Abstract 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 identification of Euglena sanguinea as the responsible species for the latter. Water, tissues, and maggots from dead specimens were analyzed to assess physico-chemical characteristics, pla