The movement of carbon (C) across land-water boundaries is a critical factor for metabolism in freshwater ecosystems and the net C balance of watersheds. Because streams are a primary interface between terrestrial and freshwater ecosystems, they are ideal test beds to advance understanding of land-water C transfers and meta-ecosystem ecology (i.e., the study of multiple ecosystems linked by energy and material transfers). Our research seeks to answer: (1) What is the magnitude and variability of land-to-water C transfers and stream C emissions? (2) How does meta-ecosystem C cycling vary within and among paired terrestrial-aquatic National Ecological Observatory Network (NEON) research sites across the United States? (3) What controls C form, cycling, and fate in meta-ecosystems? Our research will advance a predictive understanding of the daily, seasonal, and annual processes controlling watershed C cycling through measurements and models that cross land-water boundaries and approaches that integrate biology, geology, and chemistry across space and time. Collaborators: D Butman (University of Washington), W Wollheim (University of New Hampshire), J Jones (University of Alaska Fairbanks), K Cawley & K Goodman (NEON). "Collaborative Proposal: MRA: Linking land-to-water transport and stream carbon cycling to inform macrosystem carbon balance" - funded by NSF DEB. Funding period: 2020-2025.

Worldwide, low-lying areas once rich in forested wetlands have been converted to agricultural production after draining and filling. Prior to their loss, the wetlands reduced flooding through water storage, provided downstream environments with an important energy source in the form of dissolved organic carbon, and played a critical role in regional carbon budgets. This research will test how spatiotemporal changes in surface and subsurface hydrology govern carbon dynamics in wetland-rich landscapes. Using coupled empirical and modeling components, we will quantify: (1) dynamics of surface water connections and surface-subsurface exchange at wetland and catchment scales; and (2) consequent hydrologic influences on wetland- and catchment-scale carbon dynamics. The study sites are on the Delmarva Peninsula of Maryland. Our research will integrate hydrologic sciences, ecosystem ecology, biogeochemistry, and restoration science; and ultimately, help inform wetland restoration and land management across the coastal plain region. Collaborators: DL McLaughlin, ER Hotchkiss, DT Scott, K Wardinski, C López Lloreda, & N Corline (Virginia Tech); CN Jones (University of Alabama); MA Palmer, J Maze, M Williams, & M Gonsior (University of Maryland). "Collaborative Research - Hydrologic Connectivity and Water Storage as Drivers of Carbon Export and Emissions from Wetland-Dominated Catchments" - funded by NSF DEB. Funding period: 2019-2025.

Watersheds are complex patchworks of different land uses, ecosystem types, and resulting biogeochemical functions. Oak Ridge National Laboratory's Watershed Dynamics and Evolution Science Focus Area (WaDE SFA) is leveraging the ongoing, rapid development of watersheds in the southeastern United States to test our understanding of how watershed and freshwater ecosystem functions respond to land cover and climate change.  The WaDE SFA collaboration will link hill-slope and stream reach processes with river network signals and earth system models to develop a "predictive understanding of the hydro-biogeochemical processes and feedbacks that control solute mobilization and export from headwater catchments with heterogeneous land cover (Theme 1), resultant feedbacks between flow, solute concentrations, and stream function in stream corridors (Theme 2), and the emergent patterns in stream metabolism at network scales (Theme 3)." 

ER Hotchkiss and M Beall are External Collaborators on Theme 2 of the WaDE SFA Project (current funding support: 2024-2027).

Increased salt loading to inland waters is a "wicked environmental problem" with unknown consequences for freshwater ecosystem function. We are involved with collaborations focused on different aspects of freshwater salinization impacts and solutions, including two NSF-funded collaborations:

• Carbon cycling and food web energy transfer in salinized headwater streams. Collaborators: SA Entrekin, DL McLaughlin, SH Schoenholtz, C Zipper (Virginia Tech) & T Brown (UVA-Wise). Funded by Virginia Tech's Global Change Center & NSF DEB. NSF funding period: 2023-2026.

• Common Pool Resource Theory as a Scalable Framework for Catalyzing Stakeholder-Driven Solutions to the Freshwater Salinization Syndrome. Collaborators: SB Grant, T Schenk, MA Rippy, & M Edwards (Virginia Tech); TA Birkland (North Carolina State University); S Kaushal (University of Maryland College Park); & J Gomez-Velez (Vanderbilt). Funded by NSF GCR. Funding period: 2020-2025. Project Website.

Past salinization-related collaborations include:

• Salt dilution and flushing during high flow events in a stream draining a mixed-use landscape. Collaborators: V Lakoba, L Wind, SE DeVilbiss, M Lofton, K Bretz, A Weinheimer, C Moore, C Baciocco, & WC Hession (Virginia Tech).

• Subsidy-stress response of bacterial respiration and nutrient uptake to freshwater salinization. Collaborators: SE DeVilbiss, MK Steele, & BD Badgley (Virginia Tech).

While flow discontinuities within river networks (e.g., wetlands, reservoirs, confluences, intermittent surface waters) are ubiquitous, we rarely account for ecosystem heterogeneity in network-scale measurements or simulations of water quality and ecosystem processes (e.g., carbon metabolism, nutrient uptake). Indeed, much of our current understanding of freshwater ecosystem function is still informed by predicted physical, chemical, and biological shifts from upstream to downstream ecosystems along a simplified stream-river continuum (i.e., the river continuum concept). Ongoing research is measuring biogeochemical consequences of discontinuities within river networks, including work led by graduate students working with me on NSF-funded projects who have developed dissertation research chapters addressing discontinuity knowledge gaps in addition to the core objectives of our NSF collaborations. A working group funded in 2019 is interested in "Ecosystem Mosaics and Broad-Scale Biogeochemistry".  Collaborators K Bretz, S Plont, KX Pérez Rivera (Virginia Tech), AM Helton (University of Connecticut), CT Solomon (Cary IES), SE Jones (Notre Dame), and others.

Time series of water chemistry, climate, hydrology, and ecosystem processes (carbon metabolism, nutrient cycling, food web dynamics) can identify changing patterns and controls of carbon and nutrient variability, transport, and fates within and among ecosystems. Considering the spatial and temporal dynamics of linked nutrient, carbon, and water cycles will help us better understand how the efficiency of ecosystem- and catchment-scale carbon and nutrient transformations may respond to environmental change. Ongoing research with KX Pérez Rivera (Virginia Tech). Additional collaborations with members of the Stream Resiliency RCN Working Group, Heterotrophic Regimes working group, and others.

Widespread changes in land use, climate, hydrology, and species composition are rapidly altering the structure and function of ecosystems. Past and ongoing research collaborations related to environmental change, in addition to ongoing research collaborations listed above, include: 

• non-native beavers and stream metabolism (e.g., García et al. 2022)

• the science and policy of biological invasions (e.g., Barney et al. 2019)

• grand challenges in freshwater science, policy, & management (e.g., O'Reilly et al. 2023, Sadro et al. 2024)

• using experimental ponds and whole-lake manipulations to identify how changes in temperature, ice cover, nutrients, organic carbon, and/or fish harvest alter ecosystem productivity, food web dynamics, and carbon cycling (e.g., Jonsson et al. 2015, Hamdan et al. 2018, Hamdan et al. 2021).

See past research for other examples of environmental change research (e.g., stream metabolic responses to experimental warming, ecological consequences of non-native species).