Groundwater discharge of legacy nitrogen at the scale of river networks
Reactive nitrogen (N) applied to land surfaces infiltrates with precipitation and accumulates in aquifers, creating a source of legacy N that is later discharged from groundwater to surface waters. Groundwater transport times can be months, decades, or even centuries longer than surface water transport times, which causes substantial lags between when N is applied to land surfaces and when it actually enters surface waters. These focused groundwater discharges can obstruct water quality management strategies that are based on reducing present-day N applications. Yet, not all N that enters the groundwater system is delivered to surface waters. Some fraction of N is removed by microbial processes during transport along regional groundwater flow paths. At the end of long groundwater flow paths, streambank and streambed (i.e., stream interface) sediments can remove N at relatively high rates, further reducing N delivery to surface waters. This project, funded by NSF-Hydrologic Sciences with co-PIs Dr. Marty Briggs and Dr. Jeff Starn (USGS), integrates extensive field measurements across a river network with groundwater models to 1) characterize the spatial patterns of focused groundwater discharge at the river network scale, 2) quantify patterns and drivers of legacy N transport through and removal within stream interface sediments, 3) scale legacy N cycling to the river channel network. PhD student Eric Moore and MS students Kevin Jackson, Adam Haynes, and Alaina Bisson lead our research efforts on this project. You can see an overview and update of this project as part of the CLEAR webinar series, recorded here.
Biogeochemical cycles from stream reaches to river networks
River networks transport, transform, and retain carbon, nutrients, and other pollutants as water flows from terrestrial landscapes to lakes and oceans. Small stream reaches are the most intensely sampled flowing waters, and we work to understand how measurements made in small stream reaches can inform whole stream- and river- network function across landscapes. Here are examples of two recent projects under this topic:
How do stream ecosystems respond to land development and climate change? With this project, we are seeking to understand what drives patterns of stream metabolism within and across streams. This large, collaborative NSF Macrosystems Biology project includes nine PIs across six Universities (Emily Bernhardt (lead), Jim Heffernan, and Brian McGlynn, Duke University; Bill McDowell, University of New Hampshire; Bob Hall, University of Wyoming; Matt Cohen, University of Florida; Emily Stanley, University of Wisconsin; Nancy Grimm, Arizona State University) and intensive data collection in five biomes coupled with extensive existing datasets of stream ecosystem metabolism across the United States. At UConn, Postdoctoral Associate Lauren Koenig will continue developing and begin testing process-based models of stream metabolic responses to disturbance regimes at the site and watershed-scales. You can read more about StreamPULSE on the project's website.
Carbon Response to Experimental Warming in Streams (CREWS)
Our goal is to understand and predict the effects of warming and other anthropogenic stressors on stream carbon dynamics. This large project, funded by NSF, examines the effects of temperature on organic carbon processing in forest stream networks, using a multi-scale design that includes a paired-catchment whole-stream warming experiment, an array of warmed streamside channels, laboratory studies of aquatic microbes, and reach- and network-scale modeling. The fieldwork will take place at the Coweeta Hydrologic Laboratory in North Carolina. Collaborators include John Benstead (lead, University of Alabama), Amy Rosemond & Seth Wenger (University of Georgia), Erin Hotchkiss (Virginia Tech), and Vlad Gulis (Coastal Carolina). PhD student Danielle Hare leads UConn's efforts on this project.
Biogeochemical regime shifts in coastal wetland landscapes
Sea level rise, increased drought severity, and hurricane intensity are increasing the susceptibility of freshwater wetlands to saltwater incursion. At the same time, many freshwater wetlands experience large loads of nutrients and other contaminants from agricultural and/or urban land uses. We are interested in how the interactions of these two chemical fronts drive patterns of greenhouse gas emissions and nutrient export to sensitive coastal areas.
Here are examples of two recent projects under this topic:
How will sea level-rise driven shifts in wetland vegetation alter ecosystem services?
The overarching objective of this project is to quantify how shifts in vegetation may alter carbon and nitrogen cycling in Long Island Sound tidal wetlands and predict how those services change under sea-level rise scenarios. This project includes extensive field surveys of plant communities and soils, in situ "marsh organ" experiments, and spatiotemporal scaling of wetland ecosystem services in southern New England. Funded by the Long Island Sound Study, Helton is a co-PI with Beth Lawrence (PI, UConn NRE) and Chris Elphick (co-PI, UConn, EEB). In Helton's lab, former students Sean Ooi (MS 2019) and Kayleigh Granville (undergraduate Honor's 2019) collaborated on this project to quantify spatial and temporal patterns of salt marsh denitrification.
Urban runoff and saltwater intrusion in restored and unrestored tidal wetlands
Our goal is to quantify patterns of ecosystem function in natural and restored wetlands along the Connecticut coast, and to determine how sea level rise impacts the ecosystem function of wetlands in this highly urbanized landscape. We focus on the retention of nitrogen and common urban metal contaminants (Cu, Pb, Zn) as primary ecosystem functions that improve downstream water quality, and may be particularly sensitive to saltwater inundation in urban settings. Former MS student April Doroski is led this work, funded by Connecticut Sea Grant College Program (Tim Vadas, UConn ENVE, is co-PI) to quantify the effects of sea level rise on the nutrient and metal retention capacity of restored and natural wetlands on the densely populated coast of the Long Island Sound.