Proceedings of the Thirteenth International Society of Sports Nutrition (ISSN) Conference and Expo

Meeting Abstracts: Proceedings of the Thirteenth International Society of Sports Nutrition (ISSN) Conference and Expo Clearwater Beach, FL, USA. 9-11 June 201

Patterns and Controls on Nitrogen Removal and Greenhouse Gas Production in Reservoirs

The construction and operation of over 1 million dams globally has fundamentally altered the way that water and nutrients flow through river networks. The reservoirs behind dams are important sites for nutrient and carbon cycling, but the patterns and controls on this cycling are still being elucidated. In the first chapter of this dissertation, temporal patterns in dinitrogen and nitrous oxide gradients were examined in the bottom boundary layer of a small eutrophic reservoir in southwest Washington. The findings highlight the potential importance of a region called the “internal shoreline” in supporting permanent nitrogen removal (e.g. denitrification). This chapter also lays groundwork towards the development of an in situ method for measuring denitrification rates at time scales of minutes to hours and spatial scales of 10s to 100s of meters. The second chapter examines inter-annual patterns and controls on water column redox biogeochemistry in the same reservoir. Across four years, there was less water column accumulation of reduced iron and methane when spring oxygen and nitrate availability was high. We also report dynamic spikes and drops in reduced solutes and elevated methane ebullition (e.g. bubbling) during an annual autumn water level drawdown event. We suggest that water level drawdowns enhance the transport of sediment pore waters into the water column via either groundwater influx or ebullition-driven exchange. Finally, the third chapter examines the magnitude and controls on global greenhouse gas emissions from reservoirs to the atmosphere. Methane emissions constituted 78-84% of emissions on a CO2 equivalent basis and were best predicted by reservoir productivity (chlorophyll a concentrations). Finally, we estimate that greenhouse gas emissions from reservoirs are comparable in magnitude to between 1 and 4.6% of all global human-caused CO2-equivalent emissions. Given that reservoirs are human-designed and operated systems, we highlight potential management implications associated with water level drawdowns and aquatic eutrophication. The findings presented here, together with projected increases in dam construction, call for further research to better discern the role that reservoirs play in biogeochemical cycling across scales

Thermal stratification impacts microbial nitrogen removal and nitrous oxide production in a small eutrophic reservoir An in-situ approach to quantifying hypolimnetic process rates

Identifying patterns and controls on microbial N removal and N2O production is a critical step in understanding the role that freshwater systems play as both N filters and greenhouse gas sources. Efforts to discern these relationships have been limited by methodological difficulties associated with the measurement of these processes. In this study, N dynamics within the hypolimnion of a thermally stratified reservoir were examined in order to test a novel approach to measuring in situ N2 and N2O production rates wherein sub-thermocline gas accumulation rates serve as a proxy for sediment production rates. Water column stratification can significantly change bottom water chemistry and was thus expected to impact system N2 and N2O production rates. Potential denitrification rates determined by acetylene block were three orders of magnitude greater in the sediment than in the overlying water column. These assays, sediment incubation experiments, and hypolimnion NO− 3 data show that, over the course of the summer, progressive nitrate depletion at the sediment-water interface limited N2 production and associated N removal. As rates of N2 production dropped off, rates of N2O production increased, resulting in significant increases in N2O:N2 ratios as the summer progressed (p <0.05). These findings have important implications for the incorporation of seasonal patterns of aquatic N removal in N transport models. Our findings also suggest that physical disturbances, such as dam spills, may sufficiently destabilize the water column to alleviate sediment NO− 3 limitatio

Patterns and controls on nitrogen removal and greenhouse gas production in reservoirs

The construction and operation of over 1 million dams globally has fundamentally altered the way that water and nutrients flow through river networks. The reservoirs behind dams are important sites for nutrient and carbon cycling, but the patterns and controls on this cycling are still being elucidated. In the first chapter of this dissertation, temporal patterns in dinitrogen and nitrous oxide gradients were examined in the bottom boundary layer of a small eutrophic reservoir in southwest Washington. The findings highlight the potential importance of a region called the “internal shoreline” in supporting permanent nitrogen removal (e.g. denitrification). This chapter also lays groundwork towards the development of an in situ method for measuring denitrification rates at time scales of minutes to hours and spatial scales of 10s to 100s of meters. The second chapter examines inter-annual patterns and controls on water column redox biogeochemistry in the same reservoir. Across four years, there was less water column accumulation of reduced iron and methane when spring oxygen and nitrate availability was high. We also report dynamic spikes and drops in reduced solutes and elevated methane ebullition (e.g. bubbling) during an annual autumn water level drawdown event. We suggest that water level drawdowns enhance the transport of sediment pore waters into the water column via either groundwater influx or ebullition-driven exchange. Finally, the third chapter examines the magnitude and controls on global greenhouse gas emissions from reservoirs to the atmosphere. Methane emissions constituted 78-84% of emissions on a CO2 equivalent basis and were best predicted by reservoir productivity (chlorophyll a concentrations). Finally, we estimate that greenhouse gas emissions from reservoirs are comparable in magnitude to between 1 and 4.6% of all global human-caused CO2-equivalent emissions. Given that reservoirs are human-designed and operated systems, we highlight potential management implications associated with water level drawdowns and aquatic eutrophication. The findings presented here, together with projected increases in dam construction, call for further research to better discern the role that reservoirs play in biogeochemical cycling across scales

Microbial dinitrogen and nitrous oxide production in a small eutrophic reservoir: An in situ approach to quantifying hypolimnetic process rates

Nitrogen (N) dynamics within the hypolimnion of a thermally stratified reservoir were examined to test an in situ approach to measuring dinitrogen (N(2)) and nitrous oxide (N(2)O) production rates wherein hypolimnion gas accumulation is used to estimate N(2) and N(2)O production. This previously unpublished approach provides a spatially integrated, time-varying record of N transformation rates that fall well within the range of rates reported for other reservoir systems using other methods. Hypolimnion N(2) production averaged 183 mu mol N(2)-N m(-2) h(-1) with higher rates observed early in a spring stratification event (538 mu mol N(2)-N m(-2) h(-1)) and lower rates observed later in the same stratification event (90 mu mol N(2)-N m(-2) h(-1)). Sediment incubation experiments and hypolimnion nitrate (NO(3)(-)) data show that, over the course of the summer, progressive NO(3)(-) depletion at the sediment-water interface limited N(2) production and associated N removal. As rates of N(2) production dropped off, rates of N(2)O production increased (from 4.62 mu mol N(2)O-N m(-2) d(-1) to 51 mu mol N(2)O-N m(-2) d(-1), averaging 26 mu mol N(2)O-N m(-2) d(-1)), resulting in significant increases in N(2)O-N : N(2)-N ratios as the summer progressed. Also, whereas N(2) production appeared to occur predominantly at the sediment-water interface, N(2)O production was detected throughout the water column, suggesting a role for nitrification as a source of N(2)O. The use of hypolimnion accumulation to quantify N transformation rates can thus offer new insights into spatial and seasonal N transformation patterns in stratified or otherwise capped aquatic systems

Chemical mixing in the bottom boundary layer of a eutrophic reservoir: The effects of internal seiching on nitrogen dynamics

In lakes and reservoirs, the bottom boundary layer (BBL) mediates chemical fluxes between sediments and the overlying water column. At the internal shoreline, where the thermocline contacts the lakebed, the motions of internal waves can create fluctuating redox conditions and dynamic physical forcing that may support ecologically important reactions such as denitrification. We characterized physical and chemical dynamics within the internal shoreline of a eutrophic reservoir during the spring and early summer of 2012 (18 May to 18 July). An internal seiche was found to generate quasi-periodic fluctuations (periods about 12-24 h) in BBL stratification, temperature, and redox conditions. To examine possible implications for chemical mixing and microbial processing, differences between vertically offset, simultaneous BBL measurements of velocity, temperature, N-2, and N2O were made over 23 h. Vertical differences in BBL temperature, N-2, and N2O formed and collapsed during the wave cycle, with the largest differences occurring following the arrival of an internal bore. Through much of the wave cycle, chemical differences were explained by physical advection and mixing. However, chemical differences measured after bore arrival were not explained by advection, possibly owing to local production of N-2 and N2O. These results highlight the dynamic physical environment within the internal shoreline, and the potential of this zone to contribute to system wide denitrification and nitrous oxide production

Key differences between lakes and reservoirs modify climate signals: A case for a new conceptual model

Lakes and reservoirs are recognized as important sentinels of climate change, integrating catchment and atmospheric climate change drivers. Climate change conceptual models generally consider lakes and reservoirs together despite the possibility that these systems respond differently to climate-related drivers. Here, we synthesize differences between lake and reservoir characteristics that are likely important for predicting waterbody response to climate change. To better articulate these differences, we revised the energy mass flux framework, a conceptual model for the effects of climate change on lentic ecosystems, to explicitly consider the differential responses of lake versus reservoir ecosystems. The model predicts that catchment and management characteristics will be more important mediators of climate effects in reservoirs than in natural lakes. Given the increased reliance on reservoirs globally, we highlight current gaps in our understanding of these systems and suggest research directions to further characterize regional and continental differences among lakes and reservoirs

Microbial dinitrogen and nitrous oxide production in a small eutrophic reservoir: An in situ approach to quantifying hypolimnetic process rates

Nitrogen (N) dynamics within the hypolimnion of a thermally stratified reservoir were examined to test an in situ approach to measuring dinitrogen (N(2)) and nitrous oxide (N(2)O) production rates wherein hypolimnion gas accumulation is used to estimate N(2) and N(2)O production. This previously unpublished approach provides a spatially integrated, time-varying record of N transformation rates that fall well within the range of rates reported for other reservoir systems using other methods. Hypolimnion N(2) production averaged 183 mu mol N(2)-N m(-2) h(-1) with higher rates observed early in a spring stratification event (538 mu mol N(2)-N m(-2) h(-1)) and lower rates observed later in the same stratification event (90 mu mol N(2)-N m(-2) h(-1)). Sediment incubation experiments and hypolimnion nitrate (NO(3)(-)) data show that, over the course of the summer, progressive NO(3)(-) depletion at the sediment-water interface limited N(2) production and associated N removal. As rates of N(2) production dropped off, rates of N(2)O production increased (from 4.62 mu mol N(2)O-N m(-2) d(-1) to 51 mu mol N(2)O-N m(-2) d(-1), averaging 26 mu mol N(2)O-N m(-2) d(-1)), resulting in significant increases in N(2)O-N : N(2)-N ratios as the summer progressed. Also, whereas N(2) production appeared to occur predominantly at the sediment-water interface, N(2)O production was detected throughout the water column, suggesting a role for nitrification as a source of N(2)O. The use of hypolimnion accumulation to quantify N transformation rates can thus offer new insights into spatial and seasonal N transformation patterns in stratified or otherwise capped aquatic systems

Reservoir Water-Level Drawdowns Accelerate and Amplify Methane Emission

Water-level fluctuations due to reservoir management could substantially affect the timing and magnitude of reservoir methane (CH4) fluxes to the atmosphere. However, effects of such fluctuations on CH4 emissions have received limited attention. Here we examine CH4 emission dynamics in six Pacific Northwest U.S. reservoirs of varying trophic status, morphometry, and management regimes. In these systems, we show that water-level drawdowns can, at least temporarily, greatly increase per-area reservoir CH4 fluxes to the atmosphere, and can account for more than 90% of annual reservoir CH4 flux in a period of just a few weeks. Reservoirs with higher epilimnetic [chlorophyll a] experienced larger increases in CH4 emission in response to drawdown (R2 = 0.84, p < 0.01), suggesting that eutrophication magnifies the effect of drawdown on CH4 emission. We show that drawdowns as small as 0.5 m can stimulate ebullition events. Given that drawdown events of this magnitude are quite common in reservoirs, our results suggest that this process must be considered in sampling strategies designed to characterize total CH4 fluxes from reservoirs. The extent to which (and the mechanisms by which) drawdowns short-circuit connections between methanogenesis and methanotrophy, thereby increasing net CH4 fluxes to the atmosphere, should be a focus of future work

Spatiotemporal Methane Emission From Global Reservoirs

Inland aquatic systems, such as reservoirs, contribute substantially to global methane (CH4) emissions; yet are among the most uncertain components of the total CH4 budget. Reservoirs have received recent attention as they may generate high CH4 fluxes. Improved quantification of these CH4 fluxes, particularly their spatiotemporal distribution, is key to realistically incorporating them in CH4 modeling and budget studies. Here we report on a new global, gridded (0.25 degrees lat x 0.25 degrees lon) study of reservoir CH4 emissions, accounting for new knowledge regarding reservoir areal extent and distribution, and spatiotemporal emission patterns influenced by diurnal variability, temperature-dependent seasonality, satellite-derived freeze-thaw dynamics, and eco-climatic zone. The results of this new data set comprise daily CH4 emissions throughout the full annual cycle and show that reservoirs cover 297 x 10(3) km(2) globally and emit 10.1 Tg CH4 yr(-1) (1 sigma uncertainty range of 7.2-12.9 Tg CH4 yr(-1)) from diffusive (1.2 Tg CH4 yr(-1)) and ebullitive (8.9 Tg CH4 yr(-1)) emission pathways. This analysis of reservoir CH4 emission addresses multiple gaps and uncertainties in previous studies and represents an important contribution to studies of the global CH4 budget. The new data sets and methodologies from this study provide a framework to better understand and model the current and future role of reservoirs in the global CH4 budget and to guide efforts to mitigate reservoir-related CH4 emissions.Funding Agencies|NASAs Interdisciplinary Research in Earth Science (IDS) Program; European Research Council (ERC)European Research Council (ERC)European Commission [725546]; NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at NASA Ames Research Center; NASA Terrestrial Ecology and Tropospheric Composition Programs</p