Ecology and biogeochemistry of aquatic ecosystems in the anthropocene
The direct and indirect results of human activities such as climate change, urbanization, and construction/alteration of water bodies influence the streams, rivers, lakes, and reservoirs in every corner of the world. My work focuses on understanding how these anthropogenic drivers play out in the the movement and transformations of nutrients, carbon, and water in these diverse aquatic ecosystems. I combine concepts and approaches from multiple disciplines to understand the complex interactions among the biological, chemical, and hydrological processes that influence these systems.
Effects of DOM on benthic algae in northern lakes
A nutrient diffusing substrate deployment at 1 m depth in a lake in boreal northern Sweden.
How do light limitation and nutrient delivery by DOM interact to modulate the growth and function of benthic biofilms in lakes?
My current research contributes to a large, multidisciplinary research project with the aim of assessing and understanding the resilience and vulnerability of northern lakes to climate change. As a postdoctoral researcher at the Climate Impacts Research Center at Umeå University, I am asking how differences in water column DOM concentrations among arctic and boreal lakes in northern Sweden affect benthic communities' growth and function, including primary production and N cycling. We know that below a threshold of about 5 mg/L, DOM appears to have a fertilization effect for pelagic primary producers in these N-limited lakes, but at concentrations greater than 5 mg/L its absorption of light reverses this relationship (Seekell et al. 2015). My work will investigate how water column DOM in northern Swedish lakes affects the growth of benthic algae and the sources of N (mineralization of organic N vs. N2-fixation) they may use. As DOM concentrations in lakes throughout the far north increase (Monteith et al. 2007), this will contribute to better understanding and predictions for C and N cycles in these widespread ecosystems and how they may be altered over the next century of global change.
My current research contributes to a large, multidisciplinary research project with the aim of assessing and understanding the resilience and vulnerability of northern lakes to climate change. As a postdoctoral researcher at the Climate Impacts Research Center at Umeå University, I am asking how differences in water column DOM concentrations among arctic and boreal lakes in northern Sweden affect benthic communities' growth and function, including primary production and N cycling. We know that below a threshold of about 5 mg/L, DOM appears to have a fertilization effect for pelagic primary producers in these N-limited lakes, but at concentrations greater than 5 mg/L its absorption of light reverses this relationship (Seekell et al. 2015). My work will investigate how water column DOM in northern Swedish lakes affects the growth of benthic algae and the sources of N (mineralization of organic N vs. N2-fixation) they may use. As DOM concentrations in lakes throughout the far north increase (Monteith et al. 2007), this will contribute to better understanding and predictions for C and N cycles in these widespread ecosystems and how they may be altered over the next century of global change.
Lotic-lentic transitions in aquatic systems
Many aquatic ecosystems defy simple categorization as flowing (lotic) or non-flowing (lentic). Changes in the relative water surface elevation between inflowing and receiving water bodies can result in oscillations between lentic and lotic conditions, with implications for the physical, chemical, and biological conditions of the system.
 A research buoy collects temperature and conductivity data at the Fourth Creek cove of the Ft. Loudon reservoir in Knoxville, TN.
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How do pulses of stormwater affect receiving reservoirs?
Many streams, especially in populated areas, eventually flow into some kind of impoundment. This means that pulses of stormwater travel quickly through flashy urban streams and ultimately into a natural or man-made lake. How do these flashy, pulsed inputs of stormwater affect the environment of the lakes that receive them? In collaboration with Oak Ridge National Lab, and supported by a GO! Fellowship, I've been working to understand how pulses of stormwater affect the reservoirs into which they flow. Our study compared three arms of reservoirs in eastern Tennessee that span a range of development intensity and will examine the effects that storm pulses have on the physical structure and chemical environment of these reservoir coves. |
Stormwater and organic matter in the urban stream continuum
Urbanization results in changes in the sources of organic matter and the processes that transport DOM from the landscape to an urban stream. Together, these changes mean differences in the timing and composition of DOM in urban as compared to rural streams, with consequences for the ecological effects of urban DOM.
In urban catchments, designed and engineered structures like roofs, gutters, streets, and pipes are the ephemeral flowpaths that move runoff from the landscape to the stream. Roofs, roadside gutters, and catch basins store and process organic matter between storms. During storms, these engineered headwaters can be rapidly connected and deliver dissolved organic matter (DOM) and other accumulated biogeochemical products to urban streams and downstream aquatic ecosystems.
In urban catchments, designed and engineered structures like roofs, gutters, streets, and pipes are the ephemeral flowpaths that move runoff from the landscape to the stream. Roofs, roadside gutters, and catch basins store and process organic matter between storms. During storms, these engineered headwaters can be rapidly connected and deliver dissolved organic matter (DOM) and other accumulated biogeochemical products to urban streams and downstream aquatic ecosystems.
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How do processing, storage, and transport of organic matter in upland urban flowpaths determine DOM delivery to urban streams?
Together with Duke Ecology alum Joanna Blaszczak, UNC PhD student Joseph Delesantro, and the Duke River Center's Jim Heffernan, I assessed the role of the engineered headwaters as proximate sources of DOM and nitrogen to urban streams during storms. We measured the annual carbon budget for roofs, conceptualizing them as small catchments nested within the catchment of an urban headwater stream. To do this, we measured rates of leaf litter input to roofs and fluxes of C out through downspouts and decomposition. We found that roofs transform nearly one third of the leaf litter they receive into DOM that leaves through the downspout, and that scaling these fluxes of DOM from roofs and catch basins can account for a large proportion of the DOM in stormflow in the stream. Fork et al. 2018. Limnology and Oceanography Letters. |
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How does biodegradation shape the DOM that is generated in urban headwaters?
The engineered headwaters of urban streams can give us a uniquely accessible view into DOM generation at the aqua-terrestrial interface, since we know exactly where flowpaths will activate when the rain comes, and the organic matter that will be in contact with these ephemeral flows.
We measured how the amount and composition in leachates of potential DOM sources and headwater flows changed with biodegradation, and found that source leachates undergo a period of transient increase in the diversity of DOM with little change in DOC concentration. In other words, leachate DOM is "reorganized" through the production of transient degradation by-products before organic matter is removed from the dissolve pool. On the other hand, the DOM that flows along urban channels shows a continuous directional change in composition associated with the continuous removal of labile DOM over the course of our incubation experiment.
The engineered headwaters of urban streams can give us a uniquely accessible view into DOM generation at the aqua-terrestrial interface, since we know exactly where flowpaths will activate when the rain comes, and the organic matter that will be in contact with these ephemeral flows.
We measured how the amount and composition in leachates of potential DOM sources and headwater flows changed with biodegradation, and found that source leachates undergo a period of transient increase in the diversity of DOM with little change in DOC concentration. In other words, leachate DOM is "reorganized" through the production of transient degradation by-products before organic matter is removed from the dissolve pool. On the other hand, the DOM that flows along urban channels shows a continuous directional change in composition associated with the continuous removal of labile DOM over the course of our incubation experiment.
Dissolved organic carbon (DOC) concentrations in water samples (below) over the course of the storm, showing the peak of DOC concentration leading peak water level.
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How does urbanization affect the composition, concentration, and timing of DOM in urban streams?
I have paired measurements of concentration, composition (using various indices based on optical measures and EEM-PARAFAC approaches) to characterize differences in DOM from stormflow across the engineered headwaters of an urban stream. DOM in the catchment we studied shows "first flush" behavior, with the peak of DOM concentration occurring before peak water level. In addition, the composition of the DOM in the first flush seems to be markedly different (smaller, more aliphatic molecules) from the DOM in the stream at the end of the storm and at baseflow. |
Time series of water color over the course of a storm (sampled every 5 min) in an urban headwater stream in Durham, NC.
Direct and indirect effects of water column DOM on N biogeochemistry in river sediments
On the left: The blackwater Withlacoochee River has dark-colored water resulting from high concentrations of terrestrially-derived DOC that limits the penetration of light in the water column. On the right: The extremely clear water of spring-fed rivers like the Rainbow River allows highly productive, dense plant communities to grow on the river bottom.
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How does water column dissolved organic matter affect riverine nitrogen cycling?
In my master's research at Florida International University, I asked how the concentration of dissolved organic matter (DOM) in a river's water column affects denitrification in the benthic sediments. Denitrification is a microbial process that requires organic matter as a substrate in order to reduce nitrate into a gaseous form (N2O or N2). One might expect that DOM could act as fuel for denitrifying bacteria, but I found that denitrification tends to be carbon limited in rivers with higher concentrations of DOM. Not all DOM is created equal - my research suggests that denitrification in high DOM rivers may be carbon-limited because light absorption by DOM prevents the growth of benthic plants in these rivers and thereby limits labile carbon to the benthic sediments in the form of root exudates. Fork & Heffernan. 2014. Ecosystems |
