Climate Change‐Legacy Phosphorus Synergy Hinders Lake Response to Aggressive Water Policy Targets. Issue 5 (2nd May 2022) - Zia, Asim
Abstract: With mounting scientific evidence demonstrating adverse global climate change (GCC) impacts to water quality, water quality policies, such as the Total Maximum Daily Loads (TMDLs) under the U.S. Clean Water Act, have begun accounting for GCC effects in setting nutrient load‐reduction policy targets. These targets generally require nutrient reductions for attaining prescribed water quality standards (WQS) by setting safe levels of nutrient concentrations that curtail potentially harmful cyanobacteria blooms (CyanoHABs). While some governments require WQS to consider climate change, few tools are available to model the complex interactions between climate change and benthic legacy nutrients. We present a novel process‐based integrated assessment model (IAM) that examines the extent to which synergistic relationships between GCC and legacy Phosphorus release could compromise the ability of water quality policies to attain established WQS. The IAM is calibrated for simulating the eutrophic Missisquoi Bay and watershed in Lake Champlain (2001–2050). Water quality impacts of seven P‐reduction scenarios, including the 64.3% reduction specified under the current TMDL, were examined under 17 GCC scenarios. The TMDL WQS of 0.025 mg/L total phosphorus is unlikely to be met by 2035 under the mandated 64.3% reduction for all GCC scenarios. IAM simulations show that the frequency and severity of summer CyanoHABs increased or minimally decreased under most climate and nutrient reduction scenarios. By harnessing IAMs that couple complex process‐based simulation models, the management of water quality in freshwater lakes can become more adaptive through explicit accounting of GCC effects on both the external and internal sources of nutrients. Plain Language Summary: Water quality policies for freshwater systems generally do not consider synergies between legacy benthic nutrients and climate change. While some governments require water quality targets to consider climate change, few tools are available to model the complex interactions between climate change and benthic legacy nutrients. We present a novel integrated assessment model (IAM) that simulates phosphorus and potentially harmful cyanobacteria bloom dynamics in Missisquoi Bay of Lake Champlain under alternate nutrient reduction and climate change scenarios. Results illustrate biophysical and ecological challenges associated with meeting water quality standards and suppressing harmful cyanobacteria blooms due to warming water temperatures interacting with legacy pools of benthic nutrients. IAMs can inform policy for considering synergies between climate change and legacy nutrients to maintain or improve future water quality. Key Points: A novel model examines the ability of policy targets to meet Water Quality Standards (WQS) in eutrophic lakes under future climate scenarios The WQS of 0.025 mg/L total phosphorus in Missisquoi Bay is unlikely to be met by 2035 in all climate scenarios Synergies between climate change and legacy nutrients hinder significant reduction in cyanobacteria blooms despite aggressive policy targets