Stable isotopes in helophytes reflect anthropogenic nitrogen pollution in entry streams at the Doñana World Heritage Site

Nitrogen (N) loading from anthropogenic activities is contributing to the eutrophication and degradation of wetlands worldwide. Doñana (southwestern Spain), includes a dynamic marshland protected as a UNESCO World Heritage Site, which has a catchment area exposed to increasing N inputs from intensive agriculture and poorly treated urban wastewaters. Identifying the sources of N entering this iconic wetland complex is vital for its conservation. To this end, we combined multiyear (2014-2016), spatially-explicit data on N concentration in water samples with measurements on the relative abundance of N stable isotopes (δ15N) in Bolboschoenus maritimus and Typha domingensis, two dominant helophytes (i.e. emergent macrophytes) in the Doñana marsh and entry streams. Overall, plant tissues from entry streams showed higher δ15N values than those from the marsh, particularly in those streams most affected by urban wastewaters. Isotopic values did not differ between plant species. Water
samples affected by isotopically-enriched urban wastewaters and other diffuse organic N inputs
(e.g. livestock farming) had relatively high Dissolved Inorganic Nitrogen (DIN) concentrations.
In contrast, in streams mainly affected by diffuse N pollution from greenhouse crops, high DINvalues were related to isotopically-depleted N sources (e.g., inorganic fertilizers). Thus, helophytes, in combination with other parameters such as N concentration in water or land cover, can be valuable indicators of anthropogenic pressures in Mediterranean wetlands. Helophytes have widespread distributions, and can be readily sampled even when water is no longer present. However, identification of specific N sources through helophyte δ15N values is limited when key potential N sources are isotopically undistinguishable (e.g. fertilizers vs. atmospheric sources).