Influence of stormflow and baseflow phosphorus pressures on stream ecology in agricultural catchments

Abstract

Stormflow and baseflow phosphorus (P) concentrations and loads in rivers may exert different ecological pressures during different seasons. These pressures and subsequent impacts are important to disentangle in order to target and monitor the effectiveness of mitigation measures. This study investigated the influence of stormflow and baseflow P pressures on stream ecology in six contrasting agricultural catchments. A five-year high resolution dataset was used consisting of stream discharge, P chemistry, macroinvertebrate and diatom ecology, supported with microbial source tracking and turbidity data. Total reactive P (TRP) loads delivered during baseflows were low (1-7% of annual loads), but TRP concentrations frequently exceeded the environmental quality standard (EQS) of 0.035mgL during these flows (32-100% of the time in five catchments). A pilot microbial source tracking exercise in one catchment indicated that both human and ruminant faecal effluents were contributing to these baseflow P pressures but were diluted at higher flows. Seasonally, TRP concentrations tended to be highest during summer due to these baseflow P pressures and corresponded well with declines in diatom quality during this time (R=0.79). Diatoms tended to recover by late spring when storm P pressures were most prevalent and there was a poor relationship between antecedent TRP concentrations and diatom quality in spring (R=0.23). Seasonal variations were less apparent in the macroinvertebrate indices; however, there was a good relationship between antecedent TRP concentrations and macroinvertebrate quality during spring (R=0.51) and summer (R=0.52). Reducing summer point source discharges may be the quickest way to improve ecological river quality, particularly diatom quality in these and similar catchments. Aligning estimates of P sources with ecological impacts and identifying ecological signals which can be attributed to storm P pressures are important next steps for successful management of agricultural catchments at these scales.

Modeling the Ecological Impact of Phosphorus in Catchments with Multiple Environmental Stressors

“…Other studies also found urbanization to be strongly negatively linked to aquatic ecology (King et al, 2011; Teittinen et al, 2015; Golden et al, 2016; Herrero et al, 2018). This negative association with urban land cover may be linked to additional pollutants associated with surface runoff as well as more bioavailable nutrients from point sources (Ekholm and Krogerus, 2003; Ekholm et al, 2009; Richards et al, 2016; Stutter et al, 2018) likely to have a greater impact on river ecology (Shore et al, 2017). …”

Sources and Mechanisms of Low-Flow River Phosphorus Elevations: A Repeated Synoptic Survey Approach

“…The qualitative status of European waterbodies under the Water Framework Directive (WFD) [1] is determined relative to fixed quantitative thresholds of chemical concentration and physical/ecological status. Monitoring at the outlets of agricultural catchments has revealed that concentrations of nutrients, particularly phosphorus (P), in stream water become elevated during low-flow conditions and may exceed legislated thresholds [2,3]. Such elevated concentrations, while not necessarily reflective of the mean quality of the stream over the entire course of the year, may occur at critical times affecting the ecological health of indicator species such as macroinvertebrates and diatoms [2,4].…”

“…Monitoring at the outlets of agricultural catchments has revealed that concentrations of nutrients, particularly phosphorus (P), in stream water become elevated during low-flow conditions and may exceed legislated thresholds [2,3]. Such elevated concentrations, while not necessarily reflective of the mean quality of the stream over the entire course of the year, may occur at critical times affecting the ecological health of indicator species such as macroinvertebrates and diatoms [2,4]. It has been observed that low temporal resolution monitoring can lead to ambiguity in status classification [5] and in load estimation [6,7].…”

“…In six intensive agricultural sites in Ireland, [2] elevated reactive phosphorus at the catchment outlets during baseflow conditions have been reported. Although the load of P export was typically low under these conditions (≤ 7% of annual total), the concentrations exceeded the environmental quality standard (EQS) of 0.035 mg l -1 for all but one catchment (which was underlain by karst geology) greater than 32% of baseflow duration.…”

Historical and predicted variations of baseflow in China's Poyang Lake catchment

“…It is an important portion of streamflow that generates from shallow, lateral subsurface flow and deep groundwater flow and changes gradually over short‐ and long‐term scales (Nathan & McMahon, ; Rumsey, Miller, Schwarz, Hirsch, & Susong, ). Baseflow sustains river flow between events affected by rainfall–run‐off processes (Miller, Buto, Susong, & Rumsey, ; Shore et al, ; J. Zhang, Zhang, Song, & Cheng, ), which is important for understanding relationships between aquatic habitats and their environment (Ford, King, & Williams, ; Santhi, Allen, Muttiah, Arnold, & Tuppad, ). Knowledge about baseflow can improve the understanding of catchment streamflow partitioning (Mei & Anagnostou, ; Ravazzani, Gattinoni, Valentina, & Rosso, ), relieving flood events (Kinkela & Pearce, ), assessing surface–groundwater flow interactions (Arnold, Muttiah, Srinivasan, & Allen, ; Lamontagne, Leaney, & Herczeg, ; L. Zhang, Brutsaert, Crosbie, & Potter, ), and aiding water resource management (Arciniega‐Esparza et al, ; Arnold, Allen, Muttiah, & Bernhardt, ; Z. Dai, Chu, Du, Stive, & Hong, ).…”