Sewage discharges represent a very common source of anthropogenic impact in coastal areas, contributing to generate a high intake of organic matter with consequent reduction of oxygen in the water column and sediments. Among the coastal benthic communities, macrofaunal community is widely involved in the main ecological processes, therefore environmental alterations that threaten these communities, affect their stability over time, and consequently their resistance and resilience.
The main objective of this Ph.D. project was to evaluate both on a spatial and temporal scale, structural, functional, and trophic variations of the coastal macrofaunal community, influenced by the sewage discharge of urban and domestic wastewater in the Gulf of Trieste (North Adriatic Sea). The sampled macrofauna was investigated through new analytical approaches that can best highlight the community response with all its characteristics as well as its change towards a functioning evaluation of the target ecosystem. In general, the community was characterized by high biodiversity values in conjunction with the presence of stress-tolerant species which suggested that the communities are subject to a moderate level of stress. Through the functional traits analysis, the expression of traits related to semi-continuous reproduction, sediment transporters, and sub-surface depositories were increased by moving from the most distant stations to those closest to the main discharge pipeline, suggesting the presence of a continuous wastewater supply. On the contrary, in the more distant stations, a greater expression of the suspension feeder was observed, indicating the presence of lower inputs of organic matter. However, from the analysis of functional richness and β-diversity emerged that following improvement of the plant, which took place over the years studied, the community responded with an increase in the number of species, particularly in the stations near the sewage discharge loading. Furthermore, the bioturbation and bio-irrigation potential were not directly connected with the variation of allochthonous substance but rather with the granulometric characteristics of the area. On the contrary, macrofaunal secondary production also seems to have been influenced by the presence of sewage discharge. High values of the P/B̅ ratio were calculated at the stations furthest from the sewage pipelines and a negative relationship was observed between biomass and trophic efficiency (TE) values. Therefore, it appears that sewage discharge induces an increase in TE at stations close to the pipelines. In our case, the sewage discharge has determined the presence of tolerant species with small size, low biomass, high reproduction, and growth rate, which seem to have allowed a greater transfer efficiency of energy at higher trophic levels.
Overall, community analyses both on a spatial and temporal scale have shown a good degree of resilience to environmental variations induced by the pronounced contribution of organic matter, as well as different patterns of organic matter use in terms of secondary production, productivity, and TE. These aspects (P, P/B̅ and TE) were also treated for the first time in the context of macrozoobenthic communities subject to the influence of sewage and organic enrichment.
The variety of methods applied suggest how the use of integrated approaches that consider macrofaunal communities, analyzing them both from a structural and functional-trophic point of view, can provide useful tools to better understand, monitor and evaluate the functioning of coastal ecosystems, including to multiple disturbing factors.