Our research focuses on the ecology and biogeochemistry of watersheds and aquatic ecosystems, primarily through long-term studies.
We seek to answer two overarching questions: (1) how do human interactions with land use, climate, and watershed geology transform the chemistry of inland waters; and (2) how can we evaluate and enhance watershed restoration strategies to reduce water pollution?
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We collect, filter, preserve, and archive water samples from long-term monitoring sites. Samples are analyzed for biogenic elements such as carbon, nitrogen, sulfur, phosphorus, calcium, magnesium, potassium, silicon, iron, and manganese. Analysis of stable isotopes of carbon and nitrogen provide important clues regarding identification of the sources, fluxes, and transformations of chemicals in natural environments through space and time.
In the field, we measure in situ rates of microbial and ecosystem processes. We also analyze sensor data for complementary measurements of oxygen availability, temperature, conductivity, and other water quality parameters.
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In the future, we plan to build on current projects exploring the effects of the Freshwater Salinization Syndrome and human-accelerated weathering on the chemistry of inland waters; expand our studies of the interactive effects of land use, climate, and geologic change; participate in long-term research networks; and synthesize data at regional and national scales through continued service in working groups and panels.
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Our work has been supported by the National Science Foundation, the National Aeronautics and Space Administration, the Environmental Protection Agency, Maryland Sea Grant, the Chesapeake Bay Trust, and other funding sources.
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Current work in our group focuses on four primary areas:
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1. Freshwater salinization syndrome, warming rivers, and human-accelerated weathering. Our research has shown that there have been long-term increasing trends in salinization, warming, and human-accelerated weathering and alkalinization of fresh water across North America. We coined the term "human-accelerated weathering" to describe how human activities have altered weathering rates and weathering products in rivers. We then coined the term "Freshwater Salinization Syndrome" to conceptualize the suite of synergistic effects that freshwater salinization has on water quality and multiple biogeochemical cycles. Elevated major ion concentrations have important consequences for drinking water quality, corrosion of infrastructure, contaminant mobilization, and the regulation of ecosystem processes. Our ongoing work on long-term changes in the chemistry of inland waters has reached a wide audience and has stimulated discussions regarding salt pollution, which is currently unregulated in many regions of the world.
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2. The urban watershed continuum, urban karst, and urban evolution. Typically, watersheds are defined along topographic boundaries; however, we have shown that due to the expanded role of infrastructure in the urban water cycle, urban watersheds have much more dynamic boundaries across space and time that can be studied through the "urban watershed continuum" concept. We also expanded the definition of "urban karst" to include the widespread, easily weathered impervious surfaces in urban watersheds, which can influence pH, buffering capacity, toxicity of metals, and water quality. Typically, geologic materials and geologic processes are underappreciated in many ecosystem studies, even though weathering plays an important role in regulating soil and water chemistry. We also developed a concept of urban evolution, where geological, biological, and chemical cycles evolve over time in the built environment based on adapting infrastructure and management decisions.
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3. Watershed 'pulses' in chemical transport and transformation due to human-modified land use and climate. Many streams and rivers now receive large loads of carbon and nutrients from human-dominated land use. Our work has shown that streams and rovers shift between chemical "transporters" that reflect anthropogenic changes in the landscape and atmospheric deposition, to chemical "transformers" that actively change the forms and ecological effects of elements delivered further downstream. Dynamic interactions between the natural and human components of the urbanizing landscape amplify storage and transport of various chemicals in watersheds, including greenhouse gases. We are expanding global comparisons of nutrient transformations and the role of climate variability as a regulator in streams and rivers, as well as studying pulses of organic contaminants to drinking water supplies.
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4. Evaluating watershed restoration strategies. While nonpoint source pollution has increased over decades, the capacity of many suburban and urban streams to retain nutrients has been greatly impaired due to channel degradation from increased runoff and decreased hydrologic connectivity between streams and floodplain wetlands. We have been investigating stream restoration strategies to reduce river nutrient loads through empirical studies evaluating the effects of restoration on denitrification rates. We have also conducted a global review of stream restoration studies (Newcomer Johnson et al., 2016) and a series of studies investigating the potential for coupling stormwater management and stream restoration approaches. We have investigated the impacts of different geological substrates used in stream restoration on water quality (Duan et al., 2018) and have worked with other scientists and watershed managers to draft protocols for guiding stream restoration strategies to reduce nutrients in the Chesapeake Bay watershed. ​​​