Environmental Risk Assessment and Adaptive Management Implementation in Lake Simcoe, Ontario.

摘要

Addressing the problems of low deep-water oxygen concentrations and impairment of cold-water fish habitats in Lake Simcoe as a case study of a dimictic mesotrophic lake requires reduction of external phosphorus (P) loading. However, the efficiency of restoration efforts can be hindered by persistent internal P loading. This thesis develops a series of ecological and biogeochemical models, aiming at advancing our understanding of internal P recycling mechanisms in mesotrophic dimictic lakes. Special emphasis is given to sediment diagenesis processes and their interplay with the water column, macrophyte-mediated P retention, and the nutrient nearshore shunt induced by dreissenids. First, a continuous Bayesian network is presented to investigate the cause-effect relationships among physical conditions, ambient nutrient concentrations, and plankton dynamics. P sediment internal loading is subsequently quantified with a reactive-transport simulation model of the transformation of P binding forms. Sediment dynamics are then assessed under conditions of varying organic matter sedimentation and hypolimnetic oxygen levels. Finally, an integrated P mass-balance model is used to elucidate the internal P fluxes stemming from sediments, macrophytes and dreissenids. The model predicts that P diffusive fluxes from the sediments account for less than 30-35% of the exogenous P loading in Lake Simcoe. In the post-dreissenid invasion era, the limited decrease of the ice-free TP concentrations is indicative of the presence of active nutrient recycling pathways, potentially magnified by the particular morphological features and hydrodynamic patterns of Lake Simcoe, which counterbalance the direct effects of dreissenid filtration.