Author ORCID Identifier

Document Type


Date of Award


Degree Name

Master of Science (MS)



First Advisor

Jeff S Wesner


Much like the response to naturally occurring physical, chemical, and biological variables controlling phytoplankton dynamics, anthropogenic modification to those variables may have profound implications on phytoplankton density and community structure in aquatic systems. We theorized that extensive land use and river channel modifications would result in (1) an increase in basin-wide phytoplankton density in the Middle Missouri River Basin (MMRB), and (2) a shift in community structure within and downstream of reservoirs filled after 1950 by examining data collected from 2020 and 2021 across the Middle Missouri River Basin to data collected in 1950. Our results suggest that system-wide increases in algal cell density were uncommon, yet variable with only two systems showing increases with high confidence and one showing a decrease. Some systems like the Missouri River reservoir sites and the James River showed an interannual shift in dominant phytoplankton genera, while other systems shared at least one dominant genus in 1950, 2020, and 2021. Our results suggest that despite modifications to land and water use, changes in phytoplankton density and community structure are clear but not consistent across the MMRB. Chlorophyll has been used extensively in ecological monitoring as a proxy for phytoplankton density or biovolume due to the relative simplicity of processing samples. We regressed the predictor variable of total chlorophyll and response variable of algal cell density as well as the predictor variable of log10 transformed Secchi depth (Secchi) and the response variables of either total chlorophyll or algal cell density from 161 samples across nine rivers of the MMRB. A positive relationship was observed between chlorophyll and algal cell density, while an inverse relationship was observed between Secchi and either chlorophyll or algal cell density. These findings suggest that using chlorophyll as a proxy for algal cell enumeration may provide an option to monitor phytoplankton dynamics in rivers. High suspended sediment loads may have confounded the relationship between Secchi and either chlorophyll or algal cell density. Chlorophyll determined by in vivo fluorescence provides a good proxy to rapidly monitor phytoplankton dynamics in lowland rivers.


Algal cell, Chlorophyll, Dam, Missouri River, Phytoplankton, River

Number of Pages



University Of South Dakota



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