Differential responses of benthic microbes and meiofauna to fish-farm disturbance in coastal sediments

Bacterial and meiofaunal abundance and biomass and their response to the di sturbance induced by fish-farm biodeposition were investigated from March t o October 1997 on a monthly basis at two stations of the Gaeta Gulf (Tyrrhe nian Sea, Mediterranean Sea). The biopolymeric fraction of the organic matt er was characterized by high concentrations which was similar at both fish- farming-impacted and control stations. Similarly, bacteria accounted for a small fraction of the biopolymeric organic carbon (<1%), while the contribu tion due to auto-fluorescent cell biomass (i.e. prokaryotic and eukaryotic cells displaying auto-fluorescence) to the total biopolymeric carbon was qu antitatively negligible (<0.1%). Benthic bacteria appear to be sensitive to organic enrichment as their abundance increased significantly beneath the cage, whilst numbers of meiofauna was lower than in the control. Changes oc curred also in terms of individual nematode biomass that increased as resul t of the biodeposition. A particularly useful tool appeared to be represent ed by the ratio of benthic auto-fluorescent cells to bacterial abundance, b acteria to meiofaunal biomass and auto-fluorescent cells to meiofauna bioma ss. All these parameters described well the impact due to biodeposition on the benthic environment as their ratios displayed significantly higher valu es in farm sediments, but recovered rapidly (15 days) to values observed in the control (i.e. undisturbed conditions) immediately after cage removal. Changes observed in the present study highlight that the increased organic loading determined a shift of the relative contribution of the different be nthic components to the total biopolymeric carbon, so that in highly impact ed systems total benthic biomass becomes increasingly dominated by microbia l components. (C) 2001 Elsevier Science Ltd. All rights reserved.