Bathymetric variation of epiphytic assemblages on Posidonia oceanica (L.) Delile leaves in relation to anthropogenic disturbance in the southeastern Mediterranean.
Macro-suspension-feeders (predominantly ascidians, sponges, bivalves) and epifaunal suspension-feeders (hydroids, spirorbids, bryozoans, barnacles, amphipods) in Posidon~a meadows of Cockburn Sound, Western Australia, demonstrate a clear spatial distribution. Although this may be due to a number of environmental variables, this compares well with spatial patterns in phytoplankton levels, which are relatively high in Cockburn Sound (0.94 to 2.66 g chlorophyll a I-') and are generally highest at the southeastern boundary. Macro-suspension-feeder biomass was high in Posidonia meadows (28.6 to 41.3 g AFDW m-2 at the southeastern boundary, 9.6 to 15.4 g AFDW m-' dt other sites) and generally lower in bare sed~ment (0.2 to 9.3 g AFDW m-2), although on bare sediment of the Southern Flats (a site in the southwest) the introduced polychaete Sabella spal lanzan~~ reaches considerable biomass (458.9 g AFDW m-'). Heterozostera (1.2 g AFDW m2) and Amphibolis meadows (2.3 g AFDW m-') were found at only 1 site each, but appear to support a low biomass of macro-suspension-feeders. Epifaunal suspension-feeders on Posidonia leaves (hydroids, bryozoans, spirorbids, barnacles, corophiid amphipods) reached a substantial biomass (2.3 X 10b feedlng units m-' at the southeastern site; 0.6 to 0.7 x 106 units at other sites; 'feeding unlts' refers to ~ndividual polyps, zooids, etc ) . Amphibolis leaves supported similar numbers of epifaunal suspension-feeders (0.7 X 106 units m-') but Heterozostera supported far lower numbers (80 X 103 units m-'). Initial estimates indicate that the suspensionfeeding assemblages associated with Posidonia and Amphibolis meadows in Cockburn Sound are potentially able to filter the overlying water column daily, and may partially control local densities of suspended organic matter. F~ltration rates in unvegetated and Heterozostera habitats are orders of magnitude lower, so benthic invertebrate control of suspended particles In these habitats is unlikely. However, habitats dominated by the introduced polychaete S. spallanzanii, which has colonised large areas in Cockburn Sound where seagrass meadows have disappeared, have a filtering capacity of at least the same order of magnitude as that of the seagrass meadows they have replaced.