Evaluation of ecological effects of anthropogenic nutrient loading scenarios in Los Molinos reservoir through a mathematical model

Abstract

In this paper we perform a scenario analysis to assess the likely ecosystem structural shifts that might be induced by seven alternative management scenarios relative to current simulated lake dynamics of Los Molinos reservoir, a monomictic lake located in Argentina. To carry out the scenario analysis, first, we formulate, calibrate and validate a mechanistic biogeochemical model to characterize the nutrients and phytoplankton dynamics of the reservoir under study. The model includes a set of partial differential algebraic equations, which results from dynamic mass balances on main phytoplankton groups, nutrients, dissolved oxygen and biochemical demand of oxygen. We consider the main sources of nutrient loading and the dominant phytoplankton groups. The parameter estimation model is a constrained dynamic optimization problem that has been formulated within an equation oriented control vector parameterization environment. We estimated sixteen parameters, previously identified as the most influential ones. Numerical results for calibration and validation phases show good agreement with field data, and then, we use the proposed model as a prospective tool. The obtained results for the designed nutrient loading scenarios indicate the importance of the impacts of current livestock practices and sewage/collective septic tanks discharges on the water quality. Model predictions indicate that the combination of reduction in nutrient loading from the sewage of residential areas in the lake shore, and implementation of measures to mitigate nutrient exports from livestock production will render the most beneficial effects over the studied ecosystem, thus promoting the improvement of the current ecological state of the reservoir. This is the first time such an approach has been applied to the study of Los Molinos catchment. Thus, the proposed ecological model constitutes a management tool to assess the anthropogenic impacts over this freshwater resource.