Fine scale physical-biological interactions during a shift from relaxation to upwelling with a focus on Dinophysis acuminata and its potential ciliate prey
19 pages, 11 figures, 2 tables.-- This is an open access article under the CC BY-NC-ND licenseWind 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-Rías” (17–21 June 2013) was carried out in the Galician Rías 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 °C isotherms and turbulence levels (ε) of
10−8–10−6m2 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. acuminata, 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 (μ)
of D. acuminata was associated with increased turbulence (ε < 10−4m2 s−3) near the surface and a sharp drop
of temperature (> 2°C in about 8 h). In contrast, high division rates (μmin∼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.This work and the sampling cruise were funded by project ASIMUTH
(EC FP7-SPACE-2010-1 grant agreement number 261860). Additional
support came from Spanish project REMEDIOS (MINECO, Programa
RETOS, CTM2016-75451-C2-2-R), and the EU Interreg Atlantic Area
PRIMROSE (EAPA_182/2016), an IOC-SCOR GlobalHAB endorsed
project. Patricio A. Díaz had a PhD student fellowship from BECAS–CHILE, National Commission for Scientific and Technological
Research (CONICYT) and is now funded by projects PAI79160065 (The
Attraction and Insertion of Advanced Human Capital Program) and
REDES170101 (International Cooperation Programme), CONICYT,
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