Influence of Guadalquivir estuary on aquatic biogeochemistry and isotopic signature of the saltmarshes of Doñana National Park (SW Spain)

Doñana National Park (DNP) hosts one of the most important wetlands in Europe. In 1998, the release of a toxic sludge from storage ponds at a nearby mine, led to the construction of a dyke on the natural levee of the Guadalquivir river, to protect the vast Doñana marshland against the contaminated tailings. During almost two decades, hydrology in the marshes mostly depended on the rainfall regime and to a lesser extent on an underground aquifer. Over 2014, several actions were carried out to restore connectivity with the river, allowing the entry of estuarine waters. This work presents biogeochemical and isotopic data collected during spring-summer 2017 in the flooded marshes and in the estuary. The aquatic biogeochemistry of both systems was analysed to assess the effect of the permeability actions on the biogeochemical status of the marshes. Nitrogen and carbon stable isotopes ratios were examined in water samples and, for the first time, in saltmarsh plants, to assess the main nitrogen sources. Hence, this study provides a preliminary analysis on vegetation isotope signature and its suitability as nitrogen tracer. Additionally, meteorological data and Remote Sensing NDWI-images were collected to observe the hydrodynamic variations over the 2016-2017 hydrological cycle. Our findings suggested that the flooding regime in the marshland is still driven by rainfall, although the entry of estuarine waters in Doñana marshes was evidenced through both remote sensing and in situ biogeochemical data. Isotopic measurements identified human and animal waste as main sources of nitrogenous nutrients. On the other hand, water C stable isotopes signature reflected the mesoeuthrophic–hypereutrophic conditions of the estuary.