Initial fish-farm impact on meiofaunal assemblages in coastal sediments ofthe Western Mediterranean

We studied the initial impact of organic loads due to the biodeposition of a new fish farm in a non-impacted coastal area of the Tyrrhenian Sea (Weste rn Mediterranean). Sediment chemistry and meiofaunal assemblages were inves tigated from July 1997 to February 1998 on a monthly basis at two stations: the first was located under the fish farm, while the second was at about 1 km distance, and served as control. Variations in the biochemical composit ion of the sedimentary organic matter and meiofaunal community structure we re also related to changes in reared fish biomass, The presence of the cage -induced rapid changes in the benthic compartment: the sediments reached ra pidly (i.e., after only 6 weeks) reducing conditions. A significant accumul ation of biopolymeric carbon was observed beneath the cage both at the begi nning of this study and 5-7 months after cage disposal and appeared to be r elated to the fish-farm production cycle. Sedimentary proteins and lipids r esulted to be good descriptors of medium-term fish-farm impact. Changes in the sediment conditions beneath the cage determined a significant reduction of the total meiofaunal density ton average for 70%). Most meiofaunal taxa , including nematodes, copepods and ostracods displayed a significant decre ase in the farm sediments within 3 months after cage installation, Kinorhyn chs appeared extremely sensitive to reducing conditions of the sediments an d disappeared almost completely, whereas polychaete densities remained unva ried. After initial impact meiofaunal assemblages responded to organic enri chment recovering, at least partially, their apparent structural characteri stics. These data indicate that studies carried out several months after ca ge deployment might underestimate the actual impact on benthic assemblages. (C) 1999 Elsevier Science Ltd. All rights reserved.