Video and image analysis techniques to monitor mussel (Mytilus edulis) reefs development under different hydrodynamic conditions.

Pablo Lanza

Coastal erosion has become a major threat to coastal civilisations due to sea level rise, aggressive climatic conditions, and obsolete coastal defence strategies. The Coastbusters project faces this threat in a sustainable and cost-efficient way by developing nature-based solutions. Coastbusters aims to build shellfish reefs, which can stabilise the loose sandy sediment, limiting the depletion of sand caused by winter storms, , while enhancing ecosystem functioning and services. In this study, we used 2 image techniques (Sediment Profile Imaging and transect video surveys) to extract information about the biogeochemistry, the sea floor and the epifauna of the benthic ecosystem around the reef development areas. Although mussel reefs did not develop in 2020, remarkable differences were found between the 2 study areas (sheltered, exposed). These areas were characterised by different hydrodynamic conditions. The one sheltered by coastal sandbanks was dominated by finer sediments and higher biological activity (burrows, fauna, and biological beds). Conversely, the area more exposed to open sea, was dominated by coarser sediments and fewer organisms were found. Seasonal differences in biogeochemical parameters were found. Identified epifauna were compared between both areas, with special attention to blue mussels’ predators. Lastly, this thesis provides insights into the image techniques and tailor-made protocols used for the data analyses, so they can be used within the Coastbusters monitoring of the coming years (2021-2022).

Key words: Coastal protection, shellfish reefs, blue mussel, hydrodynamic conditions, SPI, video survey, transect dive.