Spatial and seasonal variability in benthic impact of mussel farms: Predicting and mitigating impacts using ambient oxygen conditions

Environmental impacts are a cause for concern when developing and expanding aquaculture and to be sustainable potential negative effects need to be addressed. The intensity and extent of these impacts likely vary among sites and seasons, depending on multiple factors including the physical and biological setting and operational aspects. Using a combination of sampling techniques, we investigated the spatial variability in epibenthic impacts in eleven commercial mussel farms, on the Swedish west coast. We found increased levels of organic content, changes in epibenthic macrofauna and increased cover of Beggiatoa sp., a documented indicator of hypoxia. The extent of these impacts was generally limited to the extent of the farms. Because the cover of Beggiatoa sp. was particularly clear and because oxygen conditions in the sediment is of great importance to the structure and function of these habitats, we analysed spatial patterns using an index of the benthic footprint (BFI) accounting for both intensity and extent of impacts. In the summer, the BFI varied strongly among farm-sites and subsequent analyses showed that it highly correlated with ambient bottom oxygen concentration. Repeated sampling during early spring, however, showed that impacts were quickly reversible also in the most impacted sites. Thus, we conclude that in Swedish coastal waters the benthic footprint calculated on the % cover of Beggiatoa sp. is highly dependent on ambient oxygen concentration. We suggest that knowledge about spatial and temporal patterns of oxygen in the bottom water can be used to predict the severity of impacts and provide an important criterion in a site-selection process aimed at developing a sustainable food industry. [Display omitted] •Impacts of mussel farms were studied in 11 farming sites on the Swedish coast.•The intensity and extent of impacts varied among farms, regions and seasons.•The impact typically decreased rapidly within 10–30 m away from the farm.•The cover of Beggiatoa sp. was a particularly useful indicator of benthic impact.•We developed an index incorporating both intensity and extent of impacts.