Implications of global change on biogeochemical cycling in aquatic ecosystems across biomes

Date of Award

Spring 2023

Project Type

Dissertation

Program or Major

Earth and Environmental Sciences

Degree Name

Doctor of Philosophy

First Advisor

William H McDowell

Second Advisor

Adam S Wymore

Third Advisor

Ruth K Varner

Abstract

Freshwater ecosystems reflect the biogeochemical signals of terrestrial ecosystems, atmospheric chemistry, and regulate export of materials to coastal oceans, making them integral components of global elemental cycles. Lakes and streams actively process, retain, and transform inputs of nutrients and organic carbon, thereby playing an important role in providing ecosystem services, supporting aquatic food webs, and contributing to greenhouse gas emissions. Inland waters face multiple stressors as a result of global change. Understanding how these ecosystems will respond to the ongoing impacts of global change across globally distinct biomes is crucial for predicting shifts in ecosystem structure and function, as well as their contributions to global biogeochemical cycles.In this dissertation, I assessed the response to and role of lakes and rivers in ongoing global change across biomes at varying spatial and temporal scales. I used a combination of field experiments, targeted synoptic sampling, and analysis of long-term data, high-frequency data from in situ sensors to better understand how global change impacts biogeochemical cycling in inland waters. To assess the extent to which fluvial ecosystems in underrepresented biomes influence climate change through emission of greenhouse gases, I used a five-year record of dissolved greenhouse gases from tropical montane watersheds. I found that streams were consistent sources of carbon dioxide, methane, and nitrous oxide to the atmosphere. Carbon dioxide and nitrous oxide concentration were associated with inputs from the surrounding landscape while methane was related to in-stream oxygen availability and lithology. Next, I used experimental additions of carbon and nitrogen in small Arctic streams to assess how nitrogen cycling may be impacted by future climate-induced changes in carbon and nitrogen inputs. Streams showed preferential uptake of ammonium over nitrate, and the addition of carbon increased ammonium uptake by over 50%. Freshwater ecosystems in the northeastern United States are experiencing increased variability in climate, warming winters, and increased frequency and severity of drought. I examined the impact of two severe droughts on stream carbon and nitrogen cycling using a combination of synoptic sampling and data from high frequency in situ sensors across a network of streams and rivers in New Hampshire. Droughts consistently impacted carbon and nitrogen concentrations, typically resulting in an accumulation of nitrogen and a decline in carbon. Finally, I assessed how the relative influence of drivers of variability in lake dissolved organic carbon (DOC) concentrations changes over more than three decades and what role warming winters play in regulating DOC in subsequent seasons. I found that the influence of climate and site-level differences in lake characteristics became more pronounced over time while the influence of atmospheric deposition decreased. With respect to winter, results suggest that a shift to rainier winters may lead to increased spring DOC concentrations in lakes. Collectively, these dissertation chapters provide a better understanding of the response of carbon and nitrogen cycling in inland waters to ongoing global change at varying spatial and temporal scales and emphasize the importance of ongoing monitoring and experimental studies to assess the impact of global change on these vital ecosystems.

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