Simulating Lags, Tipping Points and Cross Scale Interactions in Integrated Socio-Environmental Systems: Evaluating the Impacts of Early vs. Delayed Nutrient Reductions under Alternate Hydro-Climatic Scenarios in Missisquoi Bay, 2000-2050

While a growing body of modeling and experimental research has identified the importance of lags, tipping points and cross-scale interactions in understanding the dynamics of integrated socio-environmental systems, many issues about incorporating these dynamic features in policies aimed at sustainable management of water quality in freshwater lakes remain unresolved. Situated in a Social Ecological Systems (SES) theoretical and empirical framework, this paper addresses the following question: How do early versus delayed policy actions affect the evolution of threshold-based state variables in SES that interact across multiple spatial and temporal scales? We investigate this question in the SES context of Clean Water Act mandated Total Maximum Daily Load (TMDL) policies guiding the management of water quality in transboundary Missisquoi Bay of Lake Champlain, shared between the US and Canada. An integrated SES model simulates the cascading impacts of global climate change and early vs. delayed nutrient management interventions in the focal SES from 2000 through 2050. Twelve future climate simulations (4 global climate models (GCMs) x 3 emissions scenarios) and 11 stakeholder-informed early versus delayed nutrient reduction scenarios drive a distributed hydrological model (RHESSys) that simulates 132 daily time series of riverine discharge and nutrient loads entering Missisquoi Bay of Lake Champlain. A high-resolution system of biogeochemistry and hydrodynamic lake models simulates water quality, including four phytoplankton groups, in the shallow bay. Simulations suggest that early action to reduce external loading of nutrients may avoid phase transitions of SES into undesirable hyper-eutrophic state, while delayed action may lead to difficult-to-reverse hypereutrophic regimes. Further, current TMDL driven nutrient reduction policies will trigger phase transitions in summer and shoulder season months due to sustained high water temperatures above thresholds that promote cyanobacteria dominance. The freshwater lakes and bays thus face severe limits to adaptation. Computational models of integrated socio-environmental systems can build stakeholder foresight about the choice of nutrient reduction policy goals under alternate hydro-climatic and land management scenarios.