Comparing Drivers of Spatial Variability in U.S. Lake and Stream Phosphorus Concentrations.

Decision makers need to know the drivers of surface water phosphorus (P) concentrations, the environmental factors that mediate P loading in freshwater systems, and where pollution sources and mediating factors are co‐located to inform water quality restoration efforts. To provide this information, publicly available spatial data sets of P pollution sources and relevant environmental variables, like temperature, precipitation, and agricultural soil erodibility, were matched with >7,000 stream and lake total P observations throughout the conterminous United States. Using three statistical approaches, consisting of (a) correlation, (b) regression, and (c) machine learning techniques, we identified likely drivers of P concentrations. Surface water concentrations in streams were more strongly correlated and effectively predicted by annual fertilizer and manure input rates and agricultural legacy sources compared to that of lakes. This observation suggests that streams may be more immediately responsive to improvements in agricultural nutrient management. In contrast, lake concentrations, though still positively associated with agricultural input and surplus variables, may be more influenced by historic erosional inputs, internal lake recycling, and other environmental factors. Thus, lake TP concentrations may not be as immediately responsive as streams to improvements in phosphorus management. Both stream and lake P concentrations will potentially increase because of warming temperatures and forest recovering from past acidification, putting even further pressure on existing water quality restoration efforts to meet nutrient loading reduction targets. The identified spatial data sets and relationships elucidated in this effort can inform the placement and development of watershed restoration strategies to reduce excess P in aquatic systems. Plain Language Summary: Over the past century, increasing agricultural production and urbanization increased the amount of phosphorus entering lakes and streams across the United States. Increased nutrient inputs have degraded water quality, and substantial effort has been put into decreasing phosphorus loads to surface water. Decision makers need to know the drivers of lake and stream phosphorus concentrations and the environmental factors (e.g., soil characteristics, precipitation, temperature, etc.) that moderate phosphorus loss to surface water to optimize restoration plans and consider future challenges. This work highlights that stream concentrations may be more immediately responsive to improvements in farm nutrient management, but like lakes, the loss of legacy phosphorus from erodible soils may delay or even offset anticipated improvements. Future climate change, particularly increases in air temperature, will likely result in increased phosphorus concentration in lakes and streams thus putting further pressure on rural and urban stakeholders to decrease phosphorus loss. The statistical evidence summarized in this work, combined with the publicly available maps of pollution sources and environmental factors, can help guide watershed managers on where and how to implement watershed restoration plans, communicate expected watershed responses to management actions, and anticipate future challenges in the Anthropocene. Key Points: Three statistical techniques identified major environmental and anthropogenic drivers of lake and stream phosphorus concentrationsStreams may be more responsive to shifts in nutrient inputs, whereas lakes will likely show a lagged response due to internal recyclingWarming temperatures and more intense summertime precipitation will likely increase aquatic phosphorus concentrations in the future [ABSTRACT FROM AUTHOR]