Phosphorus supply and floodplain design govern phosphorus reduction capacity in remediated agricultural streams

Abstract

Abstract. Agricultural headwater streams are important pathways for diffuse sediment and nutrient losses, requiring mitigation strategies beyond in-field measures to intercept the transport of pollutants to downstream freshwater resources. As such, floodplains can be constructed along existing agricultural streams and ditches to improve fluvial stability and promote deposition of sediments and particulate phosphorus. In this study, we evaluated 10 remediated agricultural streams in Sweden for their capacity to reduce sediment and particulate phosphorus export and investigated the interplay between fluvial processes and phosphorus dynamics. Remediated streams with different floodplain designs (either on one side or both sides of the channel, with different width and elevation) were paired with upstream trapezoidal channels as controls. We used sedimentation plates to determine seasonal patterns in sediment deposition on channel beds and floodplains and monthly water quality monitoring. This was combined with continuous flow discharge measurements to examine suspended sediment and particulate phosphorus dynamics and reduction along reaches. Remediated streams with floodplains on both sides of the channel reduced particulate phosphorus concentrations and loads (−54 µg L−1, −0.21 kg ha−1 yr−1) along reaches, whereas increases occurred along streams with one-sided floodplains (27 µg L−1, 0.09 kg ha−1 yr−1) and control streams (46.6 µg L−1). Sediment deposition in remediated streams was five times higher on channel beds than on floodplains and there was no evident lateral distribution of sediments from channel to floodplains. There was no effect from sediment deposition on particulate phosphorus reduction, suggesting that bank stabilization was the key determinant for phosphorus mitigation in remediated streams, which can be realized with two-sided but not one-sided floodplains. Further, the overall narrow floodplain widths likely restricted reach-scale sediment deposition and its impact on P reductions. To fully understand remediated streams' potential for reductions in both nitrogen and different phosphorus species and to avoid pollution swapping effects, there is a need to further investigate how floodplain design can be optimized to achieve a holistic solution towards improved stream water quality.

Recognizing Agricultural Headwaters as Critical Ecosystems

“…28−30 Thus, agricultural headwaters capturing secondary and legacy pollution are one of the key points of intervention to focus remediation measures such as constructed/reconnected floodplains and remeandering. Reported solute and sediment retention rates during low-to-medium magnitude flow conditions 24,31,32 in remediated agricultural headwaters are within similar order of magnitude compared to values reported for the edge-of-field buffer strips and wetlands. 33 Thus, remediation of agricultural headwaters not only can improve their function but also have cascading impacts on water quality and ecology of downstream ecosystems.…”

“…The continuous release of legacy nutrients into aquatic environments, together with higher occurrence of extreme hydrological events, can control water quality in the long term and override positive effects of BMPs and catchment remediation. − Thus, agricultural headwaters capturing secondary and legacy pollution are one of the key points of intervention to focus remediation measures such as constructed/reconnected floodplains and remeandering. Reported solute and sediment retention rates during low-to-medium magnitude flow conditions ,, in remediated agricultural headwaters are within similar order of magnitude compared to values reported for the edge-of-field buffer strips and wetlands . Thus, remediation of agricultural headwaters not only can improve their function but also have cascading impacts on water quality and ecology of downstream ecosystems. ,,, However, restoration of agricultural headwaters is underrepresented in management compared to catchment remediation (e.g., edge-of-field wetlands) and restoration of larger rivers (Figure c).…”

“…Catchment water storage can be increased through mitigation measures, such as ponds, wetlands, or controlled drainage. In agricultural headwaters, there is a scope to adapt bed roughness through vegetation management, remeandering, or floodplain construction that can effectively regulate in-channel water velocity and residence times, dampen rainfall-runoff response, and provide additional ecological and water quality benefits …”

“…33 Thus, remediation of agricultural headwaters not only can improve their function but also have cascading impacts on water quality and ecology of downstream ecosystems. 6 , 10 , 24 , 31 However, restoration of agricultural headwaters is underrepresented in management compared to catchment remediation (e.g., edge-of-field wetlands) and restoration of larger rivers ( Figure 1 c). This together with knowledge gaps related to the functioning of agricultural headwater catchments and streams has led to poor and slow water quality improvements and growing skepticism among stakeholders implementing BMPs and catchment remediation measures.…”

“…In agricultural headwaters, there is a scope to adapt bed roughness through vegetation management, remeandering, or floodplain construction that can effectively regulate inchannel water velocity and residence times, dampen rainfallrunoff response, 23 and provide additional ecological and water quality benefits. 24 ■ AGRICULTURAL HEADWATERS CONTROL WATER…”