​I'm an ecosystem ecologist studying the movement and transformations of nutrients and carbon. These processes have implications ranging from water quality to fish production to greenhouse gas emissions. In my work, I combine concepts and approaches from multiple disciplines to understand the complex interactions among the biological, chemical, and hydrological processes that influence aquatic ecosystems. My past and present research asks how the supply of organic matter (OM) to streams and rivers affects nutrient cycling and is affected by urbanization. As I continue to build my research program, I will ask about the biogeochemical consequences of water management through studies that connect water management and ecology. I hope to collaborate with other biologists and ecologists to link the diversity of traits and organisms to ecosystem function and with social scientists to study feedbacks between ecology and human institutions. In doing so, I will broaden the impact of my research and expand opportunities for student involvement in my research program.

Postdoc research: DOM effects on benthic algae and nitrogen cycling in northern lakes

How do light limitation and nutrient delivery by DOM interact to modulate N cycling and primary production?

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, I am asking how differences in water column DOM concentrations among arctic and boreal lakes in northern Sweden affect benthic 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.

PhD research: Stormwater and Organic Matter in the Urban Stream Continuum

How do processing, storage, and transport of organic matter in upland urban flowpaths determine DOM delivery to urban streams?

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.
​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, M.L., J.R. Blaszczak, J.M. Delesantro, and J.B. Heffernan. 2018. Engineered headwaters can act as sources of dissolved organic matter and nitrogen to urban stream networks. Limnology and Oceanography Letters. 
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Below: A series of water samples collected from an urban stream in Durham, North Carolina during a storm on Nov. 17, 2014. Above: Dissolved organic carbon (DOC) concentrations in the same water samples over the course of the storm, showing the peak of DOC concentration leading peak water level.
How do location, transport, and processing affect the lability and composition of DOM in urban streams? 
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Urbanization of catchments 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.

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, pairing these measurements with a bioavailability incubation to link composition to mineralization rates and to observe changes in the composition of urban DOM associated with microbial processing.

So far, I've found that DOM 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. Furthermore, the DOM pool from across the urban headwaters grows more homogenous as microbial processing proceeds.
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A research buoy collects temperature and conductivity data at the Fourth Creek cove of the Ft. Loudon reservoir in Knoxville, TN.
How do pulses of stormwater affect receiving reservoirs?
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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'm working to understand how pulses of stormwater affect the reservoirs into which they flow.  Our study compares 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.

Master's Research:

Blackwater and Spring rivers in northern Florida
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.

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.
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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, M.L. and J.B. Heffernan. 2014. Direct and indirect effects of organic matter source and concentration on denitrification in Florida rivers. Ecosystems 17: 14-28. doi: 10.1007/s10021-013-9705-9 
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