MS Thesis Defense - Environmental Science and Policy

MS Thesis
Department of Environmental Science and Policy
College of Science
George Mason University

Candidate: Hannah Toney
Defense Date and Time: April 10, 2026 at 9:00am
Defense Location: Zoom (registration required)
Title: Annual patterns of photosynthetic performance, pigment composition and toxin production of the mat-forming cyanobacterium Microseira wollei in the tidal freshwater Potomac River (USA)

Thesis Director: Dr. Rosalina Stancheva Christova
Committee: Dr. Jennifer L. Saleerno and Dr. Diego Valderrama

ABSTRACT

We studied the seasonal dynamics of the filamentous, toxigenic cyanobacterium Microseira wollei in the freshwater tidal Occoquan River, where it forms extensive benthic prostrate mats and drifting spherical aggregates (“balls”), collected monthly from August 2024 to August 2025. For each mat type, we analyzed photosynthetic rates using ¹⁴C uptake, pigment concentrations (chlorophyll a, phycocyanin, phycoerythrin), organic mass, saxitoxin content, and morphological variation. Microseira wollei exhibited peak physiological performance in summer (July–August), but retained measurable photosynthetic activity and continued saxitoxin production throughout winter, indicating a persistent ecological risk. These findings emphasize the importance of year-round cyanobacterial monitoring in freshwater systems.