UC Davis

This dissertation explores freshwater lake ecosystems, especially their productivity, vulnerability, and food web architecture, through the lenses of landscape limnology and community ecology. There is a growing necessity to quantify ecosystem productivity (Chapter 1), climate change resilience (Chapter 2), and the mechanisms that govern lake food web structure and function (Chapter 3). Studies on these topics will enable conservation management of lakes at all scales, including adaptation to global environmental change.

The first chapter develops five USA reservoir classification systems ranging in complexity to understand the distribution of fish biomass in different reservoir types. This framework is leveraged to predict pools of fish biomass in unsampled reservoirs across the USA. Results provide evidence that reservoirs, while undeniably ecological catastrophes, hold massive pools of freshwater fisheries biomass and may have higher ecological and ecosystem services value than previously realized. Results provide a new vehicle for upscaled estimates of ecosystem productivity, ecological resilience, a snapshot of inland fisheries biomass potentially available for human consumption, and identifies how reservoir biomass changes over time.

The second chapter describes velocity of change in heat accumulation rates across high elevation lake landscapes in the contiguous United States. Mountain lake landscapes are sensitive to climate change, yet the velocity at which they are undergoing thermal change is poorly understood. This uncertainty presents challenges for managers interested in building ecological resilience to climate change. Developed velocity of change metrics provide compelling evidence that lower elevation mountain landscapes are undergoing temperature change more rapidly, and that each of the ten primary mountain ranges in the United States exhibit unique trends in velocity of change. Products from this work include climate vulnerability classifications for mountain lake landscapes across the USA, based on velocity of change, that managers can use in conservation prioritization frameworks.

The third chapter uses an extensive field study to test applicability of two critical ecological theories in mountain lakes; that is that food chain length and community niche complexity scales predictably with either ecosystem size or productivity. Using data collected from 36 mountain lake food webs over three summers in the Sierra Nevada, California, USA, patterns of food web complexity in relation to lake volume were directly tested. Overall, there is limited support for the ecosystem size hypothesis, and weak-to-no support for the productive space hypothesis. Rather, food web architecture was highly context-dependent. Aquatic and terrestrial insects dominated contributions of ecological energy flows to consumers in most lakes. Fish consumer diet reliance on key food sources varied along a lake ecosystem size gradient. From small to large lakes, there is apparently increasing reliance on terrestrial insects and periphyton, decreasing reliance on aquatic plants, and constant, but again high, reliance on aquatic insects. These results are useful in assessing how oligotrophic mountain lake ecology differs from other more well-studied regions, and for better managing mountain lake landscapes and ecosystems.

Collectively, these chapters provide novel information on the ecology of North American lakes, with a special focus on mountain landscapes. These science products will aid conservation management of lake landscapes overall. Further, they are important examples of the use of lesser-studied lake types as models for ecological study.