Observations on benthic assemblages impacted by hydraulic dredges in the Southern Adriatic Sea (Central Mediterranean Sea)

The structure of benthic assemblages impacted by hydraulic dredges in Margherita, Barletta, Lesina, and Varano areas (Southern Adriatic Sea) have been explored through a multivariate analysis conducted on abundance data (N/100 m2) collected during two summer surveys, in 2018 and 2019. Overall, 134 hauls were performed and approximately equally split over the two years. The differences in the benthic assemblages composition were investigated both between the areas and within each area between the two years. The similarity in the species composition among the areas in each year was investigated by means Sorensen-Dice similarity coefficient. Several multivariate techniques (Principal COordinate analysis - PCO and SIMPER) were carried out to explore the benthic assemblages. Finally, diversity indices (Shannon-Weiner's and Pielou's evenness) were calculated for each haul and Tukey's range test was performed. The highest Sorensen-Dice similarity values were found between Margherita and Barletta in 2018 and between Lesina and Varano in 2019. Clear differences between Lesina and Varano with respect to the Barletta and Margherita areas, were highlighted through the PCO analysis within each year. This could be due to the geographical isolation of these areas' pairs from each other. Changes in the benthic assemblages' structure were detected through PCO analysis for Lesina and Margherita, where species more or less sensitive to disturbance changed in abundance between 2018 and 2019. Differently, benthic assemblages in Barletta and Varano did not show clear temporal differences. Diversity in investigated benthic assemblages did not show significant temporal differences. This condition could be due to several anthropogenic stressors (harbors, urban structures) moderately impacting during the year, which could support a stable condition of intermediate disturbance on the benthic assemblages. Further studies should be carried out to establish whether these changes in benthic assemblages are mainly driven by environmental or anthropogenic impacts.