Jed Howrey | 2025 I.S. Symposium

Name: Jed Howrey
Title: Evaluating The Interactive Effects of Elevated pH and Hypoxia on Growth of The Diatom Thalassiosira pseudonana: Implications for Harmful Algal Blooms in The Laurentian Great Lakes
Major: Environmental Studies
Minors: Economics; German
Advisor:Dr. Carlo Moreno

In the Laurentian Great Lakes, eutrophication, characterized by excessive nutrient inputs from fertilizer runoff, is a major issue. Harmful algal blooms (HABs) which result from eutrophication pose significant ecological and economic threats to aquatic ecosystems. Hypoxia (i.e., dissolved oxygen < 2.0 mg O2 L−1) and elevated pH are adverse environmental conditions which commonly occur in ecosystems facing HABs, however their combined effects on phytoplankton growth are still not well understood. One group of phytoplankton species affected by HABs are diatoms, a critically important type of algae which produce around 40% of Earth’s breathable oxygen. The aim of this study is to investigate the synergistic effects of hypoxia and elevated pH on the growth of Thalassiosira pseudonana, a model diatom species. A laboratory study was conducted to determine differences in cell density, cell size, and specific growth rates between varying pH and dissolved oxygen conditions.

Results demonstrated that pH alone and hypoxia alone both significantly decreased cell density over time, with differences appearing after about 10 days. Additionally, after 10 days, there was a significant interaction between elevated pH and hypoxia. There were no significant interactive or main effects on cell size. The results also indicated that hypoxia alone significantly lowered specific growth rate, and that elevated pH and hypoxia had a significant interactive effect. A major implication of these results is that previous literature, which only tested the effect of a single factor, likely understated the true impact that HABs have on diatom populations. Given the growing frequency and intensity of Great Lakes HABs as well as the importance of diatoms to freshwater ecosystems, my results could be used to develop more accurate nutrient loading targets that account for impacts to diatoms.

Posted in Symposium 2025 on May 1, 2025.