Author ORCID Identifier:

https://orcid.org/0009-0005-0228-4590

Date of Graduation

5-2026

Document Type

Thesis

Degree Name

Master of Science in Crop, Soil & Environmental Sciences (MS)

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor/Mentor

Speir, Shannon

Committee Member

Shogren, Arial

Second Committee Member

Befus, Kevin

Third Committee Member

Brye, Kristofor

Fourth Committee Member

Daniels, Michael

Keywords

Biogeochemistry; Non-Perennial; Nutrients; Reservoirs; Streams

Abstract

Wetting and drying cycles are a fundamental, but underexplored, control on nutrient mobilization and transport in aquatic ecosystems. Such intermittency impacts approximately 60% of the world's streams and rivers, and the extent is expected to increase with future climate change. Additionally, aging reservoir infrastructure is driving restoration efforts, like large-scale drawdowns, that can expose benthic sediments to extended dry periods before refilling. Despite the prevalence of wet-dry cycles, how they influence nutrient transport and mobilization across different aquatic ecosystems remains poorly understood. To address this gap, this thesis presents two complementary studies exploring nutrient dynamics in Arkansas aquatic ecosystems that experience wetting and drying cycles. First, I studied Brush Creek, an intermittent stream in Northwest Arkansas, sampling four nested sites along the mainstem for nitrate-nitrogen (NO3-N) and soluble reactive phosphorus (SRP) from October 2024 to September 2025. Using the loadflex package in R, I estimated daily nutrient concentrations and loads using continuous discharge measurements and grab sample data. With these data, I sought to quantify how flow state structured nutrient propagation and variability across space and time. I found that longitudinal surface disconnection does not interrupt nutrient propagation, but reroutes propagation through subsurface flows, lengthening travel time from one to three days, while maintaining load coupling (NO3-N: r = 0.62; SRP: r = 0.74). Additionally, I quantified changes in loads from the most upstream sampling site to the watershed outlet, documenting a 98% and 96% reduction in NO3-N and SRP loads, respectively, during the disconnected period. This suggests that lateral sources accounted for the majority of NO3-N and SRP loading during connected flows, and mean load differences between the headwater and outlet sites were 10,390 kg d-1 for NO3-N and 7.3 kg d-1 for SRP. Moreover, N:P molar ratios severely exceeded the Redfield ratio (N:P = 16:1) for all sites and both connectivity periods, indicating extensive stoichiometric P limitation across the watershed. Finally, variability in nutrient loading decreased with surface disconnection, whereby coefficients of variation (CV) dropping from 97-180% during connection to 14-127% during disconnection for NO3--N loads. As such, this suggests flow state served as a key control on nutrient dynamics in Brush Creek. These results demonstrated that drying does not simply interrupt nutrient transport, but also reorganizes the key pathways through which transport occurs, challenging traditional binary models of connectivity for intermittent streams. Second, I examined the nutrient mobilization risk of full-reservoir drawdown at Lake Conway in central Arkansas. Here, I sampled benthic sediments at six sites distributed across Lake Conway to assess sediment nutrient concentrations and conducted experimental simulations using sediment cores to determine the risk of N and P loss during storms in the dry period and during refilling. I found that patterns of sediment deposition driven by legacy nutrient accumulation and differences in exposure during drawdown influenced the spatial distribution of N and P concentrations in both site water (when present) and sediments. Additionally, reduction-oxidation reactions (redox) and hydrologic controls drove seasonal patterns in sediment N and P concentrations, with summer nutrient concentrations in sediments (SRP = 0.52 ± 0.07 µg g-1; NH4-N = 34 ± 8 µg g-1) exceeding sediment concentrations in fall and winter. Finally, simulations revealed that any rewetting event released N and P from sediments, regardless of experiment type, as , posing a risk to downstream water quality. My findings indicate that drying can potentially enhance the risk of downstream nutrient loss from reservoirs, with rewetting events serving as a key control on mobilization. Across both studies, wet-dry cycles emerged as the primary control on nutrient dynamics, reorganizing nutrient transport pathways in streams and triggering mobilization events in reservoirs experiencing drawdown. Overall, my findings have direct implications for nutrient management and monitoring in aquatic ecosystems that experience wet-dry conditions.

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