Assessing the influence of seaweed and shellfish aquaculture on benthic communities in the western English Channel

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Abstract
Seaweed and shellfish aquaculture is increasing globally, offering potentially sustainable food sources with low environmental impacts and possible benefits, including habitat creation and restoration of degraded areas. Aquaculture siting in new areas often requires empirical evidence to support licencing applications; however, environmental impact assessments for seaweed and shellfish farms on underlying benthic (seabed) habitats are currently lacking. Here, we examined whether initiating kelp cultivation (predominantly Saccharina latissima ) in previous European lobster ( Homarus gammarus ) farming sites within a larger blue mussel ( Mytilus edulis ) farm influenced macrobenthic community structure and sediment composition in St Austell Bay, southwestern England, UK. After 2 seasons of kelp cultivation, no effects on sediment composition or macrofaunal abundance, biomass, diversity or assemblage composition were recorded. There was no significant difference in total macrofaunal abundance between M. edulis , H. gammarus or S. latissima culture areas; however, the aquaculture areas hosted higher macrofaunal abundance compared with reference areas outside the farm. Macrofaunal diversity did not differ between lobster/seaweed and reference areas, although it was significantly lower in mussel culture areas, corresponding with spatial variability in sediment composition over the ~70 ha site. Sediment composition was consistent within each treatment area over time, indicating no change due to the farm between 2017 and 2022. Overall, this study found that small-scale seaweed cultivation in a relatively dynamic and open coastal environment had minimal impacts on benthic communities after 2 growing seasons. More region-specific long-term surveys are recommended to confirm whether these findings apply to seaweed and mussel farming more widely.

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