Date of Graduation

5-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Biological and Agricultural Engineering

Advisor

Marty Matlock

Committee Member

William Reed Green

Second Committee Member

Robert Morgan

Third Committee Member

Julian Fairey

Keywords

Algal growth, Eutrophication, Internal Loading, Littoral Zone, Phosphorus, Sedimentation

Abstract

Eutrophication of surface waters not only impacts the environment but also water treatment processes, the most significant of which is from the effects of algae. During peak algal growth in many southern U.S. reservoirs, inflows that bring nutrients are at an annual minimum, and phosphorus released from bed-sediments is trapped in the hypolimnion. Littoral areas, described as the most productive zone of the lake, may be a possible source of phosphorus that fuels algal growth in the reservoir. I studied an isolated shallow cove in the War Eagle Creek arm of Beaver Lake in Northwest Arkansas to measure, quantify, and model the dynamics and transport of nutrients in littoral zones. A summary of the research problem, the statement of purpose, and associated hypothesis are provided in Chapter 1. Chapter 2 provides an overview of eutrophication as it relates to drinking water treatment, reservoirs, and zones within reservoirs. Also reviewed is the relationship of phosphorus (P) to reservoir eutrophication, internal loading as a source of P, and P cycling and transport within a reservoir. In Chapter 3, I present my study of sediment cores, temperature profiles, and dissolved oxygen (DO) profiles of the cove to determine the redox condition of the water overlying the bed-sediments and the associated sediment P release rates. Phosphorus release rates from bed-sediment incubations were as high as 2.02 mg m-2 d-1 under aerobic conditions and 4.05 mg m-2 d-1 under anaerobic. Variability in bottom DO concentrations of the littoral zone likely contribute to a cycling of aerobic and anaerobic conditions by changing the redox state at the sediment-water interface. When compared to whole-lake P release averages conducted for Beaver Lake, eutrophic coves were a disproportionate source of P within the reservoir, producing 6.5 times more P per unit area. Chapter 4 describes a study of how sediment P release and algal growth in littoral areas are connected. Sediment composition, equilibrium P concentration (EPCo), and aerobic P release rates were characterized for several sites within the cove. Core aerobic P release rates and EPCo both confirmed the release of P under aerobic conditions; however, it appears that algal demand sustained low dissolved P concentrations in the waters of the cove. This created a nutrient cycle where algae impose a nutrient gradient favoring the release of P by keeping concentrations of the waters overlying sediments below the EPCo. Chapter 5 describes a model developed to simulate thermoconvective flow and transport mechanisms between littoral and pelagic zones. Diurnal fluctuation in flow for surface and bottom waters indicated water moves in and out of the cove on a daily timestep. Overall, the general direction of flow was in along the bottom of the cove, and out along the top. The model and in situ P concentrations revealed that P is transported from bottom waters as they enter the cove to surface waters as they leave the cove. This process applied to all littoral areas of the reservoir would result in a major P transport mechanism to surface waters in late summer. Chapter 6 provides a synthesis of the three studies. Unique processes of the littoral zone make it a disproportionate source of P within a reservoir. P loads brought into the littoral zone along the bottom and P released out of littoral sediments are then available for transport out of the surface of the littoral zone. This P would then be available to be used by algae and replenish P losses in the epilimnion which occur through sedimentation. Our model and sampling indicate that late summer blooms of algae in Beaver Lake are likely fueled in part by P transport out of the littoral zone. The fundamental contribution of this research is the recognition of littoral areas as a significant source of internal P loading to fuel algal growth in reservoirs.

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