Assessing the ecological stress in a Garonne River stretch, southwest France

In order to assess the level of ecological stress caused by the pollution from local disturbances in a stretch of the Garonne River, France, we applied the Abundance-Biomass Comparison (ABC) index, using fish assemblages. Data were collected in a 10-year span (1992–2002) in a reference site and in two pollution-exposed sites. The ABC index mean value in the reference site (S1) was 0.03 ± 0.002 (95% Confidence Interval – CI); for the polluted sites (S2 and S3), the values were −0.09 ± 0.002 (95% CI) and −0.12 ± 0.002 (95% CI), respectively. The ABC index showed that, besides flow variations, both downstream sites are statistically different (p < 0.05) from the reference site, but all three seem to be under moderate stress. Furthermore, we related our ABC scores to water quality and flow regime variables in the reference site and one of the polluted sites by means of a cluster analysis. The results showed that, in the reference site, the ABC scores are closely related to the flow regime, while in the polluted site, downstream a urban area, ABC is related to water quality variables such as phosphates and total phosphorous. We argue that ecological indicators can help decisions on environmental damage liability.