SPATIAL-DISTRIBUTION AND SEASONAL-VARIATION OF SUBTIDAL POLYCHAETE POPULATIONS IN THE MONDEGO ESTUARY (WESTERN PORTUGAL)

The Mondego estuary consists of two arms, north and south. The north a rm is deeper, while the south arm is largely silted up, causing freshw ater to flow primarily by the north arm. Environmental stress is sever e. essentially due to harbour activities, salt-works, aquaculture farm s. and nutrients and chemicals discharged from agricultural areas of t he lower Mondego river valley. From December 1989 to September 1990 a study of the subtidal macrofaunal community was carried out. Polychaet es were found to be the most abundant macrofaunal group of the Mondego estuary subtidal community, representing about 34.5 % of the species and 63 % of the individuals sampled. Both biological and physicochemic al data were analysed by principal components analysis (PCA), and data on the Polychaete fauna was also analysed with regard to spatial and temporal variations of total abundance. The Mondego estuary seems to p resent an impoverished subtidal Polychaete fauna with regard to the nu mber of species, although the south arm presents enhanced populations. In the inner areas of the south arm. the Polychaete fauna is characte rized by abundant populations of Amage adspersa, followed by more spar se populations of Capitella capitata, Heteromastus filiformis, Polydor a ciliata, and Streblospio shrubsolii. Moreover, the number of species was roughly stable through the year, and Polychaetes exhibited by far the most elevated abundances found in the estuary. In the north arm, the Polychaete fauna was characterized by Hediste diversicolor and Str eblospio shrubsolii, almost restricted to the upstream section, follow ed by the sporadic occurrence of other species. The north arm exhibite d an extremely impoverished Polychaete fauna, both in number of specie s and total abundance. as compared to the south arm. In the downstream area of the south arm and near the mouth of the estuary the Polychaet e fauna was characterized by the occurrence of a mixture of uncommon s pecies e.g. Eulalia sp., Eteone picta, Glycera convoluta, Lagis koreni , Nephthys spp., and Spio decoratus, reflecting a stronger marine infl uence. The subtidal Polychaete fauna appears to be controlled primaril y by physicochemical factors, especially sediments type and salinity. Biologically, the two arms of the estuary constitute distinct sub-syst ems. with dissimilar physicochemical characteristics. Furthermore, tim e intervals between dredging operations along the north arm are appare ntly inadequate to allow infaunal recovery in the north arm, and exten sive eutrophication seems to be taking place in the south arm.