This work aims to establish a physico-chemical indicator of freshwater quality with respect to biological quality in order to facilitate the modeling of aquatic systems with a management perspective. In the context of the diversity-stability debate and relying on previous work, we proposed the hypothesis that unsteady systems which are unstable in terms of water composition are less favorable to biological diversity. We tested this hypothesis by considering the distribution of the conductivity/calcium ratio as a function of the flow with respect to the Global Biological Index based upon the determination of macroinvertebrates. We used the results obtained in rivers from the Geneva region (19 rivers, 30 sampling sites, sampled monthly for one year for physico-chemical parameters and four times a year for mean biological index determination).
In rivers (or sampling stations) with biological indexes superior to 13 (considered as not or only moderately polluted) the conductivity/calcium ratio remains stable independently of the flow, whereas in rivers (or sampling stations) presenting poor biological diversity (biological index less than 13), we observed not only changes in the conductivity/calcium ratio but also the deterioration of the stability for the given flow ranges.
The conductivity/calcium index relies on two easily measured parameters, its relationship with the flow can easily be modeled, thus allowing not only the modeling of the water quality response to watershed management, but also the determination of the critical flow ranges where inputs will have the worst impact on the biology of the receiving system. We are now in the process of applying the index to numerical modeling of the impact of urbanization on water quality in rivers from the Geneva region.