The effects of water injection dredging on low-salinity estuarine ecosystems: Implications for fish and macroinvertebrate communities

Subaqueous dredging is a management activity undertaken globally to improve navigation, remove contaminants, mitigate flood risk and/or generate aggregate. Water Injection Dredging (WID) is a hydrodynamic technique involving the turbation and downstream displacement of fine sediments using vessel-mounted water jets. Despite the technique being widely applied internationally, the environmental and ecological effects of WID are poorly understood. For the first time, this study used a Before-After-Control-Impact (BACI) experimental design to assess the effects of WID on water physicochemistry, and macroinvertebrate and fish communities within a 5.7 km-long reach of tidal river. WID targeted the central channel (thalweg) to avoid disturbance of the channel margins and banks. Mean but not peak turbidity levels were substantially elevated, and dissolved oxygen levels were reduced during periods of WID, although effects were relatively short-lived (approximate to 3 h on average). Dredging resulted in significant reductions in benthic macroinvertebrate community abundance (particularly taxa that burrow into fine sediments), taxonomic richness and diversity. In contrast, minor changes were detected in marginal macroinvertebrate communities within and downstream of the dredged reach following WID. Reductions in fish taxonomic richness and diversity were recorded downstream of the dredged reach most likely due to behavioural avoidance of the sediment plume. No visibly stressed or dead fish were sampled during dredging. Results suggest that mobile organisms and marginal communities were largely unaffected by thalweg WID and that the technique represents a more ecologically sensitive alternative to traditional channel margin mechanical dredging techniques.

Automated summary

Subaqueous dredging, including Water Injection Dredging (WID), is a globally undertaken management activity aimed at improving navigation, removing contaminants, mitigating flood risk, and generating aggregates. However, the environmental and ecological effects of WID are poorly understood. This study used a BACI experimental design to evaluate the impacts of WID on water physicochemistry, macroinvertebrate and fish communities. The results showed that WID had significant but relatively short-lived effects on water turbidity, dissolved oxygen levels, and benthic macroinvertebrate community abundance, while marginal communities and fish taxonomic richness downstream of the dredged reach were less affected.