This thesis examines the production and distribution of dissolved gaseous mercury (DGM) in freshwater ecosystems and its relationship to mercury volatilization. The importance of volatilization was assessed within a multidisciplinary mercury mass balance for Big Dam West Lake (BDW) Kejimkujik Park, Nova Scotia. The magnitude of volatilization was found to be approximately double the direct wet deposition over lake and wetlands, and 27% of the direct wet deposition to the terrestrial catchment. Over the entire basin area the mass of mercury volatilized is 46% of the mass deposited by wet deposition. A new method of continuous (5 minute) DGM analysis was developed and tested. The detection limit for DGM was 20 fmol L-1 with 99% removal efficiency. Control experiments showed that there was no interference due to methyl mercury, which is present in similar concentrations to DGM. Experiments comparing continuous DGM analysis with discrete DGM analysis showed that the results are not significantly affected by typical variations in water temperature (4--30°C), oxidation-reduction potential (135--355 mV), dissolved organic carbon (4.5--10.5 mg L-1), or pH (3.5--7.8). The continuous analysis was within 4.5% of the discrete analysis when compared across 12 samples analyzed in triplicate. Diurnal patterns for dissolved gaseous mercury (DGM) and mercury flux were measured (using this new DGM method and a Teflon flux chamber method) in two lakes with contrasting dissolved organic carbon (DOC) concentrations in Kejimkujik Park, Nova Scotia. Consistently higher DGM concentrations were found in the high DOC lake as compared to the low DOC lake. Cross-correlation analysis indicated that DGM dynamics changed in response to solar radiation with lag-times of 65 and 90 minutes. An examination of current mercury flux models using this quantitative data indicated some good correlations between the data and predicted flux (r ranging from 0.27 to 0.83) but generally poor fit (standard deviation of residuals ranging from 0.97 to 3.38). This research indicates that DOC and wind speed may play important roles in DGM and mercury flux dynamics that have not been adequately accounted for in current predictive models. The distribution of DGM in the water columns of shallow and deep freshwater lakes was investigated in Lake Ontario and several small freshwater lakes. When DGM concentrations were expressed on an areal basis, DGM concentrations above the thermocline in Lake Ontario average 1.5 ng m-2 and in small freshwater lakes it ranged between 0.1 and 0.8 ng m -2. Further, it was demonstrated that the majority of DGM in large freshwater lakes such as Lake Ontario exists below the thermocline where photochemical oxidation and reduction processes cannot occur. The depth profiles indicate that vertical mixing in the water column may alter the DGM concentration in the upper epilimnion, and that turn over in deep lakes may result in a transfer of large concentrations of DGM from the hypolimnion into the epilimnion. In addition, the results indicate that microbial processes may be an important factor regulating DGM in the water column of freshwater lakes, particularly in the hypolimnion. (Abstract shortened by UMI.)

We present two novel stable isotope methods for measuring lake metabolism and compare the results to traditional techniques. The delta 18O method measures planktonic gross primary production (GPP) from dissolved oxygen concentrations, isotopes and respiration (R) and the delta 13C method measures "whole-lake" GPP and R from dissolved oxygen and carbon concentrations and isotopes. All three methods showed GPP was greater than R over the ice-free season and estimates of GPP were not significantly different. There was also no significant difference in R as measured by bottle incubations and the delta13C method. However, the delta 13C method does not account for inputs of external carbon which will result in underestimation of R and overestimation of GPP. In systems with significant allochthonous carbon inputs, the delta13C method cannot be accurate unless these inputs are accounted for. The delta18O method was used to measure metabolic parameters of twenty-one northern temperate lakes and showed GPP dominated over R during the ice-free season. GPP and R were most strongly correlated with lake temperature, which in turn is a function of the amount of solar radiation received by the lake. Our results imply that it is this solar radiation that drives planktonic gross primary productivity, which in turn drives the majority of planktonic respiration. Variation in dissolved organic carbon only explained 8% of the variation in planktonic R, while variation in planktonic GPP explained approximately 80% of the variation in planktonic R. Despite general autotrophy in the lakes, they were generally oversaturated in CO2 during the ice-free season, on average 252+/-25%. However, we found little evidence to conclude that this was the result of an excess of in situ respiration over production. The magnitude of the annual excess of R over GPP was not sufficient to account for the flux to the atmosphere. Moreover, carbon evasion was not a function of respiratory flux, nor did the isotopic signature of dissolved CO2 in the lakes present evidence of respiration. Groundwater inputs of carbon dioxide represent a plausible source for carbon dioxide oversaturation in some but not all of the lakes sampled.

No description available.

No description available.

I determined that Corbicula fluminea can be used as a bioindicator on the lower Colorado River. I analyzed tissue samples for trace element concentrations. Selenium and arsenic were elevated above U.S. background levels at 89% and 83% (respectively), of the sites. Selenium concentrations were significantly higher in backwaters than at river sites. Selenium in clams predicts the contamination state of a site 78% of the time. There is a strong correlation between selenium concentrations in clams and selenium concentrations in vascular aquatic plants (r²) and carnivorous birds (r²). The white morph of C. fluminea is more prevalent at northern and backwater sites than the purple morph. Selenium levels in clams at several sites exceeded levels that have been shown to result in teratogenicity for birds in laboratory studies. Birds that eat clams in the study area could have increased risk of lowered reproductive success.

The dynamics of crappie populations in southwestern reservoirs are not well documented. In order for managers to enhance a crappie fishery, they need to understand the factors that control populations. I examined potential limiting factors for black crappie in 4 sequential reservoirs on the Salt River, Arizona. Only in Roosevelt Reservoir, the uppermost reservoir, were crappie relatively abundant. I identified 3 possible limiting factors. Apache, Canyon, and Saguaro reservoirs lacked sufficient spawning substrates. All 4 reservoirs lacked sufficient cover. Finally, the lower 3 reservoirs are less productive during the fall according to chlorophyll a levels than Roosevelt Reservoir. Low productivity may have resulted in fewer zooplankton and possibly a limited forage base for juvenile crappie. Options such as spawning platforms, artificial cover, fertilization, and stocking are discussed as ways to improve the crappie fishery in these reservoirs.

acase@tulane.edu

Because of the variation in the temperature of maximum density with depth, it is known that the upper (above 200m) and lower water columns circulate separately in deep temperate lakes. While near surface water overturns twice per year, the deep waters often mix only partially during a complete annual cycle. It is believed that deep convection is triggered by storm surge forcing some of the relatively cold upper water column downwards through its compensation depth, so that it becomes unstable and sinks The scale and intensity of the resulting deep water forming plumes are studied numerically. A high resolution model based on the non hydrostatic Boussinesq equations is used. Deep water formation is simulated by applying a statically unstable initial temperature profile over various regions of the domain at depth below 300m. The grid spacing is small enough to resolve individual plumes which carry surface water to the lake bottom. The domain of study is also large enough that the geostrophic Eady wave break up of newly formed deep water can be observed It is found that when the initial instability is applied in mid basin remote from the solid sidewalls, relatively little fresh deep water is formed, and the fluid quickly reaches a geostrophic and hydrostatic balance. Vigourous vertical mixing ceases after two days, and the resulting baroclinic fluid has a well defined and predictable scale. In contrast, when the initial instability is applied immediately adjacent to a solid boundary, vigourous plume motion continues for the duration of the numerical simulations (twelve days), producing a much greater volume of fresh deep water A parametric study investigates the scale and intensity of this boundary mixing. It is found that increased horizontal diffusion coefficients, buoyance parameter or a reduced Coriolis acceleration increase the rate of deep water renewal / acase@tulane.edu

Effect of implementing best management practices on water and habitat quality in the Upper Strawberry River Watershed, Fulton County, Arkansas, USA

Lake Tanganyika, East Africa, is one of the world's unique freshwater ecosystems. This lake is approximately 10 million years old and contains an extremely diverse fauna with high levels of endemicity. Most of the biodiversity in the lake is concentrated along the rocky shoreline, and this dissertation focused on the land-water interactions that may affect the littoral ecosystem as human population densities increase. First, the potential of carbon and nitrogen stable isotopes as indicators of human impact was examined. Sedimentary organic matter from four river deltas with contrasting patterns of land use was used to develop a predictive relationship between terrestrial patterns and geochemical signals. Stable isotope analyses of cores taken at two disturbed and two undisturbed watersheds were consistent with patterns seen in surface sediments. These results suggest that nitrogen isotopes are a better indicator of land use while carbon isotopes are useful in determining changes in vegetation patterns. To evaluate the effect of deforestation on ecosystem dynamics, epilithic metabolism was measured at several sites over a period of one year. Epilithic net productivity did not differ significantly between the disturbed and undisturbed watersheds, but epilithic respiration and algal biomass were significantly higher at the disturbed site. Further evidence of disturbance was given by greater amounts of inorganic material on the rocks and lower ambient oxygen concentrations at the impacted site. Finally, in order to identify species more susceptible to changes in benthic productivity, the food web structure of the pelagic and littoral food webs was elucidated using stable isotopic analyses. Diet specificity is not apparent in the littoral food web, suggesting that broad dietary preferences may help stabilize this persistent food web during environmental perturbations. The isotopic structure of the pelagic food web suggests that upwelling of deeper waters is an important nutrient source for the food web, which is consistent with current nutrient and hydrodynamic models.