Abstract

Wind reversals and quick transitions from relaxation to upwelling in coastal areas cause major changes in water column structure, phytoplankton distribution and dominance, and rates of physiological processes. The cruise "ASIMUTH-Rias" (17-21 June 2013) was carried out in the Galician Rias and adjacent shelf, at the time of a DSP outbreak, to study small-scale physical processes associated with late spring blooms of D. acuminata and accompanying microzooplanktonic ciliates with the overall objective of improving predictive models of their occurrence. The cruise coincided with the initiation of an upwelling pulse following relaxation and deepening of a previously formed thin layer of diatoms. A 36-h cell cycle study carried on 18-20 June showed the vertical excursions of the thin layer, mainly delimited by the 13.5-14 degrees C isotherms and turbulence levels (epsilon) of 10(-8)-10(-6)m(2)s(-3), as well as marked changes in phytoplankton composition (increased density and dominance of diatoms). There was no evidence of daily vertical migration of D. acuminate, which remained in the top layer during the cycle study, but the opposite was observed in the ciliate populations. Dinophysis and its potential prey (Mesodinium species) cell maxima overlapped after midday, when the ciliate moved to the surface, suggesting an "ambush" strategy of Dinophysis to catch prey. A remarkable decline (from 0.65 to 0.33 d(-1) ) in division rates (mu) of D. acuminate was associated with increased turbulence (epsilon < 10(-4 )m(2)s(-3)) near the surface and a sharp drop of temperature ( > 2 degrees C in about 8 h). In contrast, high division rates (mu(min) similar to 0.69 d(-1)) persisted at a mid-shelf station where environmental conditions below the mixed layer were more stable. The onset of upwelling pulses appears to have a double negative effect on the net growth of Dinophysis populations: a direct physical effect due to advective dispersion and an indirect effect, decreased division rates. The latter would be caused by the rapid cooling of the mixed layer, and the increased turbulence at the surface resulting in shear stress to the cells. The short-term impact of upwelling pulses (and the winds promoting it) on the physiology of Dinophysis and its ciliate prey, and the role of mid-shelf populations of Dinophysis as a relatively undisturbed reservoir for the inoculation of subsequent blooms are discussed.