The performance of biological indicators in assessing the ecological state of streams with varying catchment urbanisation levels in Coimbra, Portugal

Abstract

The growth of human populations has resulted in the expansion of metropolitan areas and changes in land use, both of which affect watersheds and streams. The ecological integrity of streams is likely to be negatively affected by urbanisation, compromising freshwater ecosystem services. The aim of this study was to assess how efficient structural and functional indicators are in evaluating the ecological conditions of water in urban stream ecosystems. Two urban streams crossing the city and one stream crossing a suburban area of Coimbra, Portugal were selected. Total impervious area (TIA) was used as an indicator of urbanisation. Physical and chemical parameters of water were measured and analysed within the Water Framework Directive (WFD/2000/60/EC). Benthic macroinvertebrates were used as structural indicators, and the IBMWP biotic index (modified) and the Portuguese IPtIS index were calculated. The decomposition rates of oak (Quercus robur) and alder (Alnus glutinosa) leaves were used as indicators of functional quality. Biotic indices and litter decomposition rates indicated poor ecological conditions in the urban streams compared to the suburban stream, consistent with the degree of urbanisation. The decrease in ecological quality in urban streams most likely reflected decreases in dissolved oxygen and increases in water temperature and conductivity. We emphasise (a) the need to combine physical and chemical data with biological data and (b) the high performance of a novel functional indicator based on litter breakdown rate as an accurate, efficient and integrative measure of ecological integrity in urban streams.