Vertical distribution of zooplankton biomass during internal tidal forcing under mesoscale conditions of upwelling and relaxation

The vertical distribution and variability of zooplankton biomass were evaluated during a period of internal tides, when mesoscale conditions were changing from upwelling to relaxation. The distribution of zooplankton biomass was estimated using ADCP backscatter by comparing with in situ vertically stratified net hauls. Overall, the greatest concentration of zooplankton biomass occurred in the deeper layers and during the upwelling period. Wavelet spectral analysis confirmed a dominant semidiurnal internal tidal signal in temperature, in currents and in estimated zooplankton biomass at most depths regardless of mesoscale conditions. The semidiurnal signal in biomass was stronger at depth than at the surface. The shoaling and sinking of the thermocline modulated the distribution of zooplankton biomass during the internal tide for both mesoscale periods. During the cold phase of the internal tide, when the thermocline shoaled, zooplankton biomass increased in shallow waters and decreased at depth, with the reverse found during the warm phase, with the depression of the thermocline. However, zooplankton biomass also showed a diurnal signal in surface layers during the upwelling period, which was correlated with the intensity of the wind when the thermocline was shallower. This study showed, in this coastal area, that the distribution of zooplankton biomass was partly modulated by the depth of the thermocline at a semidiurnal frequency due to the internal tide, but was also further affected by upwelling-favorable winds modulating the thermocline depth over longer time scales.