Benthic macroinvertebrate communities of the Swan Estuary Marine Park

Emily J Stout

Benthic macroinvertebrates perform essential ecosystem functions, including as a food source for higher-order consumers such as birds. The sheltered and productive waters of estuaries provide crucial feeding grounds for a range of resident and migratory birds. Such a situation occurs in the Swan-Canning Estuary in temperate Australia, where the Swan Estuary Marine Park was established to reduce human disturbance and protect these important habitats. This study aimed to provide contemporary information on the benthic community of the three areas of the marine park, i.e. Milyu, Pelican Point and Alfred Cove. Sediment cores were collected from subtidal seagrass, subtidal sand, and intertidal sand habitats at each marine park site and adjacent control sites in the Austral summer of 2023. A total of ~21,506 individuals representing 67 taxa were identified, with 46% being annelids. Species richness and abundance were significantly higher at the marine park sites compared to areas outside. Faunal composition differed across all sites, both inside and outside the marine park and among habitats. Specifically, Milyu marine park site had significantly higher species richness and abundance than the corresponding control site. These differences were primarily driven by two crustacean species: the amphipod Corophium minor and the tanaid Tanais dulongii. Pelican Point marine park and control had similar fauna communities, which was credited to both sites being relatively stable with natural geographical protection and a high abundance of estuary generalist species such as Simplisetia aequisetis and Capitella 'capitata'. Alfred Cove marine park had significantly higher species abundance than the control, which was attributed to higher seagrass biomass and reduced human disturbance within the marine park. Habitat had the strongest influence on the fauna, with seagrass areas having the highest species richness and abundance, likely due to their provision of complex three-dimensional structure which provides shelter and food resources. In contrast, invertebrate communities in the intertidal habitat were depauperate and highly variable, reflecting the dynamic environmental conditions and potential predation pressures by wading birds. Comparisons were made to data collected in the marine parks 18 years earlier in the summer of 2005. Both species richness and abundance were significantly higher in 2023 than in 2005. A shift in faunal composition was evident, with 37 species only recorded in 2023, although most of the common species recorded in 2005 were still present. Between 2005 and 2023, the percentage contribution of annelids decreased and that for crustaceans and molluscs increased. This shift in fauna could reflect an improvement in the benthic communities inside the marine park. Potentially because of improved water quality, such as reduced nitrogen and phosphorus and an increase in seagrass coverage. Invertebrates are a major component of many bird diets. Hence, this increase in invertebrate density is important for avian populations, as many are under threat from various anthropogenic impacts and use the estuary as a feeding area. Hence, this study shows that the value of Swan Estuary Marine Park as a reserve and that ongoing improvements in management of the park within the waterway as a whole are essential to sustain benthic macroinvertebrates, and to promote bird resilience and survival.

Benthic macroinvertebrate communities of the Swan Estuary Marine Park

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