Online

Northrop, Amanda Claire

Biology

2021

Ph. D.

Aquatic ecosystems can undergo abrupt and long-lasting transitions from one state to another, often with negative ecological and economic consequences. With anthropogenic enrichment, aquatic ecosystems such as lakes and ponds may shift rapidly from an oligotrophic, clear water state to a eutrophic, turbid state. These shifts, or state changes, generally occur due to a phenomenon called hysteresis in which the relationship between a driving variable and ecosystem variable depend on the current state of the ecosystem. Such dynamics often make recovery difficult or impossible. Though state changes in aquatic ecosystems have been studied extensively since the 1970s, there have been few replicated experimental studies using natural aquatic ecosystems because of the long time scales over which state changes occur and ethical considerations. Further, we know little about how bacterial communities change during and after a state change. In the last few decades the Northern Pitcher Plant, Sarracenia purpurea, has been put forward as a model ecosystem for studying state changes in aquatic ecosystems. This carnivorous plant forms cup-shaped leaves that fill with rainwater and house a multi-trophic food web, including over a thousand species of bacteria. When enriched with enough organic matter, S. purpurea pitchers abruptly become hypoxic due to increased bacterial oxygen demand and remain hypoxic as enrichment continues. Though we can simulate an abrupt state change in the S. purpurea ecosystem, we know little about the ecosystem's recovery and how such abrupt shifts alter the structure and function of the bacterial community that drives dissolved oxygen in the system. Here, I used a combination of metaproteomics andmetagenomics to show that enriched S. purpurea ecosystems feature distinctly different bacterial communities than non-enriched ecosystems. Using field and greenhouse experiments, I uncovered complex hysteretic dynamics and corresponding changes in bacterial community structure and function during a state change and subsequent recovery. My work shows that enriched aquatic ecosystems differ in bacterial community structure and function after a state change and that these differences persist well after the ecosystem "recovers." I also show that hysteresis in the S. purpurea microecosystem depends on the magnitude of organic matter enrichment and suggest that bacterial communities differ between hysteric and non-hysteretic ecosystems.