Abstract
AbstractUnderstanding the interplay between hydrological flushing and biogeochemical cycling in streams is now possible owing to advances in high‐frequency water quality measurements with in situ sensors. It is often assumed that storm events are periods when biogeochemical processes become suppressed and longitudinal transport of solutes and particulates dominates. However, high‐frequency data show that diel cycles are a common feature of water quality time series and can be preserved during storm events, especially those of low‐magnitude. In this study, we mine a high‐frequency dataset and use two key hydrochemical indices, hysteresis and flushing index to evaluate the diversity of concentration‐discharge relationships in third order agricultural stream. We show that mobilization patterns, inferred from the hysteresis index, change on a seasonal basis, with a predominance of rapid mobilization from surface and near stream sources during winter high‐magnitude storm events and of delayed mobilization from subsurface sources during summer low‐magnitude storm events. Using dynamic harmonic regression, we were able to separate concentration signals during storm events into hydrological flushing (using trend as a proxy) and biogeochemical cycling (using amplitude of a diel cycle as a proxy). We identified three groups of water quality parameters depending on their typical c‐q response: flushing dominated parameters (phosphorus and sediments), mixed flushing and cycling parameters (nitrate nitrogen, specific conductivity and pH) and cycling dominated parameters (dissolved oxygen, redox potential and water temperature). Our results show that despite large storm to storm diversity in hydrochemical responses, storm event magnitude and timing have a critical role in controlling the type of mobilization, flushing and cycling behaviour of each water quality constituent. Hydrochemical indices can be used to fingerprint the effect of hydrological disturbance on freshwater quality and can be useful in determining the impacts of global change on stream ecology.
Highlights
High-frequency monitoring of freshwaters with in situ sensors enables collection of hydrochemical data at time scales matching those of hydrological, biological and chemical dynamics observed in streams (Kirchner et al, 2004)
We focus on the trend and diel component as they explained most of the variance (>90%) in the concentration time series and we assume that they represent hydrological flushing and biogeochemical cycling respectively
Our results showed that (1) there was a large variation in storm event c-q responses as almost all storm events revealed a unique pattern of c-q types based on the Hi hysteresis index and Fi flushing index, (2) each parameter had a tendency towards just one or two cq types that explain the majority of the observed variance, from 50%–60% for total reactive phosphorus (TRP), TURB and dissolved oxygen (DO) to 70%–80% for COND, NO3-N, TEMP, total phosphorus (TP), redox potential (RED) and pH, and (3) these patterns changed on a seasonal basis
Summary
High-frequency monitoring of freshwaters with in situ sensors enables collection of hydrochemical data at time scales matching those of hydrological, biological and chemical dynamics observed in streams (Kirchner et al, 2004). Our understanding of temporal drivers of the interplay between hydrological and biogeochemical processes is continually evolving with intense focus on the analysis of changes in seasonal drivers (temperature and storm event magnitude) and inter-storm variation in solute biogeochemical retention and its effect on flushing behaviour (Burns et al, 2019). The notion that biogeochemical cycling is switched off or severely dampened, depending on the magnitude of the storm event has gained prominence, with higher magnitude storm events thought to have a greater damping effect (Bernhardt et al, 2018; Raymond et al, 2016) compared to low-magnitude storm events that can preserve diel cycles (Burns et al, 2016) Despite this wealth of information on c-q patterns at high and low flows thanks to advances in high-frequency measurements, our understanding of the interplay between hydrological flushing and instream biogeochemical cycling remains limited. We synthesise our findings into a conceptual framework of solute and particulate hydrochemical behaviour under storm events of varying magnitude for a third order agricultural catchment in a groundwaterfed river system that has been the focus of detailed research on hyporheic zone processes (Binley et al, 2013; Heppell et al, 2013; Lansdown et al, 2015) and a broader study of the entire river Eden catchment (Ockenden et al, 2016; Reaney et al, 2019)
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