Emily Mulcahy, an undergraduate researcher in our lab mentored by Carla López Lloreda, was awarded a research grant from Virginia Tech's Global Change Center (GCC)! Emily is a senior at Virginia Tech majoring in Biological Sciences with minors in Wetland Science and Green Engineering. The GCC award will fund Emily's proposed research "assessing the role of phytoplankton on biogeochemistry in isolated wetlands" on the Delmarva peninsula in Maryland, USA. Congratulations, Emily!

News article celebrating Emily and the other 2023-4 GCC Undergraduate Research awardees

Carla López Lloreda recently published a paper with graduate student co-authors about their experience organizing a "flipped science fair".

From the abstract: "Flipped Science Fairs put power directly into children’s hands, inviting them to judge graduate student science fair posters. At the fair, graduate students practice communicating their research to a young audience, while children have the opportunity to see themselves as valued contributors in science. Here, we present a model for a walk-in Flipped Science Fair, designed in partnership between nine Virginia Tech graduate students and the Roanoke City Public Libraries (Roanoke, VA, USA). At our event, 27 graduate students presented posters about their research, with an audience of over 250 community members. We found that hosting the Flipped Science Fair at a public library lowered barriers to entry for participants and allowed us to reach an audience further from the university. While judging posters, children learned about a wide range of leading-edge research and had meaningful interactions with diverse scientists in small-group settings. Conversely, for graduate students, this event and associated training workshops provided an opportunity to practice communicating their research to a new audience. Throughout this article, we share our experience as graduate students collaboratively conceptualizing and organizing this community-oriented Flipped Science Fair with public library partners." Congratulations, Carla and co-authors!

Citation: Lewis, Abigail S. L., Grace O’Malley, Gates K. Palissery, Amanda Hensley, Carla López Lloreda, Claudia Perez, and Emma K. Bueren. 2023. “Flipped Science Fair Invites Children to Judge Graduate Student Posters Through a University-Community Partnership.” Journal of STEM Outreach 6 (1): 1–12. https://www.jstemoutreach.org/article/89291-flipped-science-fair-invites-children-to-judge-graduate-student-posters-through-a-university-community-partnership 

Stephen Plont, a recent PhD graduate from our lab, recently published a chapter from his dissertation on biogeochemistry at stream confluences in Water Resources Research. The paper was co-authored by Durelle Scott and Erin Hotchkiss. 

We found that:

• "Stream reaches downstream of confluences are geomorphically distinct from upstream reaches and have unique biogeochemical signatures"

• "Differences in upstream and tributary reach chemistry or drainage area do not explain non-conservative mixing of biologically reactive solutes at confluences"

• "Confluence geomorphic heterogeneity and changes to water residence time downstream may drive differences in biogeochemical processes in confluence mixing zones"

You can read more about our findings in the open access paper here and read highlights/key findings from Stephen here. Congratulations, Stephen!

Citation: Plont, S., Scott, D. T., & Hotchkiss, E. R. (2023). Biogeochemical processes are altered by non-conservative mixing at stream confluences. Water Resources Research, 59, e2022WR034224. https://doi.org/10.1029/2022WR034224

Erin Hotchkiss is co-author of an article, "Seasonality drives carbon emissions along a stream network" led by Hannah Conroy, a PhD candidate at the University of Washington collaborating on our NSF-funded Macrosystems project linking terrestrial and aquatic carbon fluxes and cycling. From the abstract: "Headwater stream networks contribute substantially to the global carbon dioxide terrestrial flux because of high turbulence and coupling with terrestrial environments. Heterogeneity within headwater stream networks, both spatially and temporally, makes measuring and upscaling these emissions challenging because measurements of carbon dioxide in streams are often limited to a few monitoring points. We modified a stream network model to reflect real measurements made under base flow and high flow conditions at Martha Creek in Stabler, WA in the US Pacific Northwest. We found that under high flow conditions, the stream network had much greater total carbon emissions than during low flow conditions (1.22 Mg C day−1 vs. 0.034 Mg C day−1). We attribute this increase to a larger overall stream network area (0.04 vs. 0.01 km2) and discharge (1.9 m3 s−1 vs. 0.005 m3 s−1) in November versus August. Our results demonstrate the need to understand the nonperennial stream reaches when calculating carbon emissions. We compared the stream network emissions with the terrestrial net ecosystem exchange (NEE) estimated by local eddy covariance measurements per watershed area (−5.5 Mg C day−1 in August and −2.2 Mg C day−1 in November). Daily stream emissions in November accounted for a much larger percentage of NEE than in August (54% vs. 0.62%). We concluded that the stream network can emit a large percentage of the forest NEE in the winter months, and annual estimates of stream network emissions must consider the flow regime throughout the year."

Citation: Conroy, H., E.R. Hotchkiss, K.M. Cawley, K. Goodman, R.O. Hall, J.B. Jones, W.M. Wollheim, & D. Butman. 2023. Seasonality Drives Carbon Emissions along a Stream Network. Journal of Geophysical Research – Biogeosciences 128: e2023JG007439. https://doi.org/10.1029/2023JG007439

Cleo Orlando, a Fralin Summer Undergraduate Research Fellowship (SURF) awardee conducting research with our lab group, presented a poster sharing results from "Exploring patterns in microbial activity across altered stream flowpaths" at Virginia Tech's Summer Undergraduate Research Conference. Cleo and mentor Katherine Pérez Rivera worked together to quantify spatiotemporal patterns in water quality and biogeochemical processes in the upper Stroubles Creek network that runs under and through Virginia Tech's campus, and includes the Duck Pond where Cleo focused her research on microbial carbon metabolism. Congratulations on a great poster presentation and thank you for a fun summer of research collaboration, Cleo!

Poster: Orlando, C., K.X. Pérez Rivera, & E.R. Hotchkiss. 2023. Exploring patterns in microbial activity across altered stream flowpaths. Virginia Tech Summer Undergraduate Research Conference, Blacksburg, VA, USA. 

Update (2023-09-18): Check out this VT news feature that highlights the experiences of the SURF researchers and mentors + a bonus photo of Cleo conducting stream sampling at the top of the article!  https://news.vt.edu/articles/2023/09/fralin-life-sciences-institure-surf-program.html 

General audience abstract from Stephen's dissertation submitted to his PhD committee:

"Streams receive and use materials from upstream and the surrounding landscape to fuel in-stream ecosystem processes (e.g. carbon and nutrient cycling). Understanding how these processes within a stream alter concentrations of carbon and other nutrients is needed to assess how the ecosystem is functioning and what the consequences are on water quality downstream. Further, predictions of how materials cycle and move at the scale of streams networks are derived from measurements at the stream reach scale. As a result, the impact of stream confluences (i.e., where two streams meet and mix) on in-stream carbon and nutrient cycling and the consequences on downstream water quality has been overlooked. In this dissertation, I seek to address the following questions: (1) How are carbon and nitrogen cycles linked in streams and how those links altered by land use? (2) How can rates of in-stream carbon cycling inform our understanding of the role of streams in landscape carbon budgets? (3) How are carbon and nutrient removal altered downstream of a stream confluence? (4) How do confluences alter water chemistry within a freshwater network? 

In Chapter 2, I showed that organic carbon and nitrate shared similar fates in streams across the United States. Organic carbon traveled longer distances before being respired in agricultural and urban streams compared to natively-vegetated streams, suggesting that human modifications to landscapes impact carbon cycling and transport in streams. In Chapter 3, I demonstrated how rates of in-stream organic carbon removal can be used to understand land-stream connections. In Chapter 4, I conducted whole-ecosystem experiments to assess how carbon and nutrient removal are altered by a confluence. I showed that carbon metabolism and phosphorus removal are suppressed downstream of a confluence and that rates of organic carbon removal are spatially variable throughout where water from the two streams are mixing. In Chapter 5, I examined potential drivers to explain patterns of organic matter and nutrient chemistry downstream of confluences throughout a stream network. Reaches downstream of confluences were physically, chemically, and biologically distinct from upstream reaches and differences in upstream and tributary reach chemistry or drainage area did not explain patterns of water chemistry downstream of confluences. 

Overall, my dissertation highlights the importance of processes within a stream in driving carbon and nutrient cycling, and how rates of whole-stream carbon cycling can be used to better understand connections between streams and the surrounding landscape. I investigate the role of stream confluences in determining the cycling and downstream fate of carbon and nutrients in freshwater networks. This work shows that ecosystem functioning and downstream water quality in freshwater networks are affected by processes occurring within streams and by the interfaces between streams and other ecosystems (e.g., land-water interfaces, stream confluences)."

General audience abstract from Kristen's dissertation submitted to her PhD committee:

"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."

Erin Hotchkiss is co-author of an article on "Variability and drivers of CO2, CH4, and N2O concentrations in streams across the United States" led by Amanda DelVecchia. From the abstract: "We present a first analysis of such a dataset collected by the National Ecological Observatory Network across 27 streams and rivers across ecoclimatic domains of the United States [and Puerto Rico]. ... CO2 and CH4 were strongly affected by physical drivers including mean air temperature and stream slope, as well as by dissolved oxygen and total nitrogen concentrations. N2O was exclusively correlated with total nitrogen concentrations. Results suggested that potential for gas exchange dominated patterns in gas concentrations at the site level, but contributions of in-stream aerobic and anaerobic metabolism, and groundwater also likely varied across sites. The highest gas concentrations as well as highest variability occurred in low-gradient, warmer, and nonperennial systems. These results are a first step in providing unprecedented, continuous estimates of GHG [greenhouse gas] flux constrained by temporally variable physical and biogeochemical drivers of GHG production."

Citation: DelVecchia, A.G., Rhea, S., Aho, K.S., Stanley, E.H., Hotchkiss, E.R., Carter, A. and Bernhardt, E.S. (2023), Variability and drivers of CO2, CO2, and N2O concentrations in streams across the United States. Limnology & Oceanography 68: 394-408. https://doi.org/10.1002/lno.12281 

Several folks in our lab group shared recent results from their dissertation research at this year's Biological Sciences Research Day, an annual event to celebrate graduate student research accomplishments in the Department of Biological Sciences at Virginia Tech. Great work, everyone!

• Kristen Bretz: How does stream intermittency affect carbon emissions and export? [poster]

• Katherine Pérez Rivera: Oxygen and temperature patterns along Stroubles Creek [poster]

• Stephen Plont: Tributary junction, what's your function? Stream confluences alter carbon and nutrient cycling in freshwater networks [poster]