Reefing the environment: implications of human-made structures on the ecological dynamics of coastal waters

By providing structural complexity, artificial reefs (ARs) support and reshape ecosystems. While their role as modifiers of benthic and fish assemblages is well documented, their effects on the surrounding water column remain less understood. Here, we compared the biological and physicochemical properties of the water surrounding four shipwrecks functioning as ARs against non-reef (nR) soft-bottom areas within the Las Grutas Underwater Park, a prominent recreational diving site in Argentine Patagonia. The relationship between ARs presence and the water environment was assessed using multivariate and univariate analyses, considering seasonal variability and coastal distance, while accounting for inherent AR traits such as size, age, orientation to currents, and associated biomass. Although seasonality and coastal distance were the main macro-structuring factors, environmental conditions differed consistently between ARs and nRs. Relative to nRs, ARs exhibited up to a four-fold depletion of dissolved inorganic nutrients, increased indicators of primary production (including higher pH and a 2.5-fold increase in chlorophyll-a concentration), and patterns consistent with enhanced recycling of dissolved chromophoric organic matter. Overall, despite modulation by the specific traits, these Patagonian ARs displayed a distinct ecological signature compared to soft-bottom areas, characterized by enhanced production and coupled with a sustained nutrient depletion. By expanding the observational focus beyond biota directly associated with the reefs, this study contributes to a more holistic understanding of how artificial structures integrate into and modify coastal ecosystem functioning, while providing relevant information for the design and effective management of artificial reef systems.