Seasonality and mortality in harbour benthic communities: the case of Leptogorgia sarmentosa (Octocorallia, Cnidaria) from a Mediterranean port

Harbours are optimal sites for studying the effects of environmental fluctuations on benthic communities due to the wide variation in hydrological variables. The species living here also face trophic and spatial competition with non-indigenous species (NIS) carried into these areas by maritime traffic. Here we investigated temporal changes in biological, physical, and water quality variables affecting a shallow-water (0-20 cm) benthic assemblage found along a dock in the Port of Genoa (Ligurian Sea, NW Mediterranean Sea) with data obtained from long-term monitoring (August 2017 to November 2022). Visual surveys and quantitative samples indicated that this community is subjected to marked seasonal ecological changes. The assemblage includes persistent species and others with unpredictable peaks in abundance, partly due to competition with NIS. The abundance, size structure, and health status (i.e. percentage of colonies with epibionts) of the gorgonian Leptogorgia sarmentosa were used as early indicators of environmental stress. Sexual reproduction data were also gathered. Mortality events of various taxa, affecting juvenile or adult colonies, are linked to disturbances related to spring algal blooms, summer heat waves, and autumn heavy rainfall (reducing salinity, increasing the organic matter, and leading to hypoxic conditions). The strong resilience of L. sarmentosa, favoured by high settlement rates, is the key to the success of this species in harbour environments. This study represents a baseline for future monitoring of Mediterranean harbour benthic communities and offers the chance to better understand the functioning of marine ecosystems in relation to anthropogenic impacts and climate change scenarios.