The first chapter Dina Liebowitz's dissertation work is now available online in Freshwater Biology.  This paper examines the relationships among environmental conditions (especially dissolved oxygen concentrations), the abundance of grazing snails, and the presence of benthic algae in Florida springs. By comparing data from field surveys both within and across springs, Dina showed that the abundance of snails (especially the genus Elimia) is the primary determinant of the abundance of algae in Florida springs.   In addition, we found that algal abundance is generally either very high or very low, a pattern that is consistent with alternative stable states.  Finally, our field survey also shows that oxygen concentrations, among other environmental controls, influences the abundance of snails.  Putting these findings together, we suggest that even temporary declines in oxygen concentrations, which occur commonly in springs, could suppress snail populations and activity and allow algae to escape grazer control.   

Most of the management attention in springs has emphasized nutrient concentrations as the major driver of algal mats, a hypothesis which we have challenged pretty strongly in an earlier paper that first articulated the oxygen-grazer-algae cascade. Dina's other chapters examine these processes experimentally, and lead to similar conclusions.  Very exciting to have this work out, and congratulations Dina!

Nutrient limitation happens when nutrient supply is insufficient to meet demand by biota, but most assessments of nutrient limitation are based only on concentration.  In streams, where metabolism is highly variable and streamflow continuously replenishes nutrient supply, concentration has been a poor predictor of nutrient limitation and autotroph biomass. We have a paper out in Freshwater Science that explores new predictors of nutrient limitation, based on nutrient spiraling theory and empirical measurements of metabolism and nutrient flux. The paper was led by Sean King, who finished his PhD at the University of Florida a few years ago, and is now at the Southwest Florida Water Management District (SWFWMD).  Congratulations Sean!

Megan Fork's paper on DOC and denitrification, published in Ecosystems, is now available online.  This work, which comes from Megan's MS thesis, uses a natural gradient in dissolved organic matter (DOC) concentration to understand how terrestrially-derived DOC influences aquatic denitrification.  We found that this DOC does not directly stimulate or inhibit denitrification, but that denitrification becomes more limited by DOC as DOC concentrations increase.  The explanation for this counter-intuitive finding is that DOC reduces light, and therefore the release of labile DOC by macrophytes and algae.  This paper provides additional evidence for the importance of in-stream primary production for denitrification in larger rivers, and has implications for how aquatic ecosystems may respond to future changes in DOC.  Read the whole thing here, and congratulations Megan! 

A cool set of underwater photographs of rivers from National Geographic.  One of our study sites (Blue Hole Spring in the Ichetucknee River, FL) is included.

I will note that Blue Hole has fairly high nitrate concentrations, despite it's otherwise excellent condition.  Which is an example of why we think that nitrate contamination may not be the whole story in Florida Springs.

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

Megan Fork successfully defended her MS thesis on June 12, 2012.  Megan's thesis, 'Direct and indirect effects of organic matter sources on denitrification in Florida Rivers,' uses natural variation in dissolved organic matter (DOM) to assess the roles of terrestrial and aquatic sources of organic matter as fuels for denitrification.  Her research shows that, as terrestrial organic matter becomes more abundant, organic matter actually becomes more limiting to denitrification.  This counter-intuitive result reflects two features of terrestrial OM: first, the complex organic molecules that make it up are not very bioavailable.  Second, the tea color of complex OM reduces light and therefore primary production within the river.  Our previous work in Florida springs has shown that this aquatic productivity fuels a lot of denitrification, so by reducing aquatic productivity, terrestrial OM reduces the amount of bioavailable OM that can drive denitrification.  

Megan will officially graduate from FIU's Department of Biological Sciences at the end of the summer, but she is already in Durham looking for another exciting questions to serve as a dissertation topic.  Congratulations Megan!

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.