The Influence of Resource Quality and Lake Physics on Zooplankton

Zooplankton are important in maintaining the ecosystem services provided by lakes. There is an extensive history assessing how different ecological variables impact zooplankton productivity in lakes; however, there is still uncertainty as to the relative control of resources, predators, and lake physics in regulating zooplankton biomass in natural lakes. My dissertation aims to add to this body of work, by assessing how the interaction of physics and resource quality impact zooplankton productivity and behavior.

I found that inputs of terrestrial dissolved organic carbon (DOC) can have a significant negative impact on zooplankton productivity mediated by reductions in mixed layer depth. However, results from my dissertation suggest that the relationship between DOC and lake productivity may be non-linear, as zooplankton productivity increased in response to experimental additions of DOC to a manipulated lake. Results from my dissertation also support the potential for a positive relationship between resource quality and DOC due to changes in the light-to-nutrient ratio within the lake. Taken together, my dissertation highlightsthe potential for DOC mediated physics to drive food web productivity in lakes, with the potential for resource quality to modulate this relationship.

In addition to regulating zooplankton biomass, resource quality and physics may play an important role in structuring zooplankton distributions and migration behavior. My dissertation uses a physiological growth model to predict optimal migration strategies for zooplankton across a set of north temperate lakes. This model, in combination with empirical measurements of zooplankton migration, point out the relative rarity of traditional migration strategies due to the influence of temperature and dissolved oxygen, but also resource quality and predator density.

In large ultra-oligotrophic lakes with very low terrestrial influence and nutrient concentrations like Lake Ontario, there is likely a small influence of lake physics on zooplankton biomass. However, there is potential for spatial heterogeneity in resource quality due to natural hydrodynamics and phytoplankton physiology. My dissertation suggests resource quality may be more homogeneous than expected, and due to the low nutrient concentration, and therefore poor resource quality throughout the lake, zooplankton are more heavily impacted by changes in resource quantity rather than quality.