Linking Azolla filiculoides invasion to increased winter temperatures in the Donana marshland (SW Spain)

Unravelling how a multiplicity of global change factors might influence the expansion of alien plants is a major goal of invasion biology. We explored the association of climatic conditions (i.e. annual rainfall and average minimum temperature during the coldest months) and water quality (orthophosphate, nitrate concentrations and electrical conductivity), with blooms of the invasive fern Azolla filiculoides Lam. in the Donana marshland (SW Spain), one of the most extensive wetlands in Europe. Azolla was first detected in Donana in 2001. Its annual cover has been quantified since then through remote sensing. In the last decade there has been a considerable increase of orthophosphate and yearly fluctuations of nitrate. The first orthophosphate increase coincided with the presence of Azolla in Donana. Since then, there has been a positive relationship between inter- annual variation in Azolla cover and minimum temperature during the coldest months. Our study shows that the Donana marshland is facing rapid eutrophication and invasion by Azolla, most notably in years with high temperatures during the winter.