Anthropogenic and Storm Impacts on a Marine Benthic Ecosystem in an Urbanised Coastal Region of Northwestern Italy

Marine benthic ecosystems along urbanised coastlines face heightened vulnerability due to the cumulative effects of chronic anthropogenic stressors. Climate change intensifies these pressures through more frequent and severe storms, while ongoing coastal development adds further stress through infrastructure projects. This study examined how soft-bottom communities in the coastal NW Mediterranean responded to two major disturbances: an exceptional storm in 2018 and the construction of a new wastewater pipeline in 2019. Sediment grain size, organic content, bacterial abundance and enzymatic activity, and metazoan communities were analysed during summer of 2018, 2019 and 2020 and in the following spring period. Hydrodynamic forcing caused a general increase in the grain size in 2019. Meiobenthos responded with a strong decline in abundance, especially crustaceans, while macrobenthos changed from a mixed deposit-feeder community to a suspension-feeder dominated one. In 2020, the improved trophic value of sediment organic matter in the pipe area favoured bacterial increase. While meiobenthos abundance slowly recovered, the differentiation increased due to macrobenthic juveniles, resulting from increased macrobenthic abundance and diversity (mainly pure deposit-feeders). A clear shift towards organic enrichment-tolerant taxa due to wastewater release was not observed, given the contemporary presence of very sensitive organisms, indicating that co-occurring stressors can lead to nonlinear responses of the communities.: