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09:05-10:45 Session 7: Hydrologic Systems
Douglas Carlson (Louisiana Geological Survey, Louisiana State University, United States)
Location: Dalton Woods Auditorium
Dependra Bhatta (LSU, United States)
Krishna Paudel (LSU, United States)
Frank Tsai (LSU, United States)
Groundwater extraction and allocation when there are stock externalities

ABSTRACT. The Mississippi River Alluvial Aquifer of North East Louisiana is facing a salt intrusion problem.  There is an upward movement of salty water from the low lying salt dome.  As such when the agricultural sector withdraws groundwater, it causes further stress to the aquifer and increases the chance of salt intrusion. In this study, we provide a framework to assess the future exploitation possibility due to saltwater intrusion. There may be exogenous or endogenous or both risk associated with water withdrawals in the aquifer. We find that precautionary optimal exploitation policy can avoid endogenous risk to maintain the higher groundwater stock, whereas knowing potential exogenous stochastic natural phenomenon has the ambiguous effect.

Key words: Endogenous risk, exogenous risk, groundwater

Zhen Xu (Louisiana State University, United States)
Y. Jun Xu (Louisiana State University, United States)
Biogeochemical connectivity of dissolved carbon across the waterscape of a low gradient watershed

ABSTRACT. Lakes and rivers are increasingly being studied in the context of biogeochemical connectivity. In this study, we analyzed dissolved carbon along the Little River-Catahoula Lake in the subtropical southern United States to investigate carbon dynamics in such a river-lake continuum. Monthly in-situ measurements and water sample collections were made at four locations from April 2015 to February 2016 to determine riverine carbon into and out of the 119 km2 lake with an average depth of approximately 11 meters. Field measurements included ambient water temperature, dissolved oxygen, pH, electrical conductivity, and chlorophyll a fluorescence. Water samples were analyzed for dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and their carbon-13 isotopes (δ13C). We found a significant, consistent decrease in DIC concentration from the river to the lake outflow (i.e., from 848 to 399 umol L-1, p = 0.035) as well as from the lake inflow to outflow (i.e., from 482 to 399 umol L-1, p = 0.045). This decreasing trend was most prevalent during the period when the lake was not affected by backwater flow from the downstream river, namely, riverine DIC concentrations declined by 27% after passing through the lake (p = 0.038). The lake acted as a sink for DIC owing to a combined effect of aquatic metabolism and air-lake CO2 exchange. Significant decrease in DOC was also found from the river to the lake outflow (i.e., from 1213 to 969 umol L-1, p = 0.019) as well as from lake inflow to outflow (i.e., from 1103 to 969 umol L-1, p = 0.043). However, the lake functioned both as a sink and as a source for DOC - during much of the year, the lake reduced DOC from the river inflow water, but it switched to functioning as a source of DOC during the warm, dry September and October. The lake seemed to be most biologically productive during these two months, striking a balance of allochthonous organic inputs and lake metabolism. The finding suggests that aquatic bioactivity is important for the sink-source behavior of in-network lakes, which can be enhanced in subtropical and tropical environments due to higher temperature.

Madeline Gill (LSU, United States)
Rodrigo Diaz (LSU, United States)
Andy Nyman (LSU, United States)
Knight Ian (LSU, United States)
Dieback of Roseau Cane (Phragmites Australis) in Coastal Louisiana
SPEAKER: Madeline Gill

ABSTRACT. Roseau cane (Phragmites australis) dieback was first noticed during the fall of 2016 in the Mississippi River Delta. By summer of 2017 dieback had been observed in several locations across coastal Louisiana and appeared to coincide with the infestation of a scale insect (Nipponaclerda biwakoensis). Roseau cane scale is native to Asia and was not previously reported in North America. The scale can be found below the leaf sheath of Roseau cane and feeds by sucking sap directly from the stem. Roseau cane found in other regions of North America is detested for its invasiveness which outcompetes beneficial native vegetation. However, throughout the Mississippi River Delta, Roseau cane provides important habitat for fish and wildlife and critical defense against salinity and wave action in water too deep for other plant species to tolerate. Roseau cane dieback in the Mississippi River Delta is resulting in patches of open water and poses a major threat to coastal integrity. Additionally, Roseau cane provides protection for oil and gas infrastructure, recreational camps, and crucial navigation channels. Loss of Roseau cane at the mouth of the Mississippi River probably will cause significant ecological and economic impacts not experienced elsewhere.

Lauris Hollis (Louisiana State University, United States)
R. Eugene Turner (Lousiana State University, United States)
The tensile root strength of Spartina patens: response to flood duration and nutrient addition
SPEAKER: Lauris Hollis

ABSTRACT. Spartina patens, a common coastal emergent macrophyte, may exhibit morphological, anatomical, and physiological adaptations to flooding that are related to aerenchyma formation. These adaptations may affect the biomechanical properties of roots and compromise soil stability and erosion rates. Nutrient addition may also lead to a reduction in root biomass and affect tensile root strength. We tested the hypothesis that the interactive effects of nutrient addition and flood duration reduce tensile root strength by conducting a 165 day greenhouse experiment using six levels of nitrogen and phosphorus addition and two different flood duration regimes. A one-way Welch’s analysis of variance (ANOVA) revealed that the tensile root strength of S. patens was significantly reduced by 3.5-day and one-week flood duration regimes (p < 0.0001) and by all six levels of nutrient addition. The tensile root strength in the experimental treatments of both main effects was significantly lower than in the controls. The results of one-way ANOVAs of tensile root strength with one main effect segregated into subsets and tested using the other main effect demonstrated that there were interactive effects of nutrient addition and flood duration on tensile root strength. These results suggest that exposure to multiple natural and/or anthropogenic stressors can weaken the belowground biomass of macrophytes and decrease the resiliency of the emergent macrophytes in coastal ecosystems.

11:00-12:15 Session 8: Carbon Dioxide in Water Systems
Douglas Carlson (Louisiana Geological Survey, Louisiana State University, United States)
Location: Dalton Woods Auditorium
Jeremy Reiman (Louisiana State University, United States)
Dr. Y. Jun Xu (Louisiana State University, United States)
Carbon Dioxide Emissions from the Lower Mississippi River
SPEAKER: Jeremy Reiman

ABSTRACT. In recent years, CO2 outgassing from rivers, or release of CO2 from the water column into the atmosphere, has been identified as a significant piece of the carbon cycle. This process has been attributed to an oversaturation of CO2 in the water column in response to large fluxes of CO2 derived from soil respiration and aquatic metabolism over-powering in-stream CO2 removal mechanisms like photosynthesis. This oversaturation results in a current estimate of global CO2 flux from rivers to the atmosphere around 0.8 Pg yr-1. While it is believed many of the world’s largest rivers consistently function as a source of CO2 to the atmosphere, previous studies have consisted of a low-resolution sampling regime and calculated CO2 values, rather than in-stream measurements. A higher resolution of data collection and direct measurements of partial pressure of carbon dioxide (pCO2) in streams, especially large rivers, will improve our ability to quantify this essential piece of the global and river carbon cycle.

This study aimed to fill in a critical knowledge gap: how much CO2 does North America’s largest river - the Mississippi River - emit? The study’s specific objectives were to (1) analyze temporal trends in partial pressure of carbon dioxide (pCO2) and CO2 outgassing in the lower Mississippi River (2) calculate the total amount of carbon outgassed from the lower Mississippi River annually (3) investigate the effects of hydrologic flood pulses on pCO2 and CO2 outgassing. To achieve the above objectives, in-situ pCO2 measurements, dissolved inorganic carbon samples, and additional in-situ ambient environmental parameters were collected monthly from December 2015 to April 2016 and December 2016 to January 2018 at two locations on a 115 river-km stretch of the lower Mississippi River. Additionally, in-situ measurements and water samples were also collected over the rise, crest, and fall of a 64-day flood pulse from April to Jun 2017 to support the flood pulse assessment. The U.S.D.A. Farm Service Agency’s National Agriculture Imagery Program (NAIP) aerial imagery was used to digitize and calculate the surface area of the Mississippi River study area, which in turn was used to calculate CO2 outgassing.

Our preliminary data reveal that all samples at both study sites were super-saturated in CO2 with respect to the atmosphere, suggesting the lower Mississippi River functions as a source of CO2 to the atmosphere across all seasons. pCO2 measurements in the Mississippi had significant (p<0.05) correlations with temperature and discharge and significant inverse correlations with dissolved oxygen and δ13CDIC. The estimated annual flux of carbon to the atmosphere through CO2 from the lower 115 km stretch of the Mississippi River was 9.96 x 107 kg C yr-1, with a larger portion of the outgassing occurring during summer months. Analysis for the influence of the 2017 flood pulse on outgassing is currently underway.

Rongjie Yang (Louisiana State University, United States)
Zhen Xu (Louisiana State University, United States)
Shiliang Liu (Louisiana State University, United States)
Yijun Xu (Louisiana State University, United States)
Solar Radiation Effects on Partial Pressure of Carbon Dioxide in Shallow Lakes
SPEAKER: Rongjie Yang

ABSTRACT. Many freshwater systems are supersaturated with dissolved carbon dioxide (CO2). These systems can act as a source of CO2 to the atmosphere, which plays an important role in the global carbon cycle. Although previous studies have reported the effect of CO2 partial pressure (pCO2) in a water body on CO2 outgassing, it is not well known how pCO2 fluctuates daily and how this fluctuation could affect the outgassing process. This is especially true for urban lakes, which often receive excess organic pollutants and nutrients from the developed surroundings, and which could be photochemically more active during daylight change. In general, there is a knowledge gap about a possible relation between light condition and pCO2 concentration in water bodies and how this relation might affect CO2 outgassing from lakes, streams and rivers. In this study, we conducted bi-weekly field measurements on pCO2 in two lakes with varied urban influences, the University Lake on the LSU campus and the Capitol Lake in downtown Baton Rouge, from November 2017 to February 2018. In each trip, ambient conditions including, in addition to pCO2, water temperature, dissolved oxygen, pH, and chlorophyll a fluorescence were recorded four time during the day (from sunrise to sunset). Additionally, water samples were collected for laboratory analysis of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and their carbon-13 isotopes (δ13C), and climatic data including solar radiation were gathered from a nearby weather station. Our results showed a strong correlation between pCO2 concentration and solar radiation for the two studied lakes. At University Lake, a clear negative linear relationship was observed between pCO2 and solar radiation in November, January and February, but such a relationship was not apparent due likely to heavy rainstorms occurred in the month. At Capitol Lake, also a negative but weaker correlation between pCO2 and solar radiation was found in November and January, but such a relation could not be seen in December and February. The difference in the pCO2-solar radiation relationship between the two studied lakes may have been resulted from the lake trophic states.

Songjie He (Louisiana State University, United States)
Y. Jun Xu (Louisiana State University, United States)
Dissolved carbon and CO2 outgassing dynamics in the freshwater-saltwater mixing zone of a coastal river entering the Northern Gulf of Mexico
SPEAKER: Songjie He

ABSTRACT. The delivery of dissolved carbon from rivers to coastal oceans is an important component of the global carbon budget. From November 2013 to December 2014, we investigated freshwater-saltwater mixing effects on dissolved carbon concentrations and CO2 outgassing at six locations along an 88-km long estuarine river entering the Northern Gulf of Mexico with salinity increasing from 0.02 at site 1 to 29.50 at site 6 near the river’s mouth. We found that throughout the sampling period, all six sites exhibited CO2 supersaturation with respect to the atmospheric CO2 pressure during most of the sampling trips. The average CO2 outgassing fluxes at site 1 through site 6 were 162, 177, 165, 218, 126, and 15 mol m-2 yr-1, respectively, with a mean of 140 mol m-2 yr-1 for the entire river reach. In the short freshwater river reach before a saltwater barrier, 0.079 ×108 kg carbon were emitted to the atmosphere during the study year. In the freshwater-saltwater mixing zone with wide channels and river lakes, however, a much larger amount of carbon (3.04 ×108 kg) was emitted to the atmosphere during the same period. For the study period, the river’s freshwater discharged 0.25×109 mol dissolved inorganic carbon (DIC) and 1.77×109 mol dissolved organic carbon (DOC) into the mixing zone. DIC concentration increased six times from freshwater (0.24 mM) to saltwater (1.64 mM), while DOC showed an opposing trend, but to a lesser degree (from 1.13 mM to 0.56 mM). These findings suggest strong effects of freshwater-saltwater mixing on dissolved carbon dynamics, which should be taken into account in carbon processing and budgeting in the world’s estuarine systems.