Date of Award

Fall 2024

Project Type

Thesis

Program or Major

Natural Resources

Degree Name

Master of Science

First Advisor

William H McDowell

Second Advisor

Adam S Wymore

Third Advisor

Lauren Koenig

Abstract

Streams are hotspots for biogeochemical cycling in freshwater ecosystems and act as transformers for elements moving throughout their watersheds. Small watersheds in particular are the ideal natural laboratory for exploring how freshwater systems react to inputs and outputs of carbon, nitrogen, and other trace elements. Carbon cycling occurs throughout watersheds at different scales and through different pathways. Organic matter enters a stream from surrounding terrestrial environments, and is subsequently consumed by microbes, macroinvertebrates, and other freshwater organisms. The fate of organic matter in a stream is diverse, but some common pathways include emission as carbon dioxide through metabolic processes, movement downstream for further transformation, or persistence within the stream reach to be taken up by primary producers or buried in river sediment. The biogeochemical fate of carbon and other elements in freshwater ecosystems is constantly influenced by climate change effects such as disturbance. Occurrences of severe hurricanes and droughts are increasing across the globe and have been especially prevalent in recent years in tropical rainforests. Understanding how tropical stream biogeochemical cycling is responding to such disturbances informs global carbon budgets and future watershed management

In this thesis I investigated how disturbances in tropical ecosystems affect carbon processing and cycling throughout a stream. In my first chapter, I assessed the effect of droughts and hurricanes on organic matter inputs to tropical streams by analyzing an existing dataset of leaf litter leachate from senesced and fresh leaves of nine common riparian Puerto Rican tree species. Senesced (i.e., brown) and freshly abscised (i.e., green) leaves from each species were used as proxies of drought and hurricane organic matter inputs, respectively. Leaf samples were leached in deionized water for one, three, and seven days to look for trends in leaching over time in the absence of stream microorganisms. I found that while most leaching occurs within the first 24 hours, there is continued leaching potential for up to three days depending on the species and leaf condition. More solutes leached from brown leaves compared to green leaves, and dissolved organic matter leachate concentration was higher than any salts or elements found in both brown and green leaf leachate. These data suggest that leaf litter inputs during droughts may contribute more organic carbon and nitrogen to streams than those occurring during hurricanes.

In my second chapter, I analyzed dissolved oxygen trends and modeled metabolism (i.e., gross primary production (GPP) and ecosystem respiration (ER)) for a Caribbean stream across a period that experienced multiple disturbances. I used seven years of high-frequency data from in-situ sensors in a headwater stream in Puerto Rico and performed time series analyses and Bayesian modeling of metabolism. Fluxes in GPP and ER over time provide insight into which biogeochemical processes are occurring during different disturbance events. I found that drivers of metabolism had different magnitudes of influence depending on the disturbance. For example, both GPP and ER decreased during droughts likely because of decreased flow and biotic activity, while GPP increased during the hurricane disturbance period probably due to increased light and primary production.

Metabolism and leaf litter leachate results together demonstrate that while increased organic matter enters streams during droughts, stream flow could restrict consumption and transformation of carbon until water depth increases. Conversely, hurricanes may be primarily affecting primary production rather than organic matter consumption because of both increased GPP and reduced carbon inputs from leaves. Understanding trends in organic matter cycling over time in a tropical rainforest stream is important for comparing biogeochemistry across biomes and provides further evidence of how disturbances change the drivers of different biogeochemical processes.

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