CONICET | Buscador de Institutos y Recursos Humanos

Some of the potential outcomes of global warming are the increase of stratification in the watercolumn, changes in both ocean circulation patterns and mixing rates. These changes could induce ahigher stress in phytoplankton cells by exposing them to higher UVR levels due to their circulationwithin a shallower upper mixed layer (UML). On the other hand, it has been shown that an increase ofCO2 concentration favored phytoplankton enhancing their growth rates. Our experiments conductedin a freshwater lagoon of a tropical region of Southern China (23o 26N, 116o 42E) were designed toassess the impact of UVR on phytoplankton photosynthesis (i.e., evaluated through measurements ofphotosynthetic quantum yield, Y). Two mixing regimes (1 rotation in 10 minutes / 1 hour down to 2.1optical depths) were imposed in cells acclimated to two CO2 concentrations - ambient level and 1100ppm. UVR was the most important variable affecting Y, followed by mixing and finally by CO2concentration. During the study period (March to May, 2005) the inhibition due to UVR was > 50%,with UV-A accounting for most of the observed impact. The rate of decrease of photosyntheticquantum yield per hour of exposure was higher under low than under high mixing speed. Cellsacclimated to ambient CO2 levels were slightly (but significantly) more sensitive to UVR than thoseacclimated to high CO2, when large dinoflagellates (i.e., Ceratium spp. and Peridinium spp.)accounted for most of biomass. On the other hand, when piconanoplankton cells (< 2um) dominated,there were no differences between ambient and high CO2 acclimated cells. Our data thus clearlysuggest that the combined impact of UVR, mixing and CO2 concentration is highly dependent on thespecies composition and / or size structure of the assemblages.

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