UC San Diego

Multiple aspects of the global environment are changing rapidly, including rising temperatures, altered biogeochemical cycles and a redistribution of biodiversity. Different aspects of environmental change may interact through synergistic processes or interference; however, how these processes magnify or dampen one another’s effects on lakes is largely unknown. My dissertation research explores the independent and interactive effects of warming and resource supply on lake food webs from multiple perspectives. Chapter 1 investigates the independent and interactive effects of temperature, supply and origin of dissolved organic material, and atmospheric nitrogen deposition on prokaryotic and eukaryotic microbial community composition and diversity in Sierra Nevada lakes. Chapter 2 focuses on stoichiometry and growth of phytoplankton communities in response to warming, nutrient addition and grazing in three Dutch lakes across a productivity gradient. Chapter 3 explores whether the competition-defense tradeoff regulates coexistence within or among members of phytoplankton communities across a productivity gradient, and how warming may alter this tradeoff. These studies collectively show that resource supply is more important than temperature in regulating microbial community composition and stoichiometry across a variety of lake ecosystems. Additionally, interactive effects of temperature, nutrient supply, and grazing on phytoplankton community stoichiometry, growth rates, biomass buildup and functional group composition depend on the trophic state and size structure of communities. Finally, turnover in communities along productivity gradients resulted in a positive correlation between nutrient and grazer limitation across taxa among lakes, but no relationship between top-down and bottom-up limitation within lakes. This result suggests that traits like small cell size that make phytoplankton more susceptible to grazing also confer strong responses to nutrient pulses in low-nutrient environments. Thus, my results provide no support for the hypothesis that costly defenses against grazing increase nutrient limitation, resulting in a tradeoff between nutrient and consumer limitation. In fact, the opposite pattern was found whereby the taxa that are most sensitive to grazing and nutrients are segregated in the least productive system, and the responses to both factors decline in more productive lakes due to increasing dominance by inedible forms. My thesis demonstrates functional associations among the traits of microbes that shape their responses to climate, resources and consumers, promote diversity at the local and regional scales, and determine how aquatic ecosystem productivity is controlled by multiple limiting factors.