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Advancing Understanding of Carbon Cycling and Coloured Dissolved Organic Matter Dynamics in European Wetlands & Coastal Ecosystems through integration of observations and novel modelling approaches

Final Report Summary - ECODOM (Advancing Understanding of Carbon Cycling and Coloured Dissolved Organic Matter Dynamics in European Wetlands & Coastal Ecosystems through integration of observations)

Project context and objectives

Despite the highly oligotrophic nature of the Eastern Mediterranean Sea, its coastal catchments, estuaries and near-shore waters are often subject to pollution and eutrophication caused by increasing coastal urbanisation, industrialisation and agricultural activities. Previous studies have highlighted that, along with an increased monitoring and control of the export of suspended particles and inorganic nutrients to the European coastal zone, there is a growing need to understand the sources and fate of dissolved organic matter (DOM) inputs, because neglecting DOM impacts on ecosystem processes will inevitably lead to management strategies that will fall short of desired outcomes. This project aims at advancing the understanding of dissolved organic matter dynamics and carbon cycling in Eastern Mediterranean coastal wetlands through an integration of previously missing comprehensive field datasets, remote-sensing observations and novel modelling approaches.

Dissolved organic matter (DOM), introduced to coastal environments mainly through river run-off and wetland discharges, plays a key role in a broad range of processes and climate-related biogeochemical cycles. Yet many questions remain about the sources and quality of DOM at the land-ocean interface, while published studies on the quality, composition and photochemical or microbial processing of DOM in coastal waters and wetland ecosystems of the Eastern Mediterranean are extremely scarce. With climate change already having a significant impact on Europe’s seas and coasts, knowledge of the linkages between climate, land-use, DOM export and coastal effects is critical for the improved protection of coastal waters. However, it is still unclear how changes in hydrological processes and coastal wetland characteristics due to changes in land use, climate, precipitation patterns and watershed management practices would affect DOM quality and fluxes into the coastal ocean, and what implications these changes would have on coastal ecosystem health and productivity.

Included amongst the main research goals is the provision of new mechanistic insights that will enhance our understanding of the fluxes, quality and transformation pathways of dissolved organic matter along the terrestrial, coastal sea, open ocean continuum, and the effects of these processes on Eastern Mediterranean coastal water photochemistry, biogeochemistry and ecosystem functioning. The specific objectives include:

-characterising variations in the quantity and quality of the dissolved organic compounds exported from different wetlands and coastal tidal marshes in SE Europe;
-developing novel and improved modelling approaches for describing and predicting the effects of photochemical processes on DOM quality and degradation;
-applying new radiative transfer modelling techniques to examine the effects of terrestrial DOM inputs on underwater light fields in coastal and shelf waters;
-linking results to remote sensing applications.

The abundance of wetlands and tidal marsh systems along the SE European coastlines, and the complex biogeochemical processes and exchanges taking place in these highly dynamic coastal margins, make this region of the Mediterranean an ideal place for addressing our objectives. Our study sites included rivers, deltas and wetland systems that vary widely in carbon sources and environmental characteristics:

- Amvrakikos wetlands, a Ramsar site and Special Protection Area under EC Directive 79/409, on the NW coast of Greece (Ionian Sea), consisting of both salt and freshwater marshes;
- Evros river and delta ecosystem, a Ramsar-protected transboundary river shared by Bulgaria, Greece and Turkey, frequently affected by flooding;
- Sperchios River and wetlands in Central Greece, where water quality, local agricultural production and fisheries are heavily affected by wastewater from industrial and other anthropogenic activities;
- Nestos wetlands in North Greece;
- Evrotas, in South Greece, a temporary river where hydrologic discontinuities strongly affect the quality and quantity of exported DOM and, subsequently, microbial and biological processes along the river's ecosystem.

The combination of hydrological, physicochemical, optical, biological and biogeochemical measurements collected as part of the project's comprehensive field campaign programme fill critical gaps in the current knowledge related to the sources, transformation and fate of organic compounds, nutrients and pollutants in the coastal Eastern Mediterranean. Measurements were performed at various locations in each studied wetland system, from the upstream part of the basin to the estuary, depending on the different hydrologic and land-use characteristics of the study catchment and the potential DOM sources in each system (agricultural drainage, riparian vegetation, sewage, freshwater or salt marsh vegetation, swamps, lagoons, etc.). Measurements were performed over a period of four years and in seasons characterised by largely different precipitation and river discharges, to study processes in terms of seasonal changes in water flow and biogeochemical exchanges.

Our measurements showed that, although the Eastern Mediterranean is an extremely oligotrophic region, dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) amounts in Eastern Mediterranean deltas and coastal wetlands can reach high values of the order of those typically measured in highly eutrophic estuarine systems. Considerably higher DOC, a CDOM, fluorescence, DOC-specific CDOM absorption were typically observed upstream in the rivers compared to the marine-end members, while the absorption spectral slope and fluorescence quantum yield were lower, making them consistent with the more aromatic / higher molecular weight (MW) signature of the terrestrial DOM components. Size-exclusion chromatography-high-performance liquid chromatography (SEC-HPLC) analysis showed higher average-weight MW and polydispersity associated with the freshwater DOM. During transport along the river and to the sea, CDOM optical properties showed strong gradients and non-conservative mixing between the freshwater and marine end-members. As discussed in our manuscripts (Tzortziou et al., 2011, and Tzortziou et al., in preparation), the observed changes in the amount and quality of DOM were due to both additional sources of humic organic compounds, associated with the presence of major anthropogenic pollution sources in most river systems, as well as DOM removal processes (both photochemical and microbial degradation).

Measurements of the fluorescence Excitation Emission Matrices (EEMs) of the CDOM collected from the different river systems we studied (Evros, Sperchios, Louros, Arachthos and Evrotas) indicate that fluorescence spectroscopy can be used as an effective monitoring tool for pollution detection and control of industry impacts on river systems (Tzortziou et al., 2008; 2009; 2012). EEMs analysis on CDOM samples collected from the Evros transboundary river revealed the presence of a major input of anthropogenic pollution between the cities of Tychero and Kipoi, associated with discharges from the Ergene River (Tzortziou et al., 2012). Many industrial plants, including leather tanning and textile industries, which normally apply treatment, discharge their wastewater into the Ergene, one of the most important tributaries flowing into the Evros River from the Turkish side. A recent ecological assessment in the Ergene River reported that the river water has severe toxicity on Vibrio fisheri. The construction of a small embankment dam upstream of the Evros delta at the beginning of the wet season (during autumn) significantly influenced freshwater discharges and carbon and nutrient dynamics along the river. Our measurements showed that, when the Evros river dam is open, the fluorescence fingerprint of the pollution input from the Ergene River persists along the full extent of the Evros River and affects the fluorescence signal of CDOM collected from the North Aegean coastal waters, suggesting the presence of strongly refractory organic carbon compounds (Tzortziou et al., 2012). Our measurements in the Sperchios River showed that in-situ deployment of advanced automated systems capable of measuring CDOM fluorescence at specific UV-visible wavelengths could be used for continuous monitoring of anthropogenic wastewater and for tracing the dispersion of effluents from paper mills. Measurements of CDOM fluorescence on samples collected from a site in the Sperchios River influenced by runoff from a paper factory revealed the presence of fluorescence centres (Exc/Em: 280/430 and 330/430), which are characteristic of compounds commonly added to laundry detergents and used during paper manufacturing (i.e. distyrl biphynl, DSBP and diaminostilbene types, DAS1 and DAS2). These results have several direct applications, and suggest that in-situ fluorescence measurements could be particularly useful in water quality monitoring and surveillance programmes that require discharges in river systems to meet national discharge standards (Tzortziou et al., in preparation).

Main results

Our chemical analyses and measurements of DOC, dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) composition and dynamics provided new data and novel information on the sources, quality, and elemental, isotopic and molecular composition of DOM in Eastern Mediterranean catchments. Our studies focused on the various biogeochemical processes taking place within the studied river systems under different hydrological conditions. The concentrations and ratios of NO3-, NH4+, PO43-, DOC, DON, DOP were evaluated on a temporal and spatial basis. Additional information on DOM composition has been provided through simultaneous measurements of carbohydrates (TCHO, PCHO), while the data on isotopic signature (d13C %o) of our samples provided insights on the importance of the various DOM sources. As discussed in our presentations by Tzortziou et al. (2010) and Pitta et al. (2010) and our two manuscripts by Zeri et al. (in review), intensive industrial and agricultural activities, and extensive use of nitrogen-based fertilisers and organophosphoric insecticides in the Evros basin result in waters enriched in DOC, DON and DOP concentrations. Elevated inorganic nitrogen loads in the Evros River, originating either from agricultural runoff (mainly as NO3-) and/or from anthropogenic inputs (mainly as NH4+), give rise to substantial autotrophic production as inferred from the high chl-a and oxygen supersautration values, especially during the summer months with low water flow. DOM composition in the Evros waters seems to be dominated by authochthonous and anthropogenic material rather than terrestrial. This is suggested by the relatively low DOC:DON and DOC:DOP ratios, the positive correlation of DOC vs chl-a and the decoupling between DOC and DON. Landsat 5 TM satellite imagery was used to develop, test and validate empirical algorithms for remote sensing retrieval of Chl-a concentrations in the Evros River. We found stronger correlation between Chl-a and the reflectance ratio TM4/TM3. Regionally specific algorithms that are based on the phytoplankton’s optical properties in the red wavelength region provide a better basis for satellite monitoring of phytoplankton blooms in these Case 2 waters (Tzortziou et al., 2009; Markogianni et al., 2012; Markogianni et al., In review).

Managing water resources, in terms of both quality and quantity, in transboundary rivers is a difficult and challenging task that requires efficient cross-border co-operation and transparency. Our measurements in the Evros river system provided detailed information on the main pollution sources and pressures in this transboundary catchment, which is shared by three countries currently having inadequate water resource management agreements. For the first time, our study applied, assessed and evaluated a groundwater pollution risk mapping technique using satellite observations (Landsat NDVI) and an extensive dataset of field measurements that cover different seasons and multiple years. We found that approximately 40 % of the Greek part of the Evros catchment is characterised as high and very high pollution risk, while 14 % of the study area is classified as moderate risk. Both modelled and measured water quality statuses of the river showed large spatiotemporal variations consistent with the strong anthropogenic pressures in this system, especially on the northern and central segments of the catchment. The pollutants identified illustrate inputs of agrochemicals and urban wastes in the river. High correlation coefficients (R between 0.79 and 0.85) were found between estimated pollution risks and measured concentrations of those pollutants that are mainly affected by anthropogenic activities rather than biogeochemical processes. The pollution risk method developed as part of our study could be used elsewhere as a decision support tool for mitigating the impact of hazardous human activities and improving the management of groundwater resources (Dimitriou et al., 2012; Dimitriou et al., in review).

The growing density of human population in European coastal regions, and especially along the Mediterranean coastline, has contributed, and will continue to contribute, to increased pressure on the environment. The research carried out here is highly relevant to European Union policymaking and research initiatives for improving our understanding and modelling of climate-related biogeochemical cycles in European coastal regions. Development of coordinated and sustained earth-observation systems and effective satellite monitoring of European seas are among the major objectives of the European Space Agency and European research in general. In order to resolve the bio-optical complexities of coastal waters from ocean colour satellite observations, studies like ECODOM are critical for understanding biogeochemical exchanges, water quality changes and ecosystem processes along the terrestrial, wetland and coastal water continuum.