Defense Date


Document Type


Degree Name

Master of Science



First Advisor

Paul A. Bukaveckas


The tidal freshwater James River exceeds chlorophyll-a (CHL-a) water quality standards and the cyanotoxin microcystin is commonly present. Efforts to control harmful algal blooms in the James have brought about significant reductions in nitrogen and phosphorus inputs, but chlorophyll levels have not changed. In order to better understand the factors constraining harmful algal blooms in this system, we performed three factorial mesocosm experiments that tested how light (ambient, enhanced), mixing (weak mixing, well-mixed), and the availability of dissolved inorganic nitrogen (pre/post NH4+ reductions) influence the abundance of phytoplankton (CHLa, particulate organic carbon [POC]) and cyanotoxin production (microcystin, anatoxin) in this system. Enhanced light treatments increased CHLa and POC in well-mixed mesocosms across all three experiments, while NH4+ treatments had little effect on CHLa and POC. Light also had the greatest effect on toxin production, where microcystin production was favored under ambient light conditions and anatoxin production was favored under enhanced light conditions. CHLa and POC results suggest that light availability constrains phytoplankton production in the James and recent reductions in NH4+ would not alone be expected to reduce CHLa concentrations in this system. Peak toxin levels observed in mesocosms were relatively low compared to other systems, suggesting that cyanobacteria blooms are unlikely to pose a significant threat in the James.


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