Predicting the likelihood of a desirable ecological regime shift: A case study in Cootes Paradise marsh, Lake Ontario, Ontario, Canada

Abstract

Abstract Environmental modelling is one of the pillars of the management process, representing an “information integrator” that brings together scientists, managers, and other stakeholders in a joint assessment of our understanding of the system being managed and the compelling knowledge gaps we seek to answer through monitoring and research. The overarching goal of the present modelling study is to offer insights into the restoration and management of Cootes Paradise marsh, one of the most degraded shallow wetlands in Southern Ontario. We use a mechanistic model to leverage our understanding of the major phosphorus biogeochemical pathways in Cootes Paradise. We also develop a network of statistical models that accommodates the spatial heterogeneity of the prevailing water quality conditions in the marsh. Combining the insights from both models, we found that a drastic reduction of point- and nonpoint-source nutrient loading could trigger a non-linear shift from the current turbid-phytoplankton dominated state to a clear-macrophyte dominated state. The restoration trajectory of the system can be profoundly modulated by the presence of a thriving macrophyte community with an enhanced ability to sequester phosphorus (i.e., the net amount of phosphorus taken up per unit of plant tissue). Critical remedial actions to re-establish the targeted macrophyte species in their native marsh habitats include both the intensification of local planting efforts and the control of invasive plant species. Another important finding of our modelling analysis is that the signature of the water quality improvements brought about by nutrient loading reductions dissipates as we move from the marsh’s western end to the central area due the presence of confounding factors, such as the hydraulic loading from Spencer Creek, internal nutrient loading, wind resuspension, and bioturbation. Given the high uncertainties associated with forecasting drastic (and costly) remedial actions, we argue in favour of a socioeconomic assessment of Cootes Paradise marsh as an ecosystem service provider to determine the benefits in terms of monetary values when we examine different courses of management options.