Inter-relationships between sediments, seagrasses, benthic invertebrates and fishes in shallow marine habitats off South-Western Australia

Abstract

Fishes, invertebrates and sediments were sampled in Posidonia and unvegetated habitats at two locations in each of the Rottnest Island, Cockburn Sound and Albany regions of southern Western Australia. Sediment particle sizes were more heterogeneous, and the level of organic matter was higher in seagrass habitats at the three most sheltered locations, while at more exposed sites little difference was apparent in sediment characteristics between vegetated and sand habitats. The species richness, abundance, biomass and estimated production of benthic invertebrates and demersal fishes were greater in vegetated than unvegetated habitats at all locations; however, large numbers of hardyheads and sprats were collected in two sandy habitats (Parker Point and Princess Royal Harbour), greatly elevating total fish production at those sites. Macrofaunal species richness and biomass remained relatively constant between the six Posidonia sites studied (a range of 101-124 species per habitat; 10.2-22.3 g AFDW m⁻² ). These faunal parameters nevertheless varied substantially between unvegetated habitats, with species richness varying over a fourfold range (17-68 species per habitat) and production varying over a 30-fold range (0.5-16.0 g AFDW m⁻² ) between sites. Fish and invertebrate production were much higher in the sheltered compared with the exposed unvegetated sites. The size-distribution of macrofauna showed a regular pattern at the different Posidonia sites, with peaks in production consistently occurring at 1 to 1.4 mm sieve size. Total production of benthic invertebrates was highly correlated with total demersal fish production, being ≈ 12 times greater across all sites. Fish are postulated to be attracted to sites with abundant prey of appropriate size.