5/18/2015  |   15:30 - 15:45   |  101B

USING MIMS TO MEASURE RIVERINE SEDIMENT, WATER-COLUMN, AND OPEN-CHANNEL DENITRIFICATION Denitrification permanently removes nitrogen (N) from aquatic ecosystems, but is difficult to quantify. Commonly-used methods either introduce artificial conditions or can be prohibitively expensive. In particular, accurate riverine estimates are lacking. We used membrane-inlet mass spectrometry (MIMS), mesocosm incubations, and a new modeling approach to partition net N2-flux in sediments and the water-column of 5 Midwestern rivers spanning a nitrate-N gradient. Sediment denitrification ranged from 0.6-1.8 mgN/m2/h, and water-column denitrification ranged from 0-5.2 mgN/m2/h, while one site exhibited net N-fixation. The importance of water-column denitrification relative to the sediment decreased with human land-use. We also estimated whole-river denitrification at one river using a diel, open-channel approach based on whole-stream metabolism methods. Whole-river denitrification was 14.7 mgN/m2/h (95% credible interval =10.0-19.6 mgN/m2/h), which is higher than combined mesocosm estimates from the same river, and LINXII 15N-tracer estimates from headwater streams. This open-channel modeling approach provides a novel, integrative method to estimate riverine denitrification rates. Our denitrification rates, measured across riverine habitats and at the whole-river scale, suggest that rivers remove N at rates comparable to, or even higher than, headwater streams.

First Last (Primary Presenter/Author), Cary Institute of Ecosystem Studies, reisingera@gmail.com;


Jennifer L. Tank (Co-Presenter/Co-Author), University of Notre Dame, tank.1@nd.edu;


Timothy Hoellein (Co-Presenter/Co-Author), Loyola University Chicago, thoellein@luc.edu;
Dr. Hoellein is a freshwater ecologist at Loyola University Chicago. His research interests are focused on ecosystem processes and biogeochemistry. His research lab explores these areas in associate with the movement and biological transformation of elements, energy, and pollution in aquatic ecosystems.

Robert O. Hall (Co-Presenter/Co-Author), Flathead Lake Biological Station, University of Montana, bob.hall@flbs.umt.edu;

5/18/2015  |   15:45 - 16:00   |  101B

TEMPORAL VARIATION OF AMMONIA UPTAKE IN A TROPICAL STREAM We conducted a series of slug nutrient additions in a Brazilian tropical stream to examine how ambient uptake rates and uptake kinetics change over time and in response to phosphorus amendments. TASCC style nutrient additions were conducted four times from June 2013 to November 2014. Each time, slug additions a conservative tracer (NaCl) combined with ammonium and ammonium + phosphate were conducted in the same 307m reach of a pristine stream (Barra Pequena) in Ilha Grande, Brazil. Additions were conducted over sequential days to minimize the influence of changing hydrological conditions. Ambient estimates of ammonium uptake lengths (Sw) and uptake rates (U) varied across time (244-720m and 0.30-0.72 mg N m-2 hr-1, respectively). Ambient Sw and U estimates varied relatively little between additions of ammonium alone and when ammonium was added with phosphorus additions. Similarly, the response of uptake rate to changing concentrations varied through time but was relatively insensitive to P addition. These results support results from nutrient diffusing substrates experiments that this stream remains N limited throughout the year, but document temporal variations in N uptake and retention.

Flavia Tromboni (Primary Presenter/Author), University of Nevada, Reno, ftromboni@unr.edu;


Eugenia Zandona (Co-Presenter/Co-Author), Universidade do Estado do Rio de Janeiro, eugenia.zandona@gmail.com;


Christine Lourenço-Amorim (Co-Presenter/Co-Author), Universidade do Estado do Rio de Janeiro, chris.lourenco@ig.com.br;


Vinicius Neres-Lima (Co-Presenter/Co-Author), Universidade do Estado do Rio de Janeiro, vinicius.lima.eco@gmail.com;


Eduardo F. Silva-Júnior (Co-Presenter/Co-Author), Universidade do Estado do Rio de Janeiro, eduardobioadventure@gmail.com ;


Rafael Feijó de Lima (Co-Presenter/Co-Author), University of Montana, rfeijod@clemson.edu;


Timothy P. Moulton (Co-Presenter/Co-Author), Universidade do Estado do Rio de Janeiro, moulton.timothy@gmail.com;


Steven Thomas (Co-Presenter/Co-Author), University of Alabama, sathomas16@ua.edu;

5/18/2015  |   16:00 - 16:15   |  101B

WHAT WE CAN AND CANNOT LEARN FROM SLUG ADDITIONS OF NUTRIENTS Researchers are increasingly using slug approaches to characterize nutrient uptake in streams. Increased application of slug techniques, as opposed to plateau methods, stems directly from the publication of the TASCC method (Covino et al. 2010). We conducted more than 50 of these additions across a broad geographic range using various nutrients and experimental designs. We use these results to provide a critical review of this method with respect to its ability to estimate uptake characteristics under ambient conditions and to characterize the relationship between uptake and nutrient concentration. TASCC results often identify differential behavior on the rising and falling limb of the relationship between enrichment concentration and estimates of uptake length (Sw) and uptake rate (U). We argue that hysteresis in these relationship reflects the relationship between U and concentration and the influence of differential uptake rates occurring along individual flowpaths. We use a combination of empirical and modeling approaches to describe these relationships, categorize and interpret different hysteresis patterns, and to provide overall recommendations for how to interpret these results.

Steven Thomas (Primary Presenter/Author), University of Alabama, sathomas16@ua.edu;


Flavia Tromboni (Co-Presenter/Co-Author), University of Nevada, Reno, ftromboni@unr.edu;


Brady Kohler (Co-Presenter/Co-Author), University of Wyoming, kohlerbrady@gmail.com;


Keeley MacNeill (Co-Presenter/Co-Author), Oregon State University, keeleymacneill@gmail.com;


Eugenia Zandona (Co-Presenter/Co-Author), Universidade do Estado do Rio de Janeiro, eugenia.zandona@gmail.com;

5/18/2015  |   16:15 - 16:30   |  101B

AMMONIUM UPTAKE KINETICS AND NITRIFICATION IN STREAMS IN ROCKY MOUNTAIN NATIONAL PARK We conducted pulse NH₄⁺ additions to investigate the kinetics and fate of NH₄⁺ in 3 sub-alpine streams in Rocky Mountain National Park. Mean ambient NH₄⁺ concentrations of the streams ranged from 0.4 to 3.5 µg L^-1 NH₄⁺ -N. Mass removal of NH₄⁺ ranged from 38-65 % and 6-33 % was immediately nitrified. We estimated dynamic uptake metrics using the TASCC method. Observed functional relationships of biological uptake with NH₄⁺ concentration were consistent within streams, though not across streams. The relationship between V_f and NH₄⁺ concentration was best described by a efficiency loss model for two streams, in which the log V_f decreased with log NH₄⁺ concentration with a slope <1. Uptake increased with NH₄⁺ concentration in one stream. A Michaelis-Menten saturation model fit our data poorly and a global half saturation constant of 800 µg L^-1 NH₄⁺ -N far exceeded our highest concentration of 120 µg L^-1 NH₄⁺ -N. A lack of saturation and varied uptake kinetics across streams indicate variation in biological processes not easily characterized by M-M kinetics.

Natalie Day (Primary Presenter/Author), University of Wyoming, nataliekateday@gmail.com;


Robert O. Hall (Co-Presenter/Co-Author), Flathead Lake Biological Station, University of Montana, bob.hall@flbs.umt.edu;

5/18/2015  |   16:30 - 16:45   |  101B

UNDERSTANDING SEDIMENT PHOSPHATE RELEASE UNDER ANOXIC CONDITIONS IN GREEN BAY, LAKE MICHIGAN Green Bay, Lake Michigan is a freshwater estuary that experiences seasonal hypoxia in the southern portion due to a number of reasons, including excess phosphorus inputs resulting in high algal production. The relationship between oxygen and phosphorus in the benthic region of Green Bay, necessary for understanding hypoxia, is not well understood for a number of reasons. One is that sediment oxygen uptake (SOU) is difficult to quantify and highly variable, both spatially and temporally. Second, phosphate release from the sediment has recently been examined in Green Bay sediments during core incubations. This release, which occurs when dissolved oxygen above the sediment is depleted, can spike local phosphate release rates from 6.7 µmol m-2 hr-1 to more than 120 µmol m-2 hr-1 under sustained anoxia. We observed continuous phosphorus concentration increases for 2-3 days, until the experiment was terminated. Our goal is to develop a simple oxygen-phosphorus model for lower Green Bay, crucial for comprehensive understanding of this hypereutrophic system.

Shelby LaBuhn (Primary Presenter/Author), University of Wisconsin-Milwaukee School of Freshwater Sciences, sllabuhn@uwm.edu;


Val Klump (Co-Presenter/Co-Author), School of Freshwater Sciences, University of Wisconsin-Milwaukee, vklump@uwm.edu;


Dirk Koopmans (Co-Presenter/Co-Author), University of Wisconsin-Milwaukee, School of Freshwater Sciences, koopmans@uwm.edu;

5/18/2015  |   16:45 - 17:00   |  101B

QUANTIFICATION OF BENTHIC SOURCES AND SINKS OF NITRATE: BUILDING ON AQUATIC EDDY COVARIANCE OXYGEN FLUX Our understanding of carbon, nitrogen, and phosphorus transformation in rivers, lakes, and marine systems has been limited in part by the spatial and temporal scales of investigation that our analytical techniques allow. With the aquatic eddy covariance technique (aka eddy correlation), oxygen fluxes are determined at high temporal resolution, under in situ hydrodynamic conditions, and at user-selectable spatial scales. We have used the technique to examine the drivers of oxygen flux over approximately 10 m^2 of cohesive, sandy, and vegetated sediments in a coastal stream and over approximately 10,000 m^2 of cohesive sediments in a hypereutrophic bay. The recent development of precise, in situ, ultraviolet nitrate detection technology has opened the door to the quantification of benthic nitrate fluxes with the same technique. The investigators who developed the technology have proven the technique in a marine setting. This presentation will describe calculated eddy covariance oxygen fluxes in a stream and in a hypereutrophic bay and their implications for future calculation of benthic nitrate fluxes at high temporal resolution, under in situ hydrodynamic conditions, and at user-selectable spatial scales.

Dirk Koopmans (Primary Presenter/Author), University of Wisconsin-Milwaukee, School of Freshwater Sciences, koopmans@uwm.edu;


Peter Berg (Co-Presenter/Co-Author), University of Virginia, pb8n@virginia.edu;


Shelby LaBuhn (Co-Presenter/Co-Author), University of Wisconsin-Milwaukee School of Freshwater Sciences, sllabuhn@uwm.edu;


Val Klump (Co-Presenter/Co-Author), School of Freshwater Sciences, University of Wisconsin-Milwaukee, vklump@uwm.edu;