Investigating the influence of mussel aquaculture on the benthic environment in a marine embayment

Eloise Wigger

Aquaculture activities are often cited as having a negative impact on the environment but there are accumulating examples that there can also be benefits. The relationship between mussel aquaculture and benthic macroinvertebrate communities has not yet been studied in Western Australia and cannot be inferred from published literature due to the need to account for site-specific environmental parameters and species. This study assessed the effect 60 years of mussel aquaculture has had on the benthic environment in Cockburn Sound, a temperate marine embayment located close to the city of Perth, Western Australia. Four replicate sediment samples were collected with a van Veen grab at five sites: in the middle of the mussel farm, at the edge (180 m from the middle), and at 60, 120 and 240 m from the farm edge. Macroinvertebrates (> 0.5 mm) were identified and counted and organic matter content, total nitrogen, total phosphorus, chlorophyll-a and phaeophytin-a concentrations, and granulometric composition determined. Organic matter and total nitrogen did not differ among sites. Significant differences were detected in total phosphorus, which increased with distance from the farm. Sediment granulometry at the site furthest from the farm differed significantly from that of the middle of the farm and contained a higher proportion of large grain sizes. A total of 122 taxa from 7 phyla were recorded across the study area with molluscs dominating every site. The average total abundance of macroinvertebrates ranged from 278 in the middle of the farm to ~550 ind. 0.05 m-2 (SE = 50.54) at the two sites furthest from the farm. While there was significantly reduced abundance in the middle of the farm, there was no loss of taxa richness and no detectable shift towards stress-tolerant species as measured by the ratio of opportunistic polychaetes to amphipods. Community composition in the middle of the farm differed significantly from all other sites, with Nematoda spp. (classed as tolerant to enrichment by AZTI’s Marine Biotic Index [AMBI]), Onuphidae sp. and Lasaeidae spp. identified as invertebrates that most strongly distinguished site differences. The community in the middle of the farm displayed the most variability between replicates, a marker for stress, while the site furthest from the farm displayed the least. Samples from outside of the farm were distinguished by gastropods and contained a greater abundance of Finella sp, which is not classed by AMBI as tolerant to enrichment These results suggest that mussel aquaculture in Cockburn Sound has influenced the macroinvertebrate community and sediment characteristics to some degree, with effects largely confined to the middle of the farm. There were, however, no strong signs of degradation.

Investigating the influence of mussel aquaculture on the benthic environment in a marine embayment

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