Six decades of change in a Mediterranean port fouling assemblage: winners and losers in the era of global change

Fouling assemblages in ports are typically dominated by euryoecious species capable of tolerating wide environmental fluctuations, suggesting that they may be little affected by seawater warming and related global factors. Some sixty years after the pioneering studies of the 1960s, the fouling of the Port of Genoa was re-investigated in the 2020s using similar methods to assess any long-term changes. During this period, annual average sea water temperature in the port increased from 17.7±0.8 °C to 19.3±0.7 °C, while pH decreased from 8.17± 0.01 to 8.03±0.002. A dramatic decline in species richness (52 to 28) and average monthly biomass (8.4±4.4 to 2.1±0.8) was observed. Out of the total of 64 species inventoried, only 16 were common to both periods, while 36 (the ‘losers’) disappeared (including once abundant species, such as the calcareous sponges Sycon ciliatum and Leucosolenia variabilis , the hydrozoan Ectopleura crocea , and the ascidian Ciona intestinalis ). The 12 species newly appeared (the ‘winners’) included 4 non-indigenous species with warm-water affinity, namely the hydrozoan Calyptospadix cerulea , the sabellid Branchiomma luctuosum , and the serpulids Hydroides dirampha and Spirobranchus arabicus . The recruitment and abundance of the three most common species, the serpulid Hydroides elegans , the barnacle Amphibalanus amphitrite, and the encrusting bryozoan Schizoporella errata decreased, remained stable or increased, respectively. Contrary to expectations, the modifications observed in the fouling assemblage were remarkable, and consistent with the overall environmental change in the Mediterranean Sea biota.