Effects of man-made structures on sedimentary oxygenation: extent, seasonality and implications for offshore renewables

Abstract

The number of man-made structures to be placed in the marine environment is set to increase massively in the near future as a consequence of the wide-scale adoption and commercialisation of offshore electricity generation. Marine renewable energy devices (MREDs) interact with their receiving environment and are de-facto artificial reefs. The Loch Linnhe Artificial Reef (LLR) complex is a large-scale experimental facility, with the main matrix consisting of 30 separate reef modules deployed in 10-30 m depth and over a gradient of hydrographic and sedimentological conditions. The LLR offers potential to examine impacts that are analogous to those likely to occur around MREDs. The extent of the impact of the LLR modules on the receiving environment was assessed by determining their effect on sedimentary redox potential, as a function of distance from the reef-edge, and season, using an innovative, hand-held, underwater redox probe. The results are commensurate with the reef-proximal baffling of water flow resulting in the entrapment of drifting phytodetritus. At the least current-exposed reef-group the expected decrease in mean redox, at 80 mm sediment depth, was 80 mV (95% CI 40, 120 mV) but this effect was not observed more than 1 m from the reef edge and only occurred during summer months (water temperature >10 °C). Redox at the reef edge, particularly during summer months, was more variable compared to redox taken at 1 m and 4 m reef-distance and was associated with the patchy distribution of phytodetrital accumulations. At all reef groups, there was no discernible difference in modelled mean redox between observations taken at 1 m and 4 m-reef distance. Artificial structures, including MREDs, may cause quite major sedimentary changes but this evidence suggests that these effects will be of limited spatial scale and, where phytodetrital accumulations occur, are only likely to be detrimental in oxygen-deficient sediments. Where these changes occur at well-flushed sites they are likely to be associated with increased infaunal biomass typical of moderate organic enrichment.

Keywords: Artificial reefs; Impacts; Marine renewable energy devices; Offshore wind-farms; Redox; Seasonality; Sedimentary oxygenation.