Resumen de From rice fields to seabirds: An integrative approach to study mercury dynamics in the Ebro Delta
Mercury pollution is of great concern for the scientific community due to their ubiquity and persistence in the environment. Its bioaccumulation and biomagnification capacities along food webs enhance their toxicity both in humans and wildlife. The Ebro Delta is one of the most important wetlands in the NW Mediterranean, both as a rice cultivation area and a biodiversity hotspot. The activity of an historical chlor-alkali plant upstream the Ebro river mouth has increased anthropogenic-derived mercury within its delta and neighboring coastal areas. Industrial operations ceased in 2017, but legacy mercury is still impacting the Ebro ecosystems. Moreover, remediation plans included the dredging of toxic sediments were performed in 2013- 2014, although the mid-term impact of such activities have never been evaluated in depth. In rice fields, mercury sinkage and methylation have been reported to occur. Also, mercury reaching coastal areas can be incorporated in local food webs reaching humans and seabird species breeding there. Monitoring mercury dynamics in these environments should be a priority from both food safety and ecosystem health perspective. In the present thesis, we used an integrative approach to study mercury pollution in the Ebro Delta and marine coastal areas. First, we evaluated mercury dynamics in rice paddies and addressed how different agricultural management practices modify metal dynamics in its different components, from soil to biota. Moreover, an in-situ mesocosm experiment was performed to evaluate the bioaccumulation potential of mercury in the rice paddy environment. In addition, we examined the potential of Audouin’s gull as a model organism to trace trends in legacy pollution in a 15 year series and the effects of soil remediation operations at the chlor-alkali site on mercury bioavailability. To further examine the mercury sources and its temporal variability, we used mercury stable isotopes to relate metal body burdens in seabirds species to mercury sources in relevant foraging areas (ricefields, mesopelagic and epipelagic marine environment). Our results indicate that legacy pollution is still ongoing in the Ebro Delta and stressed the need to continue studying rice fields as mercury cuvettes for mercury speciation and bioavailability, and the ultimate factors influencing mercury dynamics in such environments. Finally, we strongly recommend a continuous monitoring program using sensitive organisms to follow-up mercury fluctuations and the use of mercury stable isotopes, a promising tool to deepen the understanding of pollution in complex environments.
