Mercury (Hg) transport in streams is typically facilitated by dissolved organic matter (DOM), however, the dynamics of Hg and DOM during snowmelt in montane watersheds are poorly understood. Hg transport during snowmelt is widely recognized as a significant source of Hg to downstream lakes and reservoirs, such as Jordanelle Reservoir where fish consumption advisories are in effect due to elevated Hg concentrations in certain species of fish. For this study, total mercury (THg), methylmercury (MeHg), and DOM samples were collected at three sites in the upper Provo River, northern Utah, during the 2016 and 2017 water years. To evaluate Hg and DOM sources, samples were collected from snowpack and ephemeral streams in the watershed. In-situ fluorescent DOM (fDOM) data and other parameters were measured in the river to characterize high-frequency variation in water chemistry. Excitation-emissions matrices (EEMs) were used to determine changes in DOM characteristics during snowmelt. Hg concentrations increased in the upper Provo River from /L during baseflow to >;7 ng/L during the snowmelt period (~April-July), with filtered THg concentrations approximately ~75% of the unfiltered concentrations. In the watershed, filtered THg concentrations ranged from ~0.4 ng/L in snowpack to ~8 ng/L in ephemeral streams. Annual THg loading from the Provo River to Jordanelle Reservoir was approximately 1 kg/yr with ~90% of the flux occurring during the snowmelt period. High correlations between filtered THg and fDOM allowed for the development of a high frequency filtered THg proxy using in-situ fDOM sensors. DOM characteristic during the snowmelt period showed that Hg transport was facilitated by humic substances which was sourced from upland soils. Fractions of filtered methylmercury (MeHg) and filtered THg (filtered MeHG:filtered THg) were ~0.1 during baseflow and reduced to ~0.01 during snowmelt, implying that snowmelt runoff has little impact on the MeHg flux to Jordanelle Reservoir. The results suggest that Hg and DOM are flushed from soils during snowmelt, and that a significant majority of the Hg flux occurs the snowmelt period. Our study has implications for understanding Hg sources and transport mechanisms in other snowmelt dominated watersheds.

La matière organique dissoute chromophorique (CDOM) est une fraction significative du pool global de matière organique dissoute (MOD) dans les océans. La CDOM absorbe une partie de la lumière dans le domaine du rayonnement ultraviolet (UV-R) et du visible. Une fraction de cette CDOM peut émettre une fluorescence lorsqu'elle est excitée par un UV-R. Cette fraction est alors appelée matière organique dissoute fluorescente (FDOM). La CDOM a donc d'une part, un effet positif, en protégeant les cellules contre les dommages causés par les UV-R, mais d'autre part, un effet négatif en réduisant la quantité de radiation disponible pour la photosynthèse. Les propriétés optiques de la CDOM, particulièrement sensibles aux processus physiques (abiotiques) et biologiques (biotiques), renseignent sur l'intensité des processus biogéochimiques en milieux aquatiques. Des suivis de la dynamique de la CDOM en zones côtière et hauturière en Méditerranée Nord occidentale ont permis de décrire différentes tendances temporelles claires, qui vont d’une faible à une forte saisonnalité et qui peuvent être découplées des variations du pool global de MOD caractérisé par les concentrations en carbone organique dissous (COD). Dans les zones tempérées, les événements météorologiques conduisent à des changements beaucoup plus brusques dans la frange littorale que dans l’océan, où les variations tendent à être plus progressives au cours de l'année. En outre, l'apport de nutriments et de polluants dans les zones côtières est fortement influencé par l'activité anthropogénique et ces entrées ne suivent pas nécessairement de tendances saisonnières nettes. Dans la présente étude qui effectue un premier bilan de la distribution et du devenir de la CDOM/FDOM aux stations d'observation à long terme du laboratoire Arago (stations côtière SOLA et hauturière MOLA) à partir respectivement d'un suivi hebdomadaire et mensuel, nous nous sommes attachés à extraire un signal cohérent et une variabilité claire des sources des différentes fractions de la MOD entre février 2013 et avril 2014 ceci, de manière à mieux comprendre les rôles respectifs des facteurs biotiques et abiotiques. Nos observations ont ensuite pu être replacées dans un contexte synoptique d'évolution climatologique des écosystèmes méditerranéens. / Chromophoric dissolved organic matter (CDOM) is a major fraction of dissolved organic matter (DOM). CDOM absorbs light over a broad range of ultraviolet (UV-R) and visible wavelengths. A small fraction of CDOM can emit fluorescence when excited by ultraviolet radiation; so called fluorescent dissolved organic matter (FDOM). CDOM plays a key role in regulating light penetration into the ocean, absorbing high-energy electromagnetic spectrum (visible and ultraviolet light) waves. On one hand, it protects aquatic organisms of potential photo-damage; in the other hand it induces a negative effect by reducing light for photosynthesis. The optical properties of the CDOM are sensitive to biological (biotic) and physical (abiotic) processes and for this reason the colored matter can provide valuable information about the biogeochemical processes in aquatic environments. CDOM monitoring in Mediterranean coastal areas has shown different temporal trends, which go from weak to strong seasonality. Interestedly, these temporal trends were uncoupled with those of the total dissolved organic carbon. In temperate areas, episodic meteorological events can induce much more abrupt changes in the littoral than in the open sea, where changes tend to be more gradual along the year. In addition, the input of nutrients and pollutants in coastal areas is strongly influenced by the anthropogenic activity on land, and those inputs do not necessarily follow seasonal trends. In the present study, weekly and monthly samplings were performed to investigate the temporal variability in SOLA and MOLA stations, respectively. The fluctuation of different fractions of dissolved organic matter (DOM) was evaluated from February 2013 to April 2014 and referred to long time-frame databases of SOLA and MOLA stations. Inorganic nutrients and chlorophyll shown the classical seasonal patterns, with a winter period characterized by an enrichment of surface waters favoring the spring bloom, followed by a calm period that allows the summer stratification and the depletion of nutrients in the photic zone. The stratification extended until autumn winds and low temperatures eroded the thermocline. In contrast, colored DOM fractions did not follow a clear temporal trend. Interestingly, dissolved organic carbon (DOC) exhibited the highest variability in summer, when the rest of parameters showed minimum variations. To explain this mismatch we proposed a sequence of abiotic and biotic phenomena driving the DOC dynamics. In the suggested conceptual frame, DOC dynamics depended strongly on episodic meteorological events (winds, rains, etc.) along the year, except in summer, where the biological factors were more relevant. In order to better understand the influence of biological factors, we examined the temporal trends of phytoplankton composition in relation to those of the different colored DOM fractions. We found that both phytoplankton and CDOM were strongly influenced by abiotic factors such as the intrusions of fresh waters, the vertical mixing due to convection and the light exposure. However we did not find a correlation between any of the CDOM fractions and any of phytoplankton groups. In addition, we use the dust deposition database of ADEPT project (ICM-CSIC, Barcelona) to investigate the potential role of atmospheric deposition in the CDOM temporal variability, and also performed two dust addition experiments with natural plankton communities collected in the Catalan coast.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23179号 / 工博第4823号 / 新制||工||1754(附属図書館) / 京都大学大学院工学研究科都市環境工学専攻 / (主査)教授 田中 宏明, 教授 清水 芳久, 講師 中田 典秀 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM