Environmental status of a multiple use estuary, through the analysis of bethnic communities : the Sado estuary, Portugal

Abstract

Ihis work on the Sado outer estuary (western coast of Portugal) considers its bio-sedimentary characterization, and evaluates the prevailing hydrophysical and sedimentary environmental conditions. The quality status of the estuary is discussed. The thesis is based on the analysis of superficial sediments and benthic macrofauna collected from 133 sites, and on hydrodynamical data obtained from a mathematical model. The environmental variables considered were the superficial sediment temperature, granulometry and organic matter content, together with the water current velocities, flow and shear stress. The macrofaunal characterization and the analysis of the state of disturbance was based on a study of the composition, structure and spatial distribution of faunal assemblages and their relationship to the prevailing environmental conditions. The Sado was shown to be inhabited by an abundant and diversified fauna with high biomass. Two major faunal assemblages have been identified, a marine and an estuarine community, separated by a transition region. The estuarine community comprises the majority of the area and contains important subdivisions. The regions identified have been shown either to be controlled by anthropogenic inputs or to reflect mainly the effects of natural forces. The main structuring factors influencing the benthos are the hydrodynamic conditions, coupled with sediment type, and organic matter of natural and anthropogenic origin. Sedimentary organic enrichment effects have been noted in all the areas studied and in general the Sado outer estuary reveals signs of eutrophic conditions. In the more disturbed regions the macrofauna data also suggest chronic toxicity effects. Although disturbance effects due to anthropogenic inputs are suggested through the biosedimentary approach, the estuary as a whole can not be considered badly polluted. However, defaunated, impoverished and over-enriched areas, appear as a result of localised severe pollution indicating the need to improve effluent treatment systems. The methodological approach followed in this study has been shown to be effective in assessing the quality status of the Sado outer estuary. Multivariate and direct gradient analysis proved to be very useful methods and a detailed analysis of the macrofaunal species has been shown to be effective in the detection of organic enrichment effects in some of the areas. The extreme aspects of the gradients were identified by all the analytical techniques, but the univariate methods were shown to be poor at discriminating the more subtle effects.