Date of Award

Spring 2019

Document Type

Open Access Thesis

Department

Environmental Health Sciences

First Advisor

Geoffrey Scott

Second Advisor

Dwayne Porter

Abstract

Cultural eutrophication is a primary contributor to phytoplankton production in freshwater lakes from excess anthropogenic nutrient inputs, and resulting impacts on water quality, aquatic ecosystems, human and animal health are increasing worldwide. Understanding the factors that limit phytoplankton growth is an important strategy for identifying and managing nutrient sources and successfully controlling the over- enrichment of nitrogen (N) and/or phosphorus (P) in freshwater lakes. The Redfield molecular ratio of carbon (C), N and P maintains that the C:N:P molar ratio is 106:16:1 when nutrients are not limiting, and the ecosystem is balanced. A N:P molar ratio > 20:1 tends to be indicative of P limitation in freshwaters, while a N:P molar ratio < 10:1 usually indicates N limitation, regardless of freshwater or seawater. Historically, studies on freshwater lakes have emphasized control of P, but more recent studies have challenged the P limitation paradigm and emphasized control of N or duel control of N and P.

Surveillance monitoring and ambient water quality and nutrient data in Lake Wateree, South Carolina indicate elevated symptoms of excess nutrients including decreased dissolved oxygen and water clarity, elevated pH and increasing phytoplankton blooms, both spatially and temporally. This study aimed to define the nutrient limitation indicator(s) to predict phytoplankton growth in Lake Wateree using nutrient enrichment bioassays. A series of four factorial design experiments were conducted during the summer and fall (August through October 2017) for the evaluation of both separate and interactive roles of N and P during in situ incubation periods of 48 hours within the lake environment at ambient conditions. The four treatments included a control (deionized water), + N (NH4+), + P (PO43-) and + NP (NH4++ PO43-) additions and their effects on phytoplankton growth using chlorophyll-a fluorescence as the response variable.

Nutrient relationships were determined from the twenty categorical responses, and despite exceedances in P water quality criteria, the bioassays produced no P limitation or serial P limitation responses. Instead, twelve of the experimental responses (60%) were co-limitation, four responses (20%) were serial N limitation, three responses (15%) were N limitation, and one response (5%) was not significant. The results of this study are valuable in identifying the importance of each nutrient factor (N and P) and achieving successful lake management goals by reducing excess nutrients and improving water quality in Lake Wateree.

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