Excess rural and urban nutrient inputs have led to downstream water quality degradation. Landowners in a small watershed in south central Manitoba, Canada have installed small dams as flood control mechanisms. Previous work has shown these dams and reservoirs are effective at decreasing total phosphorus (P) export, however questions of permanence, daily P fluctuation, and mechanisms influencing P retention still remain. Sediment nutrient dynamics can exert an important control on water quality on daily, monthly, and yearly timescales. To help better understand spatial and temporal patterns of P retention, P sorption assays were constructed (equilibrium P concentration or EPC0) and compared monthly measurements of EPC0 in small dammed reservoirs with their natural analog, stream pools. Dammed reservoirs and stream pools both showed a strong capacity to sorb P from the water column and as such, sediment processes represent a P sink across much of the catchment. In situ high frequency P sensors were deployed to assess short-term changes in P concentrations in four dammed reservoirs. Diel changes were only apparent later in the summer (August) but what drives these changes is unknown. Dam design to optimize nutrient retention should consider factors affecting P retention, including sediment geochemistry, but also residence time, and water chemistry as potential controls on P sorption. Diel sampling results suggest that water quality monitoring regimes that rely on singular grab samples should aim to sample in the mid-morning, especially later in the summer, so as to not over or underestimate P concentrations in water bodies.

<p> The export of particulate carbon from the surface ocean into deeper water and to the seabed is a critical component of the carbon cycle. The concentrations and compositions of particulate pigments collected at different depths and sinking at different settling velocities can be used as a proxy for biologically mediated processes important to the early degradation of OM. By knowing what processes the compositional and quantitative changes in the particulate pigments represent, the POM cycle of the BaRFlux area can be better understood. It is important to understand the POM cycle because deposition of OM to the seabed is the only way that OM is sequestered. The removal of POM from the marine POM cycle is especially important to understand in subtropical gyre areas like the BaRFlux site because: 1. subtropical gyres are areas of downwelling, and therefore POM transport to the deep ocean and may increase as global warming continues. 2.the flux of CO₂ to the ocean is increasing from rising levels of atmospheric CO₂, and CO₂ removed by the biological pump will lessen processes like ocean acidification.</p><p> This thesis examines the early degradation of chloropigments in the sediment and water column in the Bermuda Rise area of the Sargasso Sea. Water column particulate samples were collected with in-situ pumps, Niskin bottles, and Indented Rotating Sphere (IRS) sediment traps, and sediment was collected by box cores during 2011-2013 to record seasonal patterns in quantity and quality of particulate pigments as a function of water column depth and particle size. Chl-a, Chl-b, and pheopigments were separated and quantified using reverse-phase High Performance Liquid Chromatography (HPLC).</p><p> The comparison of data from in-situ pumps and Niskin bottles indicates that collection method significantly affects particulate pigment data concentrations. Niskin bottle data showed total pigment concentrations 10 times greater than in-situ pump pigment concentrations at shallow depths. At depths below the euphotic zone, Niskin bottle and in-situ pump concentrations both appear similar because the particulate pigment concentrations were below the detection limit. For the BaRFlux study area, the differences in Niskin bottle and pump data are most likely from: 1. the biased particle distribution due to sampling a small volume of seawater with Niskin bottles in an area of dilute particle concentration; 2. the greater retention efficiency of picophytoplankton on Niskin GF/F filters than 1-<i>&micro;</i>m in-situ pump microquartz filters.</p><p> The compositional changes seen in small suspended particulate pigments over depth is consist with small suspended particles being consumed by shallow water zooplankton and then increasingly altered by microbial activity with increasing depth. The composition of small and large particulate pigments were compared to determine if aggregation-disaggregation was an important process. Larger suspended particulate pigments were nearly 100% Chl-a over depth and distinct from smaller suspended particulate pigments other than samples from May or June, during which particle exchange may be more important. The comparison of particulate pigment data to CTD beam transmissivity profiles suggests that the nepheloid layer consists of small suspended particulate matter rather than large particles.</p><p> Sediment trap samples were compositionally enriched in pheopigments relative to smaller bottle and pump samples, indicative of enrichment with more rapidly sinking larger zooplankton fecal pellets. The mole% of chlorophyll-a labile pigment increased with increasing settling velocity, suggesting aggregation may increase the settling velocity of particles enough to escape zooplankton feeding. The particulate pigment composition of seafloor sediment collected in August was compositionally distinct from that of suspended and sinking particulate pigments and was nearly 100% pheophorbide-a, indicating POM degradation by feeding macrobenthos.</p>

<p> The archaeological site of K&uuml;ltepe (ancient Kanesh), located in south-central Anatolia, in the present-day Republic of Turkey, was the capital city of a native Anatolian kingdom during the early Middle Bronze Age (20^th - late 18^th c. BC). Uninterrupted archaeological excavations at the site since 1948 by the Turkish Historical Society under the directorship of the late Prof. Tahsin &Ouml;zg&uuml;&ccedil; have revealed wide exposures of densely settled residential neighborhoods at the foot of a high citadel mound with palaces and temples. Archaeological evidence from the site indicates a millennium-long settlement sequence of the Early Bronze Age (EBA), predating the Level II settlement, during which a demographic explosion occurred at the site. Circumstantial evidence from Anatolia contemporary with the poorly understood levels of the EBA and direct archaeological and textual evidence from the Level II and Ib settlements of the MBA demonstrate a complex history of immigration to Kanesh. By the turn of the 2^nd millennium BC, at least five languages, namely Neshili (Hittite), Luwian, Hattian, Hurrian, and Old Assyrian were spoken in this city, as can be understood on the basis of prosopographic data. The three centuries, during which the city existed as the largest known urban site in central Anatolia, were times of political turmoil, characterized by the formation of territorial states on the Anatolian plateau, which culminated in the establishment of the Old Hittite Kingdom that was born at Kanesh. </p><p> K&uuml;ltepe/Kanesh is widely known beyond the academic circles of Ancient Near Eastern and Anatolian archaeology as an Old Assyrian Trade Colony due to the 22,500 cuneiform texts in the Old Assyrian language found in the private family archives of merchants in the residential quarters of the lower town. On the basis of these texts, the excavated areas of the lower town have been regarded as a colonial settlement (Karum) established outside the citadel walls and scholarship on Kane has been structured by colonial frameworks. Moreover, due to certain organizational principles of the Old Assyrian trade operations, which resemble free market economy, the historical evidence from Kanesh has received a great deal of attention from economic historians. On various occasions, the case of Kanesh has been cited as an ancient example of capitalism, colonialism, and World Systems that resulted in underdevelopment in Anatolia. Since the excavators' research agenda has targeted areas that bear a higher potential to yield cuneiform texts, this well-investigated mercantile district of the city has remained like an island isolated from its past and its surroundings. As such, the case of Kane represents a prime example of "the tyranny of the text" in the archaeology of Anatolia and calls for alternative perspectives beyond the straight-jacketing colonial paradigms. In recent years, the new campaign of excavations under the directorship of Prof. Fikri Kulakoglu have begun to embrace interdisciplinary and integrative research agendas, which sets a promising direction for K&uuml;ltepe studies. </p><p> In this dissertation, I place the native communities of prehistoric Anatolia at the center of my inquiries and investigate the questions of residential mobility and cultural pluralism at K&uuml;ltepe within a long-term, local perspective in relationship to the process of urbanization in the region. I use the methodological approaches of history-from-below and text-aided archaeology to counteract the interpretative biases of colonial frameworks and reconstruct a diachronic framework for demographic mobility at Kanesh in relationship to its political history. Guided by concepts borrowed from archaeology of communities that focus on the study of human interaction in face-to-face societies in light of analogies to the ethnographic record of Anatolia, I attempt to identify social, economic, and cultural distinctions of individuals and households at Kane based on the diversity of its archaeological remains, beyond a restricted notion of ethnicity. I propose a systematic research model for the reconstruction of household biographies and investigate the utility of the funerary remains from the site for demographic assessments. And finally, I present the results of the strontium and stable light isotope analyses I conducted on human tooth samples from K&uuml;ltepe graves encountered during the 2006-2010 excavation seasons, in light of which I identify local individuals, immigrants, and mixed households, and make preliminary observations on the sources of diversity in paleodiet.</p>

<p> This study used a mass balance approach by characterizing the input, output, and sink rates of N in order to assess a declared "stage III N-saturated forest" response to decreased atmospheric N deposition in western Maryland. Relying on the conceptual model of kinetic N-saturation to holistically link stream, vegetative, soil, and atmospheric compartments and the use of a novel stable isotopic technique, the study demonstrated dynamic soil NO₃-N pools, unprocessed atmospheric NO₃-N in base flow, and significant reductions in NO₃-N yield in response to decreased atmospheric N deposition. A lumped conceptual model, incorporating a dormant season NO₃-N flush, was proposed that explains forest response to decreased deposition and sheds light on the hydrologic processes that govern the storage/release of NO₃-N among years. It is proposed that this flushing mechanism prevents forests from attaining higher stages of N-saturation and predicts forests will be responsive to further reductions in N deposition. </p>

Nitrous oxide (N???O) is a potent greenhouse gas, an important driver of climate change, and its concentration in the atmosphere is rising at an unprecedented rate. Agriculture is the leading contributor of all the anthropogenic N???O sources, and the vast majority of agricultural N???O emissions originate from soil. Of all the natural N???O emissions, two-thirds originate from soil and temperate forests contribute approximately one-sixth of the natural soil emissions. Consequently, there is great interest in understanding the soil nitrogen processes responsible for N???O production so that effective policies and management practises can be implemented to successfully mitigate climate change. The stable isotopes of nitrogen (N) and oxygen (O) in soil N???O emissions are hypothesized to be useful indicators of the biogeochemical processes that produce and consume N???O, and they may be used to apportion different environmental sources. The primary objective of this thesis was to assess the utility of ???????N and ???????O values to differentiate N???O produced by nitrification and denitrification. Most of the previous research on N???O isotopes has utilized microbial cultures of single organisms; yet natural systems contain a consortium of N-metabolizing microorganisms so the relevance of this early work to natural environments is uncertain. This thesis presents the results of experimental incubations of soil from an agricultural site and a temperate forest located within Ontario, Canada. Two well-drained soils (upland), two poorly-drained soils (wetland), and one stream sediment were incubated under varying conditions (temperature, moisture, and N-availability) to achieve a wide range in the rate of N???O production. The ???????N and ???????O values of N???O produced from the different experiments were characterized and the isotope effects (??) of N???O production were calculated. Experiments were conducted in aerobic or anoxic atmospheres to stimulate N???O formation by nitrification and denitrification, respectively. The ???????N-N???O produced by denitrification in all soils was 7???35??? lower than the ???????N-nitrate (NO??????). The ???????N-N???O produced by nitrification in the upland forest soil and the agricultural soils was 28???54??? lower than the ???????N-ammonium. Nitrification in the forested wetland soil yielded higher ???????N-N???O values (?? = ???16???), which was likely caused by an increase in the ???????N-substrate. With the exception of the latter soil, there was clear ?????N-separation between the nitrification- and denitrification-derived N???O in all soils. Consequently, ???????N values can be used to apportion different environmental sources of N???O on a site-by-site basis, provided that the rates of N metabolism are known and the isotopic endmembers are well-characterized. A novel approach was employed in this thesis to help unravel the key controls of ???????O-N???O and ???????O-NO?????? formation. Different ?????O-labelled soil waters were used to demonstrate that the abiotic exchange of oxygen atoms between water and nitrite (in equilibrium) is an important control of the ???????O-N???O formed by nitrifier-denitrification and the ???????O-NO?????? formed by nitrification. O-exchange in these incubations was highly variable between soils (37???88%) and it appeared to be rate-related. Furthermore, the ???????O value of microbial NO?????? is partially controlled by ?????O/?????O fractionation that occurs during O-exchange (equilibrium fractionation) and the uptake of molecular oxygen (O???) and water (H???O) (kinetic fractionation). This research showed that the ???????O value of microbially-produced NO?????? cannot be successfully predicted in soils based upon the commonly used ???one third, two-thirds rule???, which only takes into account the ???????O values of O??? and H???O. Successful predictions of ???????O-NO?????? using this rule appear to be fortuitous and are because of the range of ???????O-H???O at natural abundance and the magnitude of the isotope effects involved. Enzyme-catalyzed (biotic) O-exchange between water and nitrite/nitric oxide in denitrification was also quantified for the first time in soils. O-exchange during denitrification was significant and variable (39???95%), but uniquely confined to narrow ranges for each soil type. Almost complete O-exchange occurred in the well-drained agricultural and forested soils (86???95%); less O-exchange occurred in the agricultural and forested wetland soils (63???70%); and even less O-exchange occurred in the agricultural stream sediment (39???51%). The magnitude of O-exchange during denitrification was independent of soil temperature and moisture for a given soil, and it was not related to the rate of N???O production. This implies that the amount of O-exchange that occurs during soil denitrification is controlled by the dominant microbial community. For the first time, estimates of the net O isotope effect were determined for N???O production by soil denitrifiers that accounted for the complicating effects of O-exchange. The net ?????O-discrimination (N???O???NO??????) ranged between +32??? and +60???, with the exception of one treatment that was cooled (?? = +17???). The O isotope separation (??) that is actually observed in natural systems is often much lower, and in some cases negative. This is because the atomic O-exchange between water and nitrite/nitric oxide effectively diminishes the net ?????O separation between NO?????? and N???O because ???????O values of environmental water are usually lower than the ???????O values of N???O-precursors. The determinants of ???????O-N???O produced by nitrification pathways are complex and there is no holistic explanation of the O isotope dynamics in the literature. This thesis provides the first systematic model to describe ???????O-N???O formation by aerobic pathways. In addition to O-exchange between water and nitrite (at equilibrium), ???????O-N???O is controlled by ?????O/?????O fractionation that results from this O-exchange mechanism, and from fractionation that occurs during ammonia-oxidation and nitrite-reduction. Although explaining ???????O-N???O values produced by nitrification is complex, reports of nitrifier-derived ???????O-N???O in the literature and this thesis are narrowly confined between +13??? and +31??? (rel. VSMOW). This is distinct from much of the denitrifier-produced ???????O-N???O, which is often ?????O-enriched and higher than +33???. In three out of the five different soils investigated in this thesis, ???????O-N???O could be used to separate N???O formed by nitrification and denitrification. There was poor ???????O separation between nitrifier- and denitrifier-derived N???O in the well-drained soils because high amounts of biotic O-exchange and reduced O isotope separations yielded lower (predicted) estimates of denitrifier-produced ???????O-N???O. On the other hand, ???????N values could be used to apportion nitrifier- and denitrifier-derived N???O sources in these soils. Thus, stable isotope ratios of N???O are a valuable and promising tool that may help differentiate nitrifier-N???O from denitrifier-N???O in natural soil environments.

Despite the importance of tropical rivers to the global carbon cycle, the nature of carbon cycling within these watersheds has been dealt with by only a handful of studies. The current work attempts to address this lack of information, using stable isotope and concentration measurements to constrain sources and sinks of carbon in two Peninsular Malaysian watersheds. The basins are located on the central-western and northeastern coasts of the Malaysian Peninsula, and are drained by the Langat and Kelantan Rivers, respectively. Water samples were collected from three points along the two rivers twice a month, in addition to the sampling of groundwater in adjacent aquifers. Principal component analyses (PCA) on water chemistry parameters in the Langat and Kelantan Rivers show the dominance of geogenic and anthropogenic influences, grouped in 4 to 6 components that comprise over 50 % of the total dataset variances. The geogenic input is reflected by components showing strong loadings by Ca, Mg, Mn, Si, and Sr, while anthropogenic influences via pollution are indicated via strong loadings by NO3, SO4, K, Zn and Cl. The carbon isotope and concentration data appear unrelated to these groups, suggesting that the riverine carbon cycle in both locations is dominated by other factors. These may include alternative sources of organic pollution, or inputs from the local vegetation and soils. The mean riverine 13CDOC of -27.8 ± 2.9 ‰ and -26.6 ± 2.2 ‰ in the Langat and Kelantan Basins, respectively, are consistent with the dominance of C3-type vegetation in both watersheds. Riverine 13CDIC signatures approach C3-like values at high DIC concentrations, with measurements as low as -19 ‰ in the Kelantan Basin and -20 ‰ observed in the Langat Basin, consistent with a biological origin for riverine DIC. However, the average 13CDIC in river water is 13C-enriched by about 10 ‰ relative to the expected C3 source in both rivers, and this 13C- enrichment appears to be largest with smaller DIC concentrations. Because of the overpressures of CO2 in the rivers, entrainment of isotopically-heavy atmospheric CO2 is not a likely explanation for the observed 13C-enrichment. Theoretically, dissolution of carbonates could be an alternative source of 13C-enriched carbon, but this lithology is scarce, particularly in the Langat watershed. The increase in DIC downstream and generally high pCO2 values in most river sections argues against aquatic photosynthesis as a primary causative factor for the observed isotopic enrichment. This elimination process leaves the speciation of riverine DIC and the evasion of CO2 as the most likely mechanisms for 13C-enrichment in DIC, via isotope fractionation during HCO3- hydration and CO2 diffusion. Potentially, methanogenic activity could also be, at least partially, responsible for the 13C-enrichment in DIC, particularly immediately downstream of the Langat Reservoir, but due to the absence of empirical data, this must remain only a theoretical proposition. The aquatic chemistry and dissolved carbon data suggests that pollution discharge into the Langat and Kelantan Rivers is the major factor that is responsible for the considerable CO2 overpressures and high DIC and DOC concentrations in the river waters, particularly in the downstream sections. This pollution is likely of biological origin, via sewage and palm oil mill effluent (POME) discharge, and therefore isotopically indistinguishable from natural C3 plant sources. Carbon budgets of the Langat and Kelantan River show CO2 degassing to be a significant mechanism of fluvial carbon loss, comprising roughly 50 %, or more, of the total riverine carbon export in both watersheds. The remainder of the river carbon is transported to the ocean in the form of DIC, DOC and POC in broadly comparable proportions. However, the combined riverine carbon export from the Kelantan and Langat Basins amount to 2 % or less of the total carbon sequestration of the watersheds. Thus, most of the sequestered carbon is returned to the atmosphere via respiration, with smaller amounts incorporated into ecosystem biomass . These results highlight the complexity of carbon cycling in tropical rivers, and agree with previous studies in showing riverine systems to be more than simple conduits of carbon from the land to the ocean.

<p>Dissolved organic matter (DOM) is an important component of nutrient cycling and energy transfer within and between ecosystems. Understanding controls over the magnitude and quality of DOM that is transferred from soils to surface water is needed to better characterize the terrestrial-aquatic carbon flux and effects of terrestrial DOM on downstream ecosystems. A meta-analysis of the response of in-stream dissolved organic nitrogen concentration (DON) to high flow events indicates that DON typically increases with flow across a wide range of ecosystem types, likely as novel DOM sources in the landscape are mobilized and transported to streams and rivers. Mechanisms controlling DOM export, including dissolved organic carbon (DOC) and DON concentrations and the quality of DOM, were examined in a small agricultural catchment in eastern Washington State. In the soil column, DOC concentration declined and source of DOM shifted from humic-like and plant-derived to microbially-derived with depth through the profile. Across seasons and years, DOM exported via drain discharge during low flows resembled that found deep in the soil profile, and DOM exported during high flows suggests topsoil and litter sources contribute to export. A simple mixing model suggests that litter leachate can contribute over 50% of DOM during peak flow. Based on modeled contributions of litter, topsoil and subsoil DOM during storm events, DOC concentration is over-predicted, except for peak flows, suggesting removal via sorption and/or microbial decomposition in the soil column control DOC export on the timescale of events. Although the character of exported DOM shifts with flow conditions, laboratory incubations suggest bioavailability to the stream sediment microbial community is consistently low, with a maximum of 7% loss over 6 days, indicating exported DOM is likely transported beyond the immediate stream reach. An analysis of anticipated effects of climate change on the flow regime in the catchment projects the wettest years to become more variable, with non-linear effects on the magnitude of DOC export. Finally I explore how climate change assessments can be incorporated into nonpoint source nutrient management plans, despite current uncertainty about the magnitude and timeframe of climate effects on nutrient loading.

Mercury (Hg)-contaminated freshwater fisheries are a global toxicological concern. Previous research suggests that the slow release of Hg in runoff from upland soils may delay the recovery of Hg-contaminated aquatic systems. Four complementary studies were undertaken in a small boreal Shield headwater catchment as part of the Mercury Experiment to Assess Atmospheric Loading in Canada and the U.S. (METAALICUS) to assess the controls on the retention and release of historically-deposited Hg (ambient Hg) and newly-deposited (spike Hg) in the soil landscape. In the first study, hydrometric and GIS-based methods were used to quantify thresholds in terrestrial water storage and their relationship to observed rainfall-runoff response. It was found that event-scale hydrologic response displayed a threshold relationship with antecedent storage in the terminal depression and predictions of event runoff improved when storage excesses from upslope depressions were explicitly routed through the catchment. In the second study, it was shown that the dominant source of ambient Hg to the lake was likely derived from shallow soil-water flowing through the lower, well-humified organic soil horizon. Throughout the catchment, ambient Hg to soil organic carbon (SOC) ratios increased with depth and the experimentally-applied spike Hg was concentrated in the surface litter layer, suggesting that the vertical redistribution of Hg in the soil profile is a function of the rate of decomposition of SOC. In the third study, canopy type was found to be a good predictor of ambient Hg and spike Hg stocks in the lower organic horizon, while drainage conditions were not, suggesting that vertical fluxes of Hg dominate over lateral fluxes in topographically-complex landscapes. Lastly, it was shown that catchment discharge, antecedent depression storage and antecedent precipitation were the best predictors of dissolved organic carbon (DOC), ambient Hg and spike Hg concentrations in catchment runoff. A comparison of DOC, ambient Hg and spike Hg dynamics for two storm events showed that distinct shifts occurred in the concentration-discharge relationship as a result of differences in antecedent moisture conditions. Combined, the results of the four studies demonstrate the need to incorporate hydrological, biogeochemical and landscape controls into predictive models of terrestrial-aquatic Hg export.

Mercury (Hg)-contaminated freshwater fisheries are a global toxicological concern. Previous research suggests that the slow release of Hg in runoff from upland soils may delay the recovery of Hg-contaminated aquatic systems. Four complementary studies were undertaken in a small boreal Shield headwater catchment as part of the Mercury Experiment to Assess Atmospheric Loading in Canada and the U.S. (METAALICUS) to assess the controls on the retention and release of historically-deposited Hg (ambient Hg) and newly-deposited (spike Hg) in the soil landscape. In the first study, hydrometric and GIS-based methods were used to quantify thresholds in terrestrial water storage and their relationship to observed rainfall-runoff response. It was found that event-scale hydrologic response displayed a threshold relationship with antecedent storage in the terminal depression and predictions of event runoff improved when storage excesses from upslope depressions were explicitly routed through the catchment. In the second study, it was shown that the dominant source of ambient Hg to the lake was likely derived from shallow soil-water flowing through the lower, well-humified organic soil horizon. Throughout the catchment, ambient Hg to soil organic carbon (SOC) ratios increased with depth and the experimentally-applied spike Hg was concentrated in the surface litter layer, suggesting that the vertical redistribution of Hg in the soil profile is a function of the rate of decomposition of SOC. In the third study, canopy type was found to be a good predictor of ambient Hg and spike Hg stocks in the lower organic horizon, while drainage conditions were not, suggesting that vertical fluxes of Hg dominate over lateral fluxes in topographically-complex landscapes. Lastly, it was shown that catchment discharge, antecedent depression storage and antecedent precipitation were the best predictors of dissolved organic carbon (DOC), ambient Hg and spike Hg concentrations in catchment runoff. A comparison of DOC, ambient Hg and spike Hg dynamics for two storm events showed that distinct shifts occurred in the concentration-discharge relationship as a result of differences in antecedent moisture conditions. Combined, the results of the four studies demonstrate the need to incorporate hydrological, biogeochemical and landscape controls into predictive models of terrestrial-aquatic Hg export.

For the purpose of evaluating the importance of the water column as a vector of mercury (Hg) to the biota of hydroelectric reservoirs, a comprehensive study was undertaken to provide continuous profiles of total dissolved Hg concentrations, (Hg$ sb{ tau} rbrack rm sb{D},$ in the water column and interstitial waters of natural and artificial environments. Water column samples were collected during the ice-free season (June to October), at various locations and depths within the La Grande-2 and Laforge-1 hydroelectric reservoirs, as well as in several neighbouring lakes. Close-interval sampling of porewaters in lake sediments, peat bogs, and flooded soils was accomplished via in situ dialysis. All Hg analyses were conducted in the field laboratory within 12 hours of sample collection, using cold vapour atomic fluorescence spectrometry. Water column (Hg$ sb{ tau} rbrack rm sb{D}$ are nearly constant, with an average value of 2.3 ng L$ sp{-1}.$ No significant concentration difference was noted regardless of: (1) the type of aquatic system, (2) the composition of the underlying soil, (3) the impoundment history, (4) the depth of the water column, or (5) seasonal changes. Preliminary results for porewater (Hg$ sb{ tau} rbrack rm sb{D}$ show them to be 2 to 3 times greater than those of the overlying water. In most profiles (Hg$ sb{ tau} rbrack rm sb{D}$ values do not show much variation with depth, away from the sediment-water interface. Despite the concentration differences near the interface, diffusion of Hg from the sediments to the water column is not apparent from this study, as satisfactory determinations of (Hg$ sb{ tau} rbrack rm sb{D}$ right at the interface were not possible. Biogeochemical processes acting near the interface (e.g. assimilation by periphyton, chelation by organic matter, methylation-demethylation, absorption on metal oxides) may be responsible for attenuating the release of dissolved Hg to the water column.