Observing, Modeling and Interpreting the Seasonal Succession of Phytoplankton in the Lower Charles River
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martens,victoria_2026_eaps_sb_thesis.pdf
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Author(s)
Martens, Victoria C.
Advisor(s)
Follows, Michael J.
Date Issued
May 2026
Publisher
Massachusetts Institute of Technology
Abstract
Seasonal succession of freshwater phytoplankton is influenced by many factors such as Phosphorus and Nitrogen availability, temperature, weather, light, and hydrological dynamics. Because the Charles River is a Eutrophic system, these forces are unbalanced and can facilitate algal blooms that render water unsafe for swimming and recreation. Despite extensive management efforts focused on reducing Phosphorus pollution, the river continues to experience harmful algal blooms on a regular basis. This study combined qualitative plankton imaging with a trait-based ecological model to explore whether seasonal phytoplankton succession in the Lower Charles can be explained primarily by nitrogen availability, phosphorus availability, and temperature. Plankton samples were collected approximately weekly during 2024 and 2025 using fine-meshed plankton nets at MIT’s sailing pavilion docks, and were analyzed using light microscopy. The images revealed a clear season succession of plankton, beginning with high diatom populations, followed by a period of increased biodiversity including green algae, mixotrophs, and cyanobacteria, which was then followed by significant late-season cyanobacterial dominance. The images season succession showed somewhat altered trends from typical oligotrohic or mesotrophic systems, including reduced green algae and an absence of a year-end diatom resurgence. To investigate which factors had the strongest effect on shaping the season succession, a simplified model was constructed using Monod kinetics, the Cardinal temperature model, and ordinary differential equations. The model included four trait-based representative phytoplankton groups: the diatom Asterionella, the diazotrophic cyanobacterium Anabaena, the non-diazotrophic cyanobacterium Microcystis, and the green alga Chlorella. Nutrient inputs from natural and human sources were estimated using publicly available watershed data, and the river was approximated as a well-mixed system with constant volume where nutrient inflows were represented as changes to chemical concentrations over time. The model successfully reproduced some general seasonal trends, including early diatom growth, increasing cyanobacterial abundance late-season, and timing of peak cyanobacterial biomass. However, it failed to accurately model some observed planktonic dynamics and biodiversity such as biomass of green algae, rate of increase of cyanobacterial populations, and year-end persistence of cyanobacteria. These differences indicate that temperature and nutrients alone are not fully able to explain phytoplankton succession in the Lower Charles, and that additional factors including hydrologic disturbance and light limitation likely suppress cyanobacterial dominance during much of the growing season, contributing to the observed biodiversity. Rainfall in the spring and early summer appeared to both increase nutrient loading and physically disturb the water column, potentially delaying bloom formation despite promoting cyanobacterial growth in the model. Overall, this thesis demonstrates that temperature and nutrients can explain some broad seasonal trends, but that disturbance of the water column and light limitation are needed to be able to predict cyanobacterial blooms.
MIT Department
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
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