The perfect storm in a coastal pond: duckweed, botulism, bird mortalities and toxic blooms of Euglena sanguinea

In summer 2024, a freshwater coastal pond in northwest Spain (A Congorza, Cangas) experienced its first massive mallard ( Anas platyrhynchos ) mortalities, repeated in 2025. The pond is covered by Lemna minor and develops a red scum during summer. Molecular, pigment and morphological analyses allowed to identify Euglena sanguinea as the responsible species for the latter. Water, tissues and maggots from dead specimens were analysed to assess physico-chemical characteristics, plankton composition, clinical signs and toxins in order to explain these mortalities. Poor water quality parameters were determined during summer 2025 with suboxic dissolved oxygen (1.5–5.2 mg L⁻¹) and high levels of biological and chemical oxygen demand (17–130 and 56–535 mg L⁻¹, respectively) revealed severe degradation, failing to meet Directive EU/2024/3019 standards. Legacy persistent organic pollutants (PAHs, PCBs, OCPs, PBDEs) appeared at low, non-lethal levels. Clinical signs and analyses confirmed avian botulism (type C and C/D toxins) as the primary cause of death. LC-HRMS detected putative euglenophycin isomers in E. sanguinea pellets. Overall, these findings indicate that avian botulism is the primary cause of mallard mortalities in A Congorza. The dominance of L. minor and recurrent E. sanguinea red blooms evidence the eutrophication and severe habitat degradation potentially contributing to a multifactorial stress fatal to A. platyrhynchos .