Meiofauna response to a dynamic river plume front

The benthic response to a plume front was studied in two areas of the north ern Adriatic (Mediterranean Sea) differently influenced by the Po River fre shwater input. Sediment samples were collected in June 1996 and February 19 97 from 12 stations. The adopted sampling strategy was able to identify the front line in real time by satellite images and to locate sampling station s along an inner-outer plume gradient in order to cover the benthic area be neath the river plume, where enhanced biological production was expected, a nd open-sea sediments not directly influenced by freshwater inputs. Meiofau nal parameters were compared to the physical conditions and to phytodetritu s inputs, organic matter accumulation and bacterial secondary production. T he sediments of the Adriatic Sea were characterised by high concentrations of phytopigments (0.6 to 13.9 mu g g(-1) for chlorophyll a and 1.2 to 17.7 mu g g(-1) for phaeopigments) and biopolymeric organic carbon (0.15 to 3.02 mg g(-1)). The plume system extended for a large sector of the northern Ad riatic. In the northern area, a large and highly dynamic plume area was cou pled with a sediment organic matter concentration significantly higher than in open-sea sediments. In the southern sector, where the plume area and th e front line did not change markedly during the year, plume-benthic couplin g was evident only in the sediments beneath the front, and corresponded to phaeopigment accumulation. Bacterial parameters and secondary production we re high and significantly higher in the frontal area than at open-sea stati ons. Meiofauna density (1342 to 8541 ind. 10 cm(-2)) did not change either by season or between areas and was significantly correlated with phaeopigme nts and bacterial secondary production. Meiofauna displayed different respo nses to plume inputs in the two sampling areas. In the northern sector, mei ofauna density was coupled with organic matter distribution and displayed h ighest values beneath the plume. In the southern sector, the densities of c opepods, turbellarians and kinorhynchs displayed highest values under the f ront in summer, and the same applied to total meiofauna density in winter. Juvenile decapods and copepod nauplii significantly increased their densiti es in sediments beneath the front. Data presented in the present study sugg est that plume inputs and frontal systems, enhancing phytodetritus accumula tion and benthic bacterial response, might influence density, composition a nd, distribution of meiofaunal assemblages. As river plumes are highly vari able systems affecting the trophic characteristics of the sediments underne ath, their dynamics should be considered when analysing mesoscale spatial c hanges of meiofaunal assemblages.