FILTERING CAPACITY OF SEAGRASS MEADOWS AND OTHER HABITATS OF COCKBURNSOUND, WESTERN-AUSTRALIA

Macro-suspension-feeders (predominantly ascidians, sponges, bivalves) and epifaunal suspension-feeders (hydroids, spirorbids, bryozoans, bar nacles, amphipods) in Posidonia meadows of Cockburn Sound, Western Aus tralia, demonstrate a clear spatial distribution. Although this may be due to a number of environmental variables, this compares well with s patial patterns in phytoplankton levels, which are relatively high in Cockburn Sound (0.94 to 2.66 g chlorophyll a l(-1)) 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 southe astern boundary, 9.6 to 15.4 g AFDW m(-2) at other sites) and generall y lower in bare sediment (0.2 to 9.3 g AFDW m(-2)), although on bare s ediment of the Southern Flats (a site in the southwest) the introduced polychaete Sabella spallanzanii reaches considerable biomass (458.9 g AFDW m(-2)). Heterozostera (1.2 g AFDW m(-2)) and Amphibolis meadows (2.3 g AFDW m(-2)) were found at only 1 site each, but appear to suppo rt a low biomass of macro-suspension-feeders. Epifaunal suspension-fee ders on Posidonia leaves (hydroids, bryozoans, spirorbids, barnacles, corophiid amphipods) reached a substantial biomass (2.3 x 10(6) feedin g units m(-2) at the southeastern site; 0.6 to 0.7 x 10(6) units m(-2) at other sites; 'feeding units' refers to individual polyps, zooids, etc.). Amphibolis leaves supported similar numbers of epifaunal suspen sion-feeders (0.7 x 10(6) units m(-2)) but Heterozostera supported far lower numbers (80 x 10(3) units m(-2)). Initial estimates indicate th at the suspension feeding assemblages associated with Posidonia and Am phibolis meadows in Cockburn Sound are potentially able to filter the overlying water column daily, and may partially control local densitie s of suspended organic matter. Filtration rates in unvegetated and Het erozostera habitats are orders of magnitude lower, so benthic inverteb rate control of suspended particles in these habitats is unlikely. How ever, habitats dominated by the introduced polychaete S. spallanzanii, which has colonised large areas in Cockburn Sound where seagrass mead ows have disappeared, have a filtering capacity of at least the same o rder of magnitude as that of the seagrass meadows they have replaced.