SPATIAL DISTRIBUTION OF THE SUBTIDAL MEIOFAUNA COMMUNITIES ALONG THE SALINITY GRADIENT IN TWO SOUTHERN EUROPEAN ESTUARIES

Abstract

Meiobenthos is an important benthic component of marine and estuarine sediments. In estuarine sediments meiofauna facilitates biomineralization of organic matter, enhancing nutrient regeneration, serves as food for a variety of higher trophic levels and exhibits high sensitivity to environmental modification. Spatial (horizontal and vertical) variations, temporal changes, abundance, species composition and fluctuations of estuarine meiofauna communities are influenced by several biotic and abiotic factors such as trophic relationships, bioturbation, oxygen, salinity, temperature and sediment grain size characteristics. The objective of this study was to analyse and compare the spatial distribution of the density and composition of subtidal Meiofauna and Nematoda communities in two southern European estuaries, exposed to different degrees of anthropogenic stress, in Portugal: Mira, a relatively undisturbed estuary, and Mondego, a system under sever anthropogenic impacts. Samples were collected along the salinity gradient of the two estuaries, from freshwater (<0.5psu) to euhaline areas (>30psu). Data were analysed in a way to describe and compare the distribution patterns of composition and density of meiofauna taxa and Nematoda communities along the salinity gradients of both estuaries and to identify the specific environmental factors structuring that distribution. In what refers to the environmental parameters, the two estuaries were different, with the Mira estuary presenting higher proportions of silt + clay and organic matter content and the Mondego estuary presenting higher percentage of dissolved oxygen and phosphate concentration. In both estuaries, meiofauna communities were characterised by the dominance of thetaxa Nematoda, Copepoda and Polychaeta. The spatial patterns of density and composition of both meiofauna and Nematoda communities reflected the salinity gradient, being these assemblages structured and influenced by this natural stressor. Besides salinity, sediment properties also influenced the communities and the responses of the communities to both anthropogenic and natural stress could not be easily differentiated. Nevertheless, different patterns of the trophic nematode structure assemblages between Mira and Mondego overlapped the salinity effects and the feeding guilds and their response could detect the anthropogenical–induced stress in these estuaries