Robert T. Hensley

Hydraulic effects on nitrogen removal in a tidal spring‐fed river

Hydraulic properties such as stage and residence time are important controls on riverine N removal. In most rivers, these hydraulic properties vary with stochastic precipitation forcing, but in tidal rivers, hydraulics variation occurs on a predictable cycle. In Manatee Springs, a highly productive, tidally influenced spring-fed river in Florida, we observed significant reach-scale N removal that varied in response to tidally driven variation in hydraulic properties as well as sunlight-driven variation in assimilatory uptake. After accounting for channel residence time and stage variation, we partitioned the total removal signal into assimilatory (i.e., plant uptake) and dissimilatory (principally denitrification) pathways. Assimilatory uptake was strongly correlated with primary production and ecosystem C:N was concordant with tissue stoichiometry of the dominant autotrophs. The magnitude of N removal was broadly consistent in magnitude with predictions from models (SPARROW and RivR-N). However, contrary to model predictions, the highest removal occurred at the lowest values of τ/d (residence time divided by depth), which occurred at low tide. Removal efficiency also exhibited significant counterclockwise hysteresis with incoming versus outgoing tides. This behavior is best explained by the sequential filling and draining of transient storage zones such that water that has spent the longest time in the storage zone, and thus had the most time for N removal, drains back into the channel at the end of an outgoing tide, concurrent with shortest channel residence times. Capturing this inversion of the expected relationship between channel residence time and N removal highlights the need for nonsteady state reactive transport models.

High-frequency measurements of reach-scale nitrogen uptake in a fourth-order river with contrasting hydromorphology and variable water chemistry (Weiße Elster, Germany)

River networks exhibit a globally important capacity to retain and process nitrogen. However direct measurement of in-stream removal in higher order streams and rivers has been extremely limited. The recent advent of automated sensors has allowed high frequency measurements, and the development of new passive methods of quantifying nitrogen uptake which are scalable across river size. Here we extend these methods to higher order streams with anthropogenically elevated nitrogen levels, substantial tributaries, complex input signals, and multiple N species. We use a combination of two station time-series and longitudinal profiling of nitrate to assess differences in nitrogen processing dynamics in a natural versus a channelized impounded reach with WWTP effluent impacted water chemistry. Our results suggest that net mass removal rates of nitrate were markedly higher in the unmodified reach. Additionally, seasonal variations in temperature and insolation affected the relative contribution of assimilatory versus dissimilatory uptake processes, with the latter exhibiting a stronger positive dependence on temperature. From a methodological perspective, we demonstrate that a mass balance approach based on high frequency data can be useful in deriving quantitative uptake estimates, even under dynamic inputs and lateral tributary inflow. However, uncertainty in diffuse groundwater inputs and more importantly the effects of alternative nitrogen species, in this case ammonium, pose considerable challenges to this method. This article is protected by copyright. All rights reserved.

On the emergence of diel solute signals in flowing waters

Biota imprint their stoichiometry on relative rates of elemental cycling in the environment. Despite this coupling, producer-driven diel solute variation in rivers and streams is more apparent for some solutes (e.g., dissolved oxygen—DO) than others (e.g., nitrate— NO3−). We hypothesized that these differences arise from atmospheric equilibration, with signals emerging and evolving differently for gaseous and nongaseous solutes. Measurements of DO and NO3 in a spring-fed river, where constant inputs isolate in-stream processing, support this hypothesis, as do results from reactive transport modeling of river solute dynamics. Atmospheric equilibration dramatically shortens the benthic footprint over which signals integrate, facilitating emergence of diel DO signals in response to in-stream metabolism. In contrast, upstream influences persist much further downstream for nongaseous solutes, confounding and potentially obscuring the diel signals from in-stream assimilatory processing. Isolating diel NO3 signals from in-stream processing requires a two-station approach wherein metabolic impacts on solute variation are measured by difference between upstream and downstream sensors. Notably, two-station inference improves markedly when hydraulic controls on signal propagation such as dispersion and storage are explicitly considered. We conclude that the absence of diel signals at a single station for nongaseous solutes such as NO3− cannot be interpreted as lack of autotroph demand or element coupling. As advances in sensors enable the acquisition of an increasingly rich array of solute signals, controlling for differences in the emergence and downstream evolution of gaseous versus nongaseous solutes will dramatically improve inferences regarding the timing and magnitude of coupled elemental processing.

Diffusion and seepage-driven element fluxes from the hyporheic zone of a karst river

The hyporheic zone (HZ) can be an important source of solutes to streams. Hyporheic solute fluxes are commonly dominated by advective exchange. However, fluxes from the HZ also may include diffusion and upward advection of ground water from underlying aquifers. We compared the relative importance of these transport mechanisms on solute budgets of a large, spring-fed river in north-central Florida using measurements of spring, river, and porewater chemistry, hydraulic gradients, and sediment hydraulic conductivity, and dilution of an injected dye (Rhodamine WT). Downstream increases in Fe, soluble reactive P (SRP), Ca2+, and Cl− concentrations of the river water suggest solute sources in addition to the major source springs. Shallow porewater concentrations of Fe, Mn, Ca2+, SRP, and Cl− were elevated relative to the river. Calculations of Fickian diffusion based on concentration gradients of these solutes indicate diffusion could account for the downstream increase in Fe concentration but only 5% of the downstream increase in SRP and <0.1% of the increases in Ca2+ and Cl−. Downstream decreases in Mn concentrations reflect in-stream retention despite predicted diffusion. Dye-trace results indicate that ∼13% of the river discharge originates from sources other than the major springs. Measured head gradients and low sediment hydraulic conductivity suggest vertical groundwater flow through the HZ is small. We used the SRP budget to partition the additional groundwater inputs between seepage through the HZ (∼3% of river discharge) and flow paths that bypass the HZ (∼10% of total river discharge). Flow paths that bypass the HZ dominated additional water delivery to the river, but diffusion, resulting from steep chemical gradients and low-permeability sediments, is an important mechanism for transporting solutes from the HZ to the river.

Flow reversals as a driver of ecosystem transition in Florida’s springs

The springs of northern Florida have degraded dramatically over recent decades, and filamentous algal mats have replaced submersed macrophytes. The unique temporal stability of these spring ecosystems has prompted narratives about the mechanisms of change in which authors focused entirely on press disturbances, such as NO3− enrichment and flow reduction. We tested the hypothesis that pulse disturbances, in the form of episodic flow reversals, are a key component of spring degradation. During flow reversals, acidic floodwaters rich in dissolved organic C (DOC) from adjacent blackwater rivers dramatically alter the light environment, and DOC respiration depletes dissolved O2 (DO), thereby creating a trophic cascade wherein negative effects on algae consumers enable algal proliferation. Synthesis of river and aquifer stages from springs in the Suwannee River basin indicates that reversal events have become more frequent with time in response to aquifer declines attributed to climate variability and groundwater pumping. This increase in frequency and duration of flow reversal is remarkably concurrent with the emergence of nuisance algal mats. Furthermore, re-analysis of existing algal abundance data suggests that potential for flow reversal is a significant predictor, but only when conditioned on spring DO concentrations. In low-DO springs, algal cover is always high, and reversals have little effect. However, in high-DO springs, algal cover is low except where reversals are likely. Our results suggest that despite remarkable temporal stability, episodic flow reversals may play a key role in regulating the ecosystem state. Focusing on these pulse events may be critical to spring protection and restoration.

Stream phosphorus dynamics of minimally impacted coastal plain watersheds

1636 Copyright

Isolating stream metabolism and nitrate processing at point-scales, and controls on heterogeneity

Stream solute signals arise from the convolution of catchment delivery, hydraulic transport, and biogeochemical processing. Drawing inference from such signals requires analytical or experimental tools to isolate the signal attributable to stream processing. We used Lexan® chambers open to the air and inserted into the sediment (benthic chambers) and high-frequency in situ sensors to measure metabolism and nutrient processing rates for a small (0.36 m2) footprint across a variety of benthic cover types. We estimated gross primary production (GPP) and ecosystem respiration (ER) based on high-resolution dissolved O2 measurements, and we estimated N retention via both autotrophic assimilation (UA) and dissimilatory removal (UD) from high-resolution NO3− measurements. We observed marked spatial variation in metabolism and nutrient retention, principally in response to autotroph cover and light during thirty-three 1-wk deployments in Gum Slough, a spring-fed river (discharge ≈ 1.5 m3/s) in northern Florida. Spatially weighted mean chamber rates were commensurate with, but slightly underpredicted, open-channel measurements. Moreover, the stoichiometry (i.e., molar C∶N from GPP and UA) matched expectations based on tissue stoichiometry and autotroph composition. Our approach enabled disaggregated measurements of metabolic processing across heterogeneous riverine settings, albeit principally where sediments were sufficiently fine-grained to limit hydraulic exchange (assessed here with a Cl− tracer). In addition, the small chamber volume allowed manipulation of myriad environmental factors, including solute concentrations via both active enrichment, or, in our case, passive depletion. We observed negligible GPP and minimal UA responses to NO3− depletion, in contrast to strong UD responses.