Colored dissolved organic matter dynamics and anthropogenic influences in a major transboundary river and its coastal wetland

Abstract

Most transboundary rivers and their wetlands are subject to considerable anthropogenic pressures associated with multiple and often conflicting uses. In the Eastern Mediterranean such systems are also particularly vulnerable to climate change, posing additional challenges for integrated water resources management. Comprehensive measurements of the optical signature of colored dissolved organic matter (CDOM) were combined with measurements of river discharges and water physicochemical and biogeochemical properties, to assess carbon dynamics, water quality, and anthropogenic influences in a major transboundary system of the Eastern Mediterranean, the Evros (or, Марица or, Meriç) river and its Ramsar protected coastal wetland. Measurements were performed over three years, in seasons characterized by different hydrologic conditions and along transects extending more than 70 km from the freshwater end‐member to two kilometers offshore in the Aegean Sea. Changes in precipitation, anthropogenic dissolved organic matter (DOM) inputs from the polluted Ergene tributary, and the irregular operation of a dam were key factors driving water quality, salinity regimes, and biogeochemical properties in the Evros delta and coastal waters. Marsh outwelling affected coastal carbon quality, but the influence of wetlands was often masked by anthropogenic DOM contributions. A distinctive five‐peak CDOM fluorescence signature was characteristic of upstream anthropogenic inputs and clearly tracked the influence of freshwater discharges on water quality. Monitoring of this CDOM fluorescence footprint could have direct applications to programs focusing on water quality and environmental assessment in this and other transboundary rivers where management of water resources remains largely ineffective.

Human impact on long‐term organic carbon export to rivers

“…In the period of 1899 to 1905, when the calibration between DOC and color was performed, the level of urbanization was relatively low, but it increased over all the period. Urbanization increases the amount of sewage effluents discharged to the river, which usually is associated with less colored carbon (given that sewage is typically composed of, among others, carbohydrates and urea, which are nonabsorbing or weakly absorbing DOC substances [ Painter and Viney , ; Tzortziou et al , ]). Therefore, we would expect a likely underestimation of the amount of observed DOC in the river pre‐1990, due to the calibration period used.…”

Convergence of Terrestrial Dissolved Organic Matter Composition and the Role of Microbial Buffering in Aquatic Ecosystems

“…Previous plant leachate degradation studies that have investigated degradation pathways have either overlooked DOM compositional changes altogether (Farjalla et al, 2001) or were limited in either the scope of plant species tested (Hansen et al, 2016) or analytical techniques (Pellerin et al, 2010). Optical absorbance and fluorescence measurements are beneficial in broadly characterizing all CDOM and can be used to infer DOM source (Tzortziou et al, 2015) and composition (Spencer et al, 2012) and can indicate changes in the quality of DOM as it is degraded over time (Fasching et al, 2014;Guillemette & del Giorgio, 2012;Hansen et al, 2016). Combined with multiparameter statistical methods such as parallel factor analysis (PARAFAC; Murphy et al, 2013;Stedmon et al, 2003) and principal component analysis (PCA; Fasching et al, 2014;Luzius et al, 2018), optical properties can link changes in DOM bioavailability with compositional transformations.…”

Environmental Drivers of Dissolved Organic Matter Molecular Composition in the Delaware Estuary

“…Positive and negative deviations from a conservative mixing line defined by high-DOC river water and the low-DOC marine endmember are frequently observed (Cadée, 1982;Middelburg and Herman, 2007;Sharp et al, 2009). Estuaries are complex systems where, in addition to mixing of fresh-and marine waters, diffuse sources such as subterranean groundwater discharge (Taniguchi et al, 2002), aeolian and anthropogenic input (Liu et al, 2005;Tzortziou et al, 2015) add to the intricacy of biogeochemical cycling. Abiotic processes shaping the DOM pool along the estuarine gradient of pH, ionic strength and turbidity include the adsorption on and desorption from mineral surfaces (Keil et al, 1994;Mayer, 1994), flocculation (Eisma, 1986), aggregation or precipitation.…”