Headwater stream network connectivity: biogeochemical consequences and carbon fate
| dc.contributor.author | Bretz, Kristen Alexandra | en |
| dc.contributor.committeechair | Hotchkiss, Erin R. | en |
| dc.contributor.committeemember | Dolloff, C. A. | en |
| dc.contributor.committeemember | McLaughlin, Daniel L. | en |
| dc.contributor.committeemember | Barrett, John E. | en |
| dc.contributor.department | Biological Sciences | en |
| dc.date.accessioned | 2023-05-05T08:00:47Z | en |
| dc.date.available | 2023-05-05T08:00:47Z | en |
| dc.date.issued | 2023-05-04 | en |
| dc.description.abstract | Headwaters may be small relative to other aquatic ecosystems, but they are neither simple nor static environments. Heterogeneous stream corridors constitute the majority of river network length and regulate cycling of carbon and oxygen as they expand and contract their connections across the landscape. Though headwater streams integrate many biogeochemical signals from the watersheds they drain and provide important ecosystem services, their diverse habitats and dynamic changes in wet length have been under- examined compared to dendritic, perennial streams. This oversight complicates efforts to identify biogeochemical patterns at larger scales. This dissertation sets out to expand our knowledge of stream biogeochemical responses to variable connections both within the channel and the wider stream corridor. First, I investigated how the presence and arrangement of different habitat patches in the stream corridor affected overall emissions of carbon dioxide (CO2) and methane (CH4) from sub-watersheds of a forested mountain stream network. To do this I measured concentration and flux of both gasses along and around 4 streams, including dry reaches and adjacent vernal pools as well as flowing water. I found that emissions were highly variable over space and time; in particular, the presence of a vernal pool enhanced total carbon emissions from the stream corridor. Next, to quantify carbon cycling and export from a non-perennial headwater stream, I monitored concentrations of CO2 and dissolved organic carbon (DOC) at the stream outlet. I found that CO2 concentration had a negative relationship with stream discharge, and that exports of both CO2 and DOC were driven by storms reconnecting isolated surface water reaches. I also found that carbon biogeochemistry of intermediate flow states were unique from driest and highest-flow conditions. Finally, to explore how isolated pools in the stream channel respond to flow decrease and cessation, I measured dissolved oxygen (DO) as well as CO2 and CH4 from persistent pools of two non- perennial streams throughout an unusually dry summer and fall. I found that hypoxia was common in all isolated pools, but swings in DO were not consistent between pools even of the same stream. In using diel changes in DO to estimate metabolism, I also found that ecosystem respiration varied by stream, but gross primary production was more driven by stream surface water connectivity. Climate change is inducing many new patterns in stream hydrology with critical implications for biogeochemical activity, from reducing durations of connectivity to causing stronger storms. Improving our understanding of how surface water and landscape connectivity both influence the movement of carbon within and through streams is essential to resolving questions about the contributions of freshwaters to the global carbon cycle. | en |
| dc.description.abstractgeneral | Headwater streams may seem inconsequential to larger ecosystem processes due to their small size. However, the majority of a river's network length, or the total length of all the streams and rivers from spring to ocean, is made up of headwater streams. The widespread presence of headwater streams over all types of land, along with the unique layout of different aquatic habitats near streams and the fact that small streams often grow and shrink in length, mean that studying headwaters can tell us many things about how energy moves through ecosystems. This dissertation explores how we can use changing headwater connectivity to understand how carbon moves through ecosystems. Connectivity in aquatic science refers to how water can move through space in ways that rocks and trees and even many animals cannot. This idea is useful because water carries things around as it moves, and its presence or absence enables reactions that are essential for the cycling of energy and nutrients. For instance, when water moves from high ground to low ground, it navigates through soil and holes in the ground; it may get slowed down at flat spots where little pools form. I measured emissions of carbon dioxide and methane from streams as well as soils, holes, and pools near mountain streams to try to understand how the path water takes influences how much carbon dioxide and methane escapes into the air. My measurements were surprisingly different depending on where and when I took them. I found that if a seasonal pond is connected to a stream channel, the stream will emit more greenhouse gasses than if the pond goes dry. Connectivity can also describe if water moves continuously along a stream, or if the stream goes dry in places and is then disconnected from different parts of itself. I asked how a stream becoming disconnected affected carbon dioxide emissions as well as the movement of dissolved organic carbon, a food source for microorganisms. I found that the less water moving through the stream channel, the higher carbon dioxide concentrations were. I also found that storms move both carbon dioxide and dissolved organic carbon out of streams quickly, even if the stream had been disconnected. Finally, I investigated the water that is left when streams disconnect. I measured dissolved oxygen, carbon dioxide, and methane in isolated pools of two disconnected streams. By tracking how microbes and algae consume and produce oxygen when a stream is not flowing, I can understand how these lifeforms adapt. I found that isolated pools frequently have very low levels of dissolved oxygen. This means that microorganisms in the pools have to use special ways of getting energy, which in turn affects how different forms of carbon move through the stream ecosystems. Headwater stream ecosystems are very sensitive to small changes in flow and precipitation; however, climate change means that streams are going dry more often than they used to. My findings contribute to our understanding of how changes in stream connectivity have many biological effects that are important for water quality and ecosystem health. | en |
| dc.description.degree | Doctor of Philosophy | en |
| dc.format.medium | ETD | en |
| dc.identifier.other | vt_gsexam:37286 | en |
| dc.identifier.uri | http://hdl.handle.net/10919/114927 | en |
| dc.language.iso | en | en |
| dc.publisher | Virginia Tech | en |
| dc.rights | In Copyright | en |
| dc.rights.uri | http://rightsstatements.org/vocab/InC/1.0/ | en |
| dc.subject | streams | en |
| dc.subject | biogeochemistry | en |
| dc.subject | carbon | en |
| dc.subject | climate change | en |
| dc.subject | carbon dioxide | en |
| dc.subject | methane | en |
| dc.subject | greenhouse gas | en |
| dc.subject | freshwater ecology | en |
| dc.title | Headwater stream network connectivity: biogeochemical consequences and carbon fate | en |
| dc.type | Dissertation | en |
| thesis.degree.discipline | Biological Sciences | en |
| thesis.degree.grantor | Virginia Polytechnic Institute and State University | en |
| thesis.degree.level | doctoral | en |
| thesis.degree.name | Doctor of Philosophy | en |
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