Climate change and watershed process interactions: Large-scale Anthropogenic changes to freshwater and nearshore coastal biogeochemical cycles

Rivers are the great connectors of the freshwater cycle, often considered the continents' "arteries." They provide essential services to humans and ecosystems valued at over $4 trillion USD annually, including drinking water, transportation channels, food security, waste assimilation, and water purification. River systems also harbour more than 10% of known biodiversity, despite accounting for less than 1% of the Earth's surface. Essential nutrient elements such as phosphorus (P), nitrogen (N), and carbon (C) are transported and transformed along river systems from source to sea, forming the basis for freshwater food webs in lakes, rivers, wetlands, reservoirs, and floodplains, and ultimately for marine food webs in estuarine and coastal environments. Rising human populations and resource-intensive lifestyles are driving increased demand for clean water at the same time as freshwater ecosystem degradation is accelerating. Enhanced nutrient loading, urbanization, land use change, and river channelization and damming have massively altered the fluxes of nutrients. The consequences of these changes can be seen worldwide, in the form of toxic algal blooms, fish kills, and in jeopardized drinking water supplies. In England and Wales alone, the annual economic impact of harmful algal blooms has been estimated to be between £75 - 114.3 million. Concurrently, the effects of climate change threaten secure water supplies internationally.

While many studies have focused on watershed-level human impacts to river systems like enhanced nutrient loading from agricultural runoff or wastewater treatment plants, very little research has been focused on determining the nature and extent of climate-driven impacts on nutrient cycles. While there is widespread evidence that climate change will massively alter hydrological flows and terrestrial biogeochemical cycles, most studies dedicated to investigate climate change effects on nutrient cycles and subsequent water quality changes are locally based and/or just focus on a single impact such as increased precipitation. The pitfall of studies that focus only on single processes is that feedback cycles that either modulate or exacerbate the magnitude of nutrient loads are neglected. These feedbacks are further compounded by additional climate change effects along the entire freshwater continuum. There is thus a strong need for continental or global-scale models that capture the redistribution of nutrient cycles, particularly those with greenhouse gas and atmospheric components. Large-scale analysis of full nutrient cycles enables the untangling of climate-driven changes to nutrient loads from source to sea, and allows prediction of consequences to ecosystem health along the entire river network and in receiving coastal zones.

This research project will couple advances in spatially-explicit computer simulation of river catchments, new global-scale hydrological datasets (MERIT-Hydro and GRADES), and AI techniques, to quantify the effects of interacting multiple stressors of climate change and direct human alterations (land use, damming) on global freshwater nutrient cycles. The resulting high-resolution, global nutrient models offer the prospect of constraining scaling laws that are relevant from the local to global scale. Such a step-change in knowledge could then be utilised by watershed managers to address/reverse problems associated with historic river catchment modifications. Without an understanding of these interacting effects along the entire LOAC, the potential for miscalculating local consequences of costly catchment management interventions to aquatic ecosystem health, and water quality and availability, will remain unacceptably high.