Hydrologic Extremes and Legacy Sources Can Override Efforts to Mitigate Nutrient and Sediment Losses at the Catchment Scale

Abstract

Combating eutrophication requires changes in land and water management in agricultural catchments and implementation of mitigation measures to reduce phosphorus (P), nitrogen (N) and suspended sediment (SS) losses. To date, such mitigation measures have been built in many agricultural catchments, but there is a lack of studies evaluating their effectiveness. Here we evaluated the effectiveness of mitigation measures in a clay soil‐dominated headwater catchment by combining the evaluation of long‐term and high‐frequency data with punctual measurements upstream and downstream of three mitigation measures: lime‐filter drains, a two‐stage ditch, and a sedimentation pond. Long‐term hydrochemical data at the catchment outlet showed a significant decrease in P (−15%) and SS (−28%) and an increase in nitrate nitrogen (NO3–N, +13%) concentrations. High‐frequency (hourly) measurements with a wet‐chemistry analyzer (total and reactive P) and optical sensor (NO3–N and SS) showed that the catchment is an abundant source of nutrients and sediments and that their transport is exacerbated by prolonged drought and resuspension of stream sediments during storm events. Lime‐filter drains showed a decrease in SS by 76% and total P by 80% and an increase in NO3–N by 45% compared with traditional drains, potentially indicating pollution swapping. The effectiveness of two‐stage ditch and sedimentation pond was less evident and depended on the prevalent hydrometeorological conditions that drove the resuspension of bed sediments and associated sediment‐bound P transport. These results suggest that increased frequency of prolonged drought due to changing weather patterns and resuspension of SS and sediment‐bound P during storm events can override the generally positive effect of mitigation measures. Core Ideas We assessed the efficiency of catchment mitigation measures to reduce water pollution. At the catchment outlet, concentrations of P and sediments decreased by 15 and 28%. Lime‐filter drains reduced P and sediments by 80% but increased nitrate by 45%. Prolonged drought and resuspension of sediments exacerbated stream nutrient export. High‐frequency data show that legacy sources can override mitigation efforts.

Hydrological and Chemical Controls on Nutrient and Contaminant Loss to Water in Agricultural Landscapes

“…Both drained and undrained plots contributed similar exports of total phosphorus and further studies from different soil, climatic and land use conditions are needed to corroborate these results. However, the study did not consider the effect of drainage on other pollutants, e.g., nitrate, which can be particularly important for achieving overall improvements in ecological status and avoiding pollution swapping effects [8].…”

“…These contributions add to our understanding of the role that climate variability has on the multitude of processes controlling nutrient export from agricultural catchments. Such knowledge lets us appreciate variability in outcomes despite mitigation efforts [8], as well as identify apparent trends that may be the result of longer-term weather variations (e.g., Mellander et al [11]).…”

“…This inherent complexity within the pollutant transfer continuum and the uptake of mitigation measures challenge our ability to design universally effective mitigation measures. A growing body of evidence shows that, while management practices help to decrease nutrient loading in agricultural catchments, their positive effects can be counterbalanced by increasing runoff attributed to changes in climatic drivers [8,11,13].…”

Phosphorus mirabilis: Illuminating the Past and Future of Phosphorus Stewardship

“…With regard to climate change, both Bieroza et al (2019) and Kleinman et al (2019) document increased mobilization of legacy P sources with increased frequency of prolonged drought or activation of latent sources during extreme storm events. The implications of interactions between legacy P and climate change, as long‐term/historical factors affecting efficacy of agricultural P mitigation, are further explored by Smith et al (2019).…”

“…This leads to a constantly shifting kaleidoscope of baselines and targets for P management ( Jarvie et al, 2013b). Bieroza et al (2019), Kleinman et al (2019), Osmond et al (2019), and Smith et al (2019) all highlight the need to address legacy P stores and mitigate the increased P losses that are expected with climate change. They also champion the need to consider potential trade-offs involving P management practices, such as between particulate and soluble forms of P ( Jarvie et al, 2017), as well as those practices that create "pollutant swapping" between P and nitrogen (N).…”