Date of Award

Summer 2023

Document Type

Open Access Dissertation


Biological Sciences

First Advisor

Jeffry L. Dudycha


Aquatic ecosystems experience a wide range of variation in resource availability; two of the resources that are most essential to phytoplankton and zooplankton are light and nutrients. Light can vary in both its color and intensity; these variations can directly impact phytoplankton by altering the amount and color of light available for photosynthesis. Nutrient variability, particularly increases in nutrient levels, also alter phytoplankton communities by favoring the growth of certain taxa such as cyanobacteria.

To understand whether phytoplankton respond differently to changes in light availability I investigated whether the diversification of cryptophytes, phytoplankton with diverse light-capturing pigments, showed evidence of trade-offs among photosynthetic traits as predicted by the gleaner-opportunist resource exploitation framework. I did not find evidence for a gleaner-opportunist trade-off in cryptophytes, indicating that cryptophytes may have evolutionarily escaped from this trade-off, or that photosynthetic trade-offs may occur among traits or across scales that we did not investigate.

I also investigated how variation in light color and phosphorus availability affect freshwater phytoplankton communities and whether these changes impact trophic transfer from phytoplankton to zooplankton. I found that phosphorus availability and light color interact to determine phytoplankton community diversity and composition, with blue light leading to the most diverse communities and broad light the least diverse communities. High phosphorus levels and broad, green, and red light favored the growth of cyanobacteria that were of low nutritional quality, resistant to digestion, and capable of producing toxins. I also found that light color had a significant effect on zooplankton survivorship and juvenile specific growth rate.

Additionally, I looked at how differences in the extent of eutrophication and brownification alter phytoplankton and zooplankton communities in lakes across the continental United States. I found that daker lakes with higher levels of phosphorus had greater levels of phytoplankton and zooplankton biomass and had phytoplankton communities that were dominated by cyanobacteria capable of forming harmful algal blooms. All lakes across this gradient had Daphnia as their most dominant zooplankton genus. Overall, variation in light and nutrient availability seems to be causing phytoplankton communities to become dominated by cyanobacteria, which then has detrimental effects on higher trophic levels such as zooplankton.


© 2023, Jake Anthony Swanson

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