Dredging disturbance imprinted in the structure of benthic communities: The case of the Port of Cervia

Dredging activities are regularly performed in front the entrance of most of the ports worldwide in order to remove subtidal sandbars, continuously accumulated due to coastal sediment dynamics, and assure minimum depth needed for access. These operations may have severe negative chemical, physical and biological impacts on marine environment (Manap & Voulvoulis, 2015) as well as high economic costs. As possible environmentally and economically friendly alternative, a new technology for seabed maintenance based on “ejectors”, intended to prevent sediment accumulation, is under development. The goal of this research was to analyse macrobenthic communities exposed to dredging activities in front of the Port of Cervia (northern Adriatic Sea) and in the nearby not affected areas, that would serve as a baseline for the evaluation of impacts of new technology that is going to be experimentally deployed in front of this port. Samples were taken in Spring 2018 in the dredged location in front of the Port of Cervia just couple of weeks after the operations and in four control locations, placed 600 m and 1200 m north and south of the impact, at 2-3 m depth range. Two sampling areas were defined in each location and at each area 4 replicated samples of sediment and fauna were taken by SCUBA diving using an aluminium frame (23.5×13.5 cm). In each sample analyses encompassed sediment grain size, percentage of organic matter, dry weight of shell debris and benthic macrofauna (after sieving on 0.5 mm mesh sieve) determination to the lowest possible taxonomic level. Univariate and multivariate analyses were done using software Primer v 6 and PERMANOVA+. Altogether 80 macrofaunal taxa were recorded. Structure of benthic communities was significantly different among sampling locations but not between areas within each location. Dredged location indicated clearly different structure of benthic communities from control ones and showed much higher variation among samples (Figure 1). Impacted communities were characterised by drastically lower species diversity and abundance (Figure 2a, 2b). SIMPER analyses (90% cut off) showed that average similarity in species composition and abundance between samples in control locations was very high (81% to 85%) and due to only three most abundant bivalve species: Lentidium mediterraneum, Donax semistriatus and Chamellea gallina. On the other hand similarity between samples in impacted location was only 37% (characterised by L. mediterraneum and C. gallina). Very high dissimilarity (75% to 80%) in species composition and abundance between impacted and control locations was due to a drastic decrease of abundance of the same three bivalve species on impacted location. Amount of shell debris and the percentage of medium sand (>250 μm) in the sediment were the parameters that had the highest correlation (Rho = 0.716) with the structure of benthic assemblages. Our results indicate that dredging activities had negative impacts both on species richness and abundance, however the settlement of opportunistic species on impacted location was not reported. The three bivalve species mostly characterising benthic assemblages on all locations belong to species sensitive to disturbance, which are characterised by relatively long life, slow growth and high biomass (Simboura and Zenetos, 2002). Communities that are characterized by sensitive species in general show strong effects and slow recoveries of dredging (Kotta et al., 2009). Moreover Chamelea gallina is edible mollusc thus negative impact of dredging on this species, might have economic implications. Several tens of taxa present in control samples weren’t recorded in impacted ones. Although these species weren’t very abundant their ecological importance in benthic communities cannot be neglected. Careful analyses of life traits of these species will have to be performed in order to understand eventual shifts in the functioning of benthic communities.