Thin layers of Pseudo‐nitzschia spp. and the fate of Dinophysis acuminata during an upwelling‐downwelling cycle in a Galician RÃa

Abstract

Fine‐resolution measurements of phytoplankton and physical parameters were made from 31 May to 14 June 2005 in the Ría de Pontevedra (Spain), which is subject to seasonal upwelling. The main objective of this work was to elucidate physical‐biological interactions leading to subsurface aggregations of toxin‐producing microalgae (Pseudo‐nitzschia spp. and Dinophysis spp.). A sequence of upwelling‐relaxation‐upwelling‐downwelling events was recorded with a moored Acoustic Doppler Current Profiler (ADCP). Thin layers (TLs) of Pseudo‐nitzschia spp. and other diatoms (up to 30 µg chlorophyll a L−1) developed and persisted in the pycnocline above cooler (12.5ºC) upwelled water but were vertically displaced and even dispersed during downwelling. The establishment of steep pycnoclines after upwelling pulses and the formation of TLs of Pseudo‐nitzschia spp. and other diatoms suggest that pycnoclines may act as retention areas for these populations. Their vertical displacement during downwelling would explain different patterns observed in the contamination of benthic resources and raft‐mussels. A decimeter‐scale segregation of Pseudo‐nitzschia micans and Dinophysis acuminata populations was observed. The population of D. acuminata, present since March 2005 in Ría de Pontevedra, was never found within the pycnocline, did not perform any significant vertical migration, and was not dispersed during upwelling. Instead, it formed patches (up to 8 × 103 cell L−1) in the warmer (15‐18ºC) surface (0‐4 m) waters associated with a diurnal thermocline, and it spread throughout the ría into a near‐surface layer during relaxation and downwelling. These results demonstrate the importance of considering species‐specific behavior to predict the location of cell maxima.

Hidden thin layers of toxic diatoms in a coastal bay

“…Examples of stratified systems where blooms of Pseudo-nitzschia have been observed are in the vicinity of the San Juan Islands McManus et al, 2003), Monterey Bay (Ryan et al, 2005;McManus et al, 2008) and Ría de Pontevedra, Spain (Velo-Suárez et al, 2008). Our study site, the northeastern bight of Monterey Bay, is a highly stratified environment that fits the GEOHAB criteria for the Stratified Systems Core Research Program.…”

Vertical distribution of division rates in coastal dinoflagellate Dinophysis spp. populations: implications for modelling

“…Dinophysis spp. often exhibit sharp heterogeneities in their vertical distribution (reviewed in , Velo-Suárez et al 2008. To minimize sampling uncertainties derived from low cell concentrations, vertical migration or patchiness (which would result in samples with inadequate numbers of target cells), in situ division rate studies of Dinophysis spp.…”

Harmful Algal Blooms in Fjords, Coastal Embayments, and Stratified Systems: Recent Progress and Future Research

“…Besides, pycnoclines may act as retention areas for non-swimming phytoplankton, such as Pseudo-nitzschia and other diatoms. The respective TLs of these Pseudo-nitzschia blooms have been observed in association with steep density gradients in the Northeast Pacific Ocean and in Monterey Bay and in pycnoclines established after upwelling pulses in the Galician Rias (Velo-Suárez et al, 2008).…”