Comparison of aquatic ecosystem functioning between eutrophic and hypereutrophic cold-region river-lake systems

Abstract Located on the same river, the degree of eutrophication in the upper and middle reaches of the Qu’Appelle River in Saskatchewan are different. While the upper Qu’Appelle is eutrophic, the middle Qu’Appelle River is hypereutrophic. To manage the river sustainably, there is a need to understand key processes governing eutrophication in both systems. In this study, a comprehensive global sensitivity analysis technique, Variogram Analysis of Response Surface (VARS), was applied to gain insights to the functioning of the two systems. Eutrophication in both systems was modelled using the Water quality Analysis Simulation Program (WASP 7.52). The performance of the model to predict key variables of eutrophication was measured using relative root mean square error. The global sensitivity analyses showed that although diffuse loading has significant influence on the systems, prevailing processes governing eutrophic state in the upper Qu’Appelle River include: nutrient and, phytoplankton cycles. Meanwhile, in the middle Qu’Appelle River a number of processes including phytoplankton cycle, nutrient cycle, diffuse loading and DO balance together sustain its hypereutrophic state.