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During the late-fall, an experiment was performed in the midriff island region of the Gulf of California to study the internal wave effect on primary production and phytoplankton biomass temporal variability. A spot east of the Ángel de la Guarda Island was monitored during 48 hours, and water samples were taken for measuring pH, dissolved oxygen, inorganic nutrients (NO3 and PO4) and chlorophyll a. Photosynthesis-irradiance curves were generated for phytoplankton collected from depths corresponding to 50% and 1% light levels. With spring tides, internal waves altered significantly the water column structure, with a change in the stratification field. Nutrient concentrations at depths > 20 m increased by as much as two-fold, with addition of > 1.0 µM for PO4 and > 7.0 µM for NO3. Chlorophyll a increased by more than 40%, between the surface and the deep chlorophyll maximum, during the same period. The assimilation number, P*m, for the 50%Eo depth decreased to near 20% of its initial value, without an apparent change in this parameter for the phytoplankton collected from the 1%Eo level. Our data support the hypothesis that P*m decreased as a result of vertical advection in the area, transporting photoacclimated phytoplankton from deeper waters toward the surface. The importance of taking these phenomena into consideration when calculating primary production for such dynamic areas using remote sensing data and chlorophyll-light models is discussed.
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