Filtering capacity of seagrass meadows and other habitats of Cockburn Sound, Western Australia

Abstract

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 spallanzan~~ 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.

Benthic grazing impact: coupling and uncoupling in relation to physical forcing

“…A traditional way is to establish clearance capacity of the dominant suspension feeder(s) in the laboratory, determine in situ densities and multiply the 2 estimates (Petersen & Riisgård 1992, Lemmens et al 1996, Roditi et al 1996, Riisgaard et al 2004). This approach neglects actual mixing conditions in the field and thus tends to overestimate grazing impact.…”

Uptake of dissolved inorganic nitrogen by shallow seagrass communities exposed to wave-driven unsteady flow

“…The interaction between water column hydrodynamics and submerged vegetated canopies affects the coupling of benthic and pelagic nutrient cycling (Lemmens et al 1996, Thomas et al 2000, Morris et al 2008) and affects stresses imposed at the sediment/ water interface (Fonseca & Fisher 1986, Fonseca & Cahalan 1992, Gacia et al 1999. Seagrass communities provide important ecosystem services: they filter the water column (Szmant & Forrester 1996, Lapointe et al 2004, feed grazing marine species (Fry & Parker 1979, Moncreiff et al 1992, Connolly et al 2005, and shelter organisms during early stages of their development (Eggleston et al 2004, Gullström et al 2008.…”

Epiphytic bryozoan community of Posidonia oceanica (L.) Delile leaves in two different meadows at disturbed and control locations

“…The variations based on species composition and coverage data were more de-scriptive than total bryozoan coverage data. According to Lemmens et al (1996), the density of total bryozoan species did not demonstrate significant differences between selected sites. According to our findings, locality, leaf side and leaf part can influence the coverage value of epiphytic bryozoan species.…”