Date Approved

11-14-2022

Graduate Degree Type

Thesis

Degree Name

Biology (M.S.)

Degree Program

Biology

First Advisor

Bopaiah Biddanda

Second Advisor

Casey Godwin

Third Advisor

Steve Ruberg

Fourth Advisor

Eric Snyder

Academic Year

2022/2023

Abstract

Bottom water hypoxia, a condition of low dissolved oxygen in the bottom waters, negatively effects lakes and freshwater and marine coastal estuaries globally. As climate change shifts climatological and ecological patterns, hypoxia continues to shift in duration and severity. In Muskegon Lake, Michigan, hypoxia occurs annually, restricting fish habitat, increasing cyanobacteria blooms, and disrupting ecological and socioeconomic prosperity. In the current study, we examined the duration and severity of hypoxia over a 11 years (2011-2021) utilizing data from the Muskegon Lake Observatory buoy (MLO, https://www.gvsu.edu/wri/buoy/). During 2021, we analyzed the dissolved oxygen (DO) concentrations observed by the MLO, conducted biweekly sampling of nutrients across the gradient of the lake ecosystem, and investigated seasonal surface and riverine loading to understand the impact these sources have on the drawdown oxygen in the hypolimnion. A new hypoxic severity index was devised to illustrate the severity of the hypoxia across the decade, with the years 2021 and 2012 being the most severe and 2015 and 2019 being the least severe. Spring air and surface water temperatures correlated with stratification strength within Muskegon Lake leading to varying levels of hypoxia. High spring precipitation and high summer algal growth correlated with high severity of hypoxia. Low spring precipitation and low summer algal growth led to reduced severity of hypoxia. Within 2021, frequent wind-mixing events, and an intrusion of cold, oxygenated upwelled Lake Michigan waters intermittently alleviated hypoxia, although never fully disrupting it, leading to a long duration of stratification and hypoxia severity throughout the year. Oxygen drawdown experiments revealed that riverine organic matter inputs to the hypolimnion contributed to the development of hypoxia more readily than surface primary production in the spring, whereas the opposite was true in the summer and fall, suggesting seasonally variable sources drive hypolimnetic hypoxia. Biweekly nutrient analyses revealed that soluble reactive phosphorus accumulated in the hypolimnion during late summer, indicative of internal phosphorus loading. Our findings on temperature and precipitation as major causative agents of hypoxia, seasonal oxygen drawdown, and internal phosphorus loading have relevance to similarly afflicted ecosystems in the Great Lakes basin and lakes and estuaries everywhere.

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