HABs in coastal upwelling systems: Insights from an exceptional red tide of the toxigenic dinoflagellate Alexandrium minutum

Alexandrium minutum blooms generally occur in semi-enclosed sites such as estuaries, harbours and lagoons, where enhanced stratification, restricted circulation accumulation of resting cysts the sediment set suitable habitat conditions for proliferation this paralytic shellfish poisoning toxigenic species. In Galician Rías Baixas (NW Iberian Peninsula), according to weekly time-series between 1994 2020, A. were recurrent small, shallow estuarine bays inside de Vigo Pontevedra, but rarely detected, if so at low concentrations, out these environments. However, from May July 2018 it developed usual small inner then spread over both (Vigo Pontevedra) causing discoloured waters during one month prolonged harvesting closures. Meteorological that period (rains / runoff higher than climatological averages, sustained temperature increment oscillating wind pattern –i.e., series upwelling-relaxation cycles), fostered optimal circumstances development extensive massive proliferation: strong vertical stratification alternation retention dispersion processes. Simulations a particle tracking model portrayed observed bloom phases: onset bay; transport within surface layer, towards interior parts Ría; all embayment. Seedbeds with high concentrations detected several months after bloom, which may have favoured flourishment following two years, markedly 2020. The present work contributes general understanding dynamics harmful algal (HABs), surveillance indicators state marine ecosystems their evolution can be derived. We hypothesize intensity frequency proliferations could increase under projected climate trends.