A model of phytoplankton development in the Lot River (France).: Simulations of scenarios

Abstract

A multidisciplinary study was conducted to predict water quality in the Lot River and to estimate the impact of nutrient inputs on algal production: measurement surveys, estimation of nutrient inputs, hydraulic modeling and biological modeling. This paper presents the biological model that simulates the evolution in phytoplankton (chlorophyll and biovolume), nutrients (N–NH 4, N–NO 3, P–PO 4) and detrital elements (N and P) in the developed stretch of the river (300 km). The model takes into account the measured or estimated nutrient inputs. The variables calculated by the hydraulic model, such as illumination and water temperature, are introduced by forcing. The model was first calibrated with measurements taken in a series of dynamic water mass monitoring studies, during the low-water periods of 1991 and 1992. The simulations correctly reproduce evolution in the different variables measured on site, through a broad range of low-water flow values (from 9–94 m 3 s −1), with a single set of coefficients. We then chose three scenarios to attempt to evaluate the impact of a modification in nutrient inputs: large-scale connection of towns to sewage treatment plants and improvements in their efficiency; the consequences of saturation of existing sewage treatment plants; the simultaneous effects of stratification of the deep downstream reaches and of high nutrient inputs linked to summer tourism activities.