Formation of thin layers of phytoplankton in the upwelling region off NW Iberia: biological growth versus physical accumulation

Thin layers of phytoplankton (TLP) are a particular case of blooms characterized by high cell densities within a narrow depth interval. Despite TLP may play an important role in the formation of harmful algae blooms (HABs), we have a limited understanding about the mechanisms responsible for their formation in the Rías Baixas (NW Spain), one of the main producers of cultured mussels in the world. In this investigation we studied the spatial and temporal variability of TLP by means of high-frequency observations of thermohaline properties, turbulent mixing, horizontal currents, inorganic nutrients, phytoplankton biomass and composition, and primary production in the Rías Baixas in summer 2018. Sharp changes in phytoplankton distribution were observed during the three-weeks cruise, when hydrographic properties rapidly evolve from downwelling to upwelling conditions. The formation of a TLP, centered at the 14.3ºC isotherm, was associated with the upwelling of cold nutrient-rich waters, which evolved to a subsurface chlorophyll maximum at the end of the cruise. Phytoplankton composition in the TLP was dominated by an assemblage of diatoms, including Pseudo-nitzschia species, Leptocylindrus danicus, Thalassionema nitzschioides, Chaetoceros lorenzianum and Cerataulina pelagica. A net accumulation rate of 0.7 d^-1, which resulted from the combination of physical and biological processes, was derived from the temporal evolution of chlorophyll-a at the TLP. This value was within the range of phytoplankton net growth rates derived from ¹⁴C-primary production measurements for the same period [0.1-1.0 d^-1]. Although other biological loss terms, such as zooplankton grazing and cells sinking, should be considered, this preliminary analysis indicates that in situ growth may account by itself for the phytoplankton biomass accumulation observed in the thin layer.