Beyond the Mass Balance: Watershed Phosphorus Legacies and the Evolution of the Current Water Quality Policy Challenge

Abstract

Increased use of phosphorus (P) fertilizers and intensive livestock production as well as increasing population densities and use of P-rich detergents have led to widespread eutrophication of both coastal and inland waters. Primary production in freshwater systems is strongly dependent on P availability (Schindler, 1974;Schindler et al., 2016), and P over-enrichment of lakes and reservoirs can lead to a proliferation of green algae, the development of toxic cyanobacterial blooms, and widespread impairment of aquatic ecosystems (Le Moal et al., 2019). Since 1950, global use of commercial P fertilizers has increased approximately four-fold (Van Vuuren et al., 2010). During this time, eutrophication has become a pervasive problem in human-impacted water bodies, from Lake Erie in North America to China's Lake Taihu (Sharpley & Wang, 2014).

Managing nitrogen legacies to accelerate water quality improvement

“…Statistical approaches, involving prediction of current-year N loads as a function of N inputs over the past few decades, have been developed to capture lag times in the Mississippi River basin 53 and the Yongan watershed in eastern China 49,54 . The number of process-based models is limited, with existing models including the Exploration of Long-tErM Nutrient Trajectories (ELEMeNT) model 14,26,27,55,56 , the nitrate time bomb model to estimate groundwater nitrate concentrations 57 , the watershed-scale LM3-TAN model 58 , SWAT-LAG, a modification of the SWAT model with addition of a groundwater travel time distribution 59 , and a hillslope-scale aquifer model, also employing groundwater travel times 60 . Use of the ELEMeNT model within the Mississippi River basin has led to estimates of multi-decadal lag times to achieve policy goals for the Gulf of Mexico, even with the most aggressive scenarios to reduce surplus N within the watershed 13,14,59 .…”

Windows into the past: lake sediment phosphorus trajectories act as integrated archives of watershed disturbance legacies over centennial scales

“…Finally, we find that the spatio-temporal variability in lake sediment P accumulation is driven by a combination of anthropogenic drivers (fertilizer input and population density) and lake morphology (lake depth, watershed to lake area ratio), with shallow lakes in regions of intensive agriculture having the largest accumulation magnitudes. This highlights the role of past watershed P inputs that can potentially build up as legacy P in lakes, and act as a long term source of P through internal loading, fuelling eutrophication and algal blooms for years after watershed inputs have ceased (Jeppesen et al 2005, Orihel et al 2017, van Meter et al 2021. Such insights about inter-region variability in legacy P accumulation in lakes is critical for the development of robust predictive models that can be used for resource managers and decision makers for the management of aquatic resources.…”

“…Despite reduction in total P loads observed in few systems (Sharpley et al 1996, Baker et al 2014, most lake P concentrations seem to have either attained stasis or continued to increase in various regions around the world (Sharpley et al 2013, Oliver et al 2017. It is now recognized that this lack of success in water quality improvement can be partially attributed to the build-up of excess P in soils, groundwater, rivers, and lake sediments over decades of agricultural intensification and increasing population densities (MacDonald and Bennett 2009, Keatley et al 2011, Haygarth et al 2014, Goyette et al 2016, Powers et al 2016, Stackpoole et al 2019, van Meter et al 2021. These legacies contribute to elevated concentrations and often decadal lags in ecosystem improvements even when inputs have ceased (Jarvie et al 2013, van Meter et al 2018, Basu et al 2022.…”

Periphyton Phosphorus Uptake in Response to Dynamic Concentrations in Streams: Assimilation and Changes to Intracellular Speciation

“…First, although assessment of long-term legacy P accumulation and depletion using a mass balance approach has been demonstrated in a number of watersheds (e.g., refs − ) few models are able to determine the spatial distribution of legacy-P within a watershed on time scales that can capture the dynamics of transient-P pools influencing the seasonal timing of P delivery to receiving water bodies. Second, due to an incomplete understanding of the chemical and physical characteristics of transient-P stores in watercourses, we have limited capacity to evaluate the risk, and timing, of P remobilization in response to changing hydrological, geochemical, or climatic drivers.…”