Changes in distribution of phytoplankton functional groups distribution, pigment composition and the realized niche of Dinophysis acuminata at the onset of an upwelling event

Dinophysis acuminata is the major cause of lengthy harvesting bans in shellfish growing areas in European Atlantic waters. In North Western Iberian shelf waters, the growing season of D. acuminata lasts throughout the upwelling season (ca. from March to September). Once environmental conditions trigger initial population growth, abundance fluctuations are coupled to the event-scale dynamics of coastal upwelling-relaxation/downwelling cycles. The “ASIMUTH-Rias” cruise (17–21 June 2013) in the Galician Rias (Pontevedra and Vigo) and adjacent shelf, during a DSP outbreak, aimed to explore small-scale physical processes associated with late spring blooms (> 103 cells L-1) of D. acuminata. The cruise coincided with the initiation of an upwelling pulse following relaxation and deepening of a previously formed thin layer of centric colony-forming diatoms. In this work, a niche (sensu Hutchinson) approach based on an Outlying Mean Index (OMI) analysis was used to describe the realized niche of D. acuminata during transient conditions. The OMI analysis was applied to fine-scale measurements of physical properties, HPLC-derived pigment composition, and phytoplankton functional groups in cross-shelf transects and in a 36-h study at a fixed station. Although having a wide niche breath, the realized niche of D. acuminata (cell maximum 5 ×103 cells L-1 between 3 and 5 m) was characterized by outflowing warmer (15-16 °C) waters with high turbulence (? < 10-6 m2 s-3), high light intensity, and low nitrate concentrations. These results show that during a transient upwelling-initiation scenario, mid-shelf waters provided a more suitable habitat for D. acuminata and the accompanying dinoflagellate populations than waters inside the ría