Effect of Shoot Density on the Infaunal Macro-invertebrate Community within aZostera marinaSeagrass Bed

A seagrass (Zostera marina) bed in south-west United Kingdom was mapped using SCUBA in July 1996. Within the bed, three seagrass density ranges (shoots m−2) were sampled, low (1–50), medium (51–100) and high (>100), to assess infaunal macro-invertebrate community structure. At each of the sites seagrass and sediment parameters were measured (shoot density, number of leaves per shoot, root-rhizome biomass, median grain size, sorting coefficient, silt fraction). As seagrass density increased, a corresponding increase in infaunal diversity was observed, multivariate analysis indicating that there were significant differences in community structure between the shoot density ranges sampled. The main factors explaining community structure were above-ground parameters (leaf number per shoot, shoot density), whilst sediment and root-rhizome measures were apparently unimportant in structuring the infaunal community. The influence of increased seagrass structural complexity on the infauna is possibly indirect (e.g. increased detrital deposition, reduced predator efficiency), however, the direct influence of root-rhizome complexity requires further investigation.