Likely controls on dissolved reactive phosphorus concentrations in baseflow of an agricultural stream

Abstract

Purpose High baseflow phosphorus (P) concentrations increase the likelihood of periphyton blooms. Several physical and chemical factors can control baseflow P concentrations such as hydraulic exchange with groundwater, particle size-sorting, redox chemistry and different sediment sources. We hypothesized that of these sources, anoxic sediments would allow P-rich groundwater to influence baseflow P concentrations the most and that the measurement of the equilibrium P concentration (EPC 0) of sediments under oxic conditions would not predict P release in anaerobic sediment or baseflow P concentrations. Materials and methods At four locations along an agricultural stream, we measured dissolved reactive P (DRP), pH, iron, manganese, sulphate, nitrate and dissolved oxygen in streamflow and hyporheic water at 0-200, 200-400 and 400-800 mm depths and P fractions and EPC 0 in sediment samples from the 0-200, 200-400 and 400-800 mm depths. Results and discussion Concentrations of DRP in streamflow and shallow hyporheic zone water increased downstream and were mirrored by concentrations in shallow sediment, EPC 0 measurements of oxic sediments and deeper hyporheic waters. Groundwater samples and the EPC 0 in deeper sediments did not show a pattern or residence time consistent with the supply of P to baseflow despite deeper sediment being anoxic and less likely to sorb upwelling P. There was also no change in pH or particle size downstream ruling out the degassing of groundwater or sediment size-sorting as an influence. However, the composition of sediment and underlying lithology of the catchment pointed to sediment downstream that was different to upstream sediment in that it could store and release more P. Conclusions Given the strong influence of sediment source on baseflow P concentrations, efforts to decrease the likelihood of periphyton blooms under baseflow should focus on reducing the erosion of P-rich sediment. Furthermore, the presence of oxic conditions in surface sediment meant that there was a relationship between EPC 0 and hyporheic water P concentrations. However, mixed oxic/anoxic conditions in deeper layer may require EPC 0 , or release rates, to be measured under reducing conditions.

Variability in fluvial suspended and streambed sediment phosphorus fractions among small agricultural streams

“…This supports our results with higher FSS Fe-P during low flow periods and lower Fe-P concentrations during high flow periods. Furthermore, McDowell et al (2020) reported a decrease in streambed Fe-P with sediment depth, which was also observed in our study. Phosphorus release due to Fe hydroxides/oxides dissolution with progressively more reducing sedimentary conditions at depth may explain this pattern (Parsons et al, 2017).…”

“…(2020) found low concentrations of H 2 O‐P in streambed sediment, with Fe‐P as the general dominating fraction at their sampling sites. These low H 2 O‐P concentrations (or corresponding fractions in other fractionation schemes) seem to be quite consistent for streambed sediment and lake sediment (Kaiserli et al., 2002; McDowell et al., 2020; Pettersson, 1998).…”

Sediment phosphorus buffering in streams at baseflow: A meta‐analysis

“…Several other methodological variables must be considered when measuring and interpreting EPC 0 . Redox status should be considered when planning benthic sediment sampling because it influences EPC 0 and its relation to DRP (Palmer‐Felgate et al., 2011; McDowell et al., 2020). If the goal is to characterize P exchange between the water column and the benthic sediments at the sediment–water interface (Figure 7), then sampling should avoid sediments deeper in the streambed or in stagnant zones that not only interact minimally with the water column but are also more likely to be anoxic.…”

“…Indeed, in slow lotic systems (e.g., riverine wetlands) or in parts of the stream channel with long residence times (e.g., deeper subsurface), there tends to be greater reducing conditions, which can influence EPC 0 (House & Denison, 2000; Palmer‐Felgate et al., 2011; Rahutomo et al., 2018). Consequently, this may lead to greater decoupling of DRP and EPC 0 in situ (McDowell et al., 2020). Whereas the redox interface in sediments is usually below where the majority of hyporheic exchange takes place for many streams (i.e., with dissolved oxygen approximately near saturation in the water column and moderate exchange with porewaters; Boano et al., 2014), reducing conditions in zones of longer retention times or where ecosystem respiration is high (Briggs et al., 2015) may nullify the sediment P buffer (Lewandowski & Nützmann, 2010; Smolders et al., 2017; Parsons et al., 2017).…”

Effect of magnetic iron-zirconium modified zeolite on the different phosphorus forms in river sediment under aerobic and anoxic conditions

“…When the sediment is in a deeper position, the water is insufficiently oxygenated and is usually in an anoxic state. Due to the reduction and dissolution of iron oxyhydroxide and the hydrolysis of calcium and phosphorus by microorganisms, phosphorus in the sediment is released into the water (Zhang et al 2018;Gao et al 2020;McDowell et al 2020). Previous studies have found that zirconium-based modified zeolite and zirconium-based modified bentonite have an effect on the unstable weakly adsorbed phosphorus (NH 4 Cl-P) and redox state phosphorus (BD-P) to the more stable metal oxide bound phosphorus (NaOH-P) and very stable residual phosphorus (Res-P) have a significant conversion effect, and significantly reduce the content of biologically available phosphorus (BAP) (Fan et al 2017;Soliman et al 2017).…”