Our new Ecological Monograph on diel variation of phosphorus in the Ichetucknee River is available online as an unformatted pre-print. I am really excited about this paper, which was led by Matt Cohen at the University of Florida. Basically this paper extends our previous research on diel variation in NO3 to estimate P uptake by vegetation in a spring-fed river, but with several interesting wrinkles. First, to understand how biotic processes influence diel variation in P, we had to correct for the precipitation of calcite in the water column, because P binds strongly to it. Second, the P budget for the Ichetucknee River suggests that there are unaccounted inputs, which is consistent with what we have found based on other elements. Third, the diel variation of P is out of phase with that of O2 and NO3, which may represent differential timing of the cellular processes that metabolize these different elements. Nonetheless, the stoichiometry of C,N,and P uptake that we estimate based on diel variation matches pretty well with the stoichiometry of the dominant plants in the river. With all these different as
Our paper on denitrification in the Floridan Aquifer was just published in the open access journal Biogeosciences. Co-authors include Andrea Albertin and Matt Cohen at the University of Florida, Brian Katz at the US Geological Survey, and Megan Fork, currently finishing her MS at Florida International University and soon to begin her PhD in the lab.
This paper is the first to provide direct measurements of denitrification over the scale of an entire regional aquifer. To achieve this, we added a large amount of existing data (mostly from co-author Brian Katz) to our own sampling of the Floridan Aquifer springs, and developed new ways to use noble gases to develop null predictions of N2 gas concentrations. Basically, we use argon and neon to estimate how much N2 should be in the water, then calculate denitrification (which produces N2 gas) by difference.
This paper also demonstrates the large effect of denitrification on groundwater N isotopes. One reason we care about this is that we use isotopes to identify sources of N: synthetic fertilizers have low or 'light' isotope signatures, while organic waste (from urban and animal husbandry sources) are 'heavy'. , we have to account for these effects or we will get the source wrong.
You can read the Duke press release here, and get the paper here. You can learn more about our other springs research here.
Our paper describing diel and longitudinal patterns of nitrogen isotopes in the Ichetucknee River was just published in the Journal of Geophysical Research - Biogeosciences. You can get it here.
Building on our previous studies of diel variation in nutrient chemistry, we found large variation in N isotopes in the Ichetucknee River, FL. Unfortunately, it appears that uptake by plants (autotrophic assimilation) and denitrifying microbes have similar effects in this system, so we were not able to discriminate between these pathways. On the plus side, we observed some novel behaviors such as diel hysteresis, and found evidence that denitrification varies over the course of the day. Check it out!
This is the homepage of the Heffernan Lab at Duke University. Here you can find all sorts of information about our research, teaching, and outreach. If you have any questions, contact Dr. Heffernan.
Dr. Jim Heffernan
I am an Assistant Professor in the Nicholas School of the Environment at Duke University. My research is focused on the causes and consequences of major changes in ecosystem structure, mostly in streams and wetlands.
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