Ricolonizzazione e recupero delle comunità macrozoobentoniche in seguito all’estrazione di sabbie relitte.

The sand-extraction operations may influence the biological characteristics of the impacted sea beds both directly, through removal, smothering and damage caused by the dredge head, and indirectly: surface and bottom plumes, changes in plankton bloom seasons, the release of nutrients and chemicals, as well as sound, can affect both the sea bottom and the water column in the immediate area around the dredging site. In this paper, we analyse the long-term effects of sand extraction on macrozoobenthic communities in an offshore area in the Northern Adriatic Sea, characterised by relict sands formed during the last Adriatic post-glacial transgression. Surveys were carried out before, during and 1, 6, 12, 18, 24 and 30 months after extraction at three impacted and seven reference stations. The operations did not influence the physical characteristics of the sediment, but they caused almost complete defaunation at dredged sites. In particular, the effects of sand extraction on the macrozoobenthic communities were particularly intense, especially during and immediately after the extraction, at stations 3 and 4, and were mainly due to the removal of fauna from the seabed. The recolonisation at the impacted station began early by larval settlement (most polychaetes, molluscs, echinoderms and other sedentary organisms) and adult migration (vagile taxa, such as the dominant crustaceans Apseudes latreillii and Ampelisca diadema). Then, starting from 24 months after dredging, the composition and structure of the macrozoobenthic assemblages become quite similar to that observed prior to sand extraction. The scant difference in the community found before and after 30 months after dredging at impacted stations were due to (1) some rare species exclusive to the first or the last survey; (2) differences in the abundances of common species, characteristic of the relict sands biocoenosis, that can be related to the natural temporal variability of the communities. The species in common between the two surveys constituted about 90% of the whole abundances at both B-Ex. and the A-Ex. 30 impacted stations. Therefore, it could be assumed that the communities of the dredged area returned to the original condition 30 months after sand extraction. This pattern of recolonisation–recovery fits well with the commonly encountered scenario where the substratum merely remains unchanged after marine aggregate extraction.