Stream phosphorus dynamics of minimally impacted coastal plain watersheds

Abstract

Understanding natural variation in stream phosphorus (P) concentrations over space and time is critical for understanding natural drivers of catchment behavior and establishing regulatory standards. Across minimally impacted benchmark streams (n = 81) in Florida, spatial variation in mean total P concentrations was large, indicating the importance of geologic controls on catchment solute dynamics. While this variation was significantly predicted by geographic regions, within regions we observed nearly comparable cross‐site variation, suggesting important finer‐scale heterogeneity in baseline catchment chemistry. Within‐site residual variation (unexplained by region or site) was as large as spatial variation, suggesting temporal variation in response to drivers such as flow may be critically important. To further explore timescales of P export variation, we collected long‐term, high‐frequency (subdaily) measurements of stream discharge (Q) and soluble reactive P (SRP) in 2 forested watersheds. We observed significant variation at annual, event, and diel timescales, all of which arise primarily from corresponding Q‐variation. Over the entire period of record, we generally observed a strong dilution signal, with SRP concentrations declining with increased Q. Despite significant SRP variation, flow variation was far larger and, thus, dominated temporal control on downstream flux. Within‐storm events, we observed strong and consistent clockwise SRP versus Q hysteresis, suggesting mobilization of proximal SRP stores. Diel variation exhibited mid‐afternoon concentration minima, Q‐controlled amplitude, and pronounced seasonal shifts in both magnitude and timing consistent with riparian evapotranspiration‐regulating lateral inputs of P‐rich groundwater. Such high‐resolution temporal signals allow identification of solute sources and provide insights into geologic and hydrologic drivers of solute variation.

Complex patterns of catchment solute–discharge relationships for coastal plain rivers

“…Riparian fluxes of P from floodplain soils may further enhance this effect when previously immobile floodplain P pools (Noe & Hupp, 2005) mobilize at high flows. Clockwise hysteresis observed in between C and Q during storms in similar coastal plain watersheds (Hensley et al, 2017) reinforces the notion that proximate sources of P mobilized under rising discharge conditions are important to our observed C-Q breakpoint patterns.…”

“…We observed strong evidence of lithologic controls on C-Q relationships. As predicted, presence of P-rich Hawthorn Group sediments was significantly correlated with higher FWCs of TP and PO 4 , and also with reduced concentration variation, suggesting that the Hawthorn acts as a relevant, and perhaps dominant, source of phosphorus to regional lakes (Cohen, Lamsal, Korhnak, & Long, 2008) and streams (Hensley et al, 2017). Similarly, Ca FWCs and presence/absence of limestone comported with expectations, with higher concentrations when carbonate exposure increases.…”

“…Decoupling of TOC and TKN behaviour at sites with low TKN β values may indicate that ammonium dilution is stronger than organic N (and organic C) enrichment. Additionally, observations of enriching responses (β > 0.2) for PO 4 /TP in 20% of P C–Q relationships contrast strong evidence of PO 4 dilution in two small coastal plain catchments (Hensley, McLaughlin, Cohen, & Decker, ). Overall, however, broad evidence for nutrient chemostasis suggests transport limitation despite a prevailing narrative that these two elements are under strong biotic control, and are thus preferentially retained on the landscape.…”

Examining sources and pathways of phosphorus transfer in a ditch‐drained field

“…As such, the indication of clockwise TDP hysteresis during larger storms in our study ditch is potentially important because it suggests TDP is rapidly transported to the ditch (Bowes et al., 2009). Faster flow pathways that arrive early in these large storm events are likely enriched in TDP (Hensley et al., 2017).…”

Effects of bedrock groundwater discharge on spatial variability of dissolved carbon, nitrogen, and phosphorous concentrations in stream water within a forest headwater catchment

“…For example, in our site, DOC concentration of bedrock springs was about 1 mgC/L (Table 1), while some previous studies have shown that groundwater in soil layer could have lower concentrations than 1 mgC/L (Osaka et al, 2010; Sebestyen et al, 2008). In addition, the stream DIP concentration is strongly affected by bedrock geology (Hensley et al, 2017; Wakamatsu et al, 2006). Therefore, the influence of bedrock groundwater discharge on the spatial distribution of stream DOC, DIN and DIP concentrations could vary greatly depending on the catchment characteristics, for example, bedrock geology and chemical properties of soil.…”