Sensitivity to solar radiation of summer phytoplankton assemblages from mid-latitudes of Patagonia

WALTER HELBLING, E; ALEJANDRA MARCOVAL, M; PAUL JANKNEGT,; MARCO DE GRAAFF,; RONALD VISSER,; J BUMA, ANITA G; VIRGINIA ESTELA VILLAFAÑE

Bath

Congreso; 12th Congress of the European Society for Photobiology; 2007

At mid-latitudes, along the Patagonian coast, summer marine phytoplankton assemblages are normally exposed to relatively high levels ofsolar radiation, especially to UVR (280-400 nm), due to a combination of low solar zenith angles, high heliophany and thermal stratificationof the water column. Under this particular radiation climate, results from short term experiments (3-6 hs) indicated that these summerphytoplankton assemblages, mainly dominated by small monads and flagellates, are comparatively more resistant to UVR exposure - interms of photosynthetic inhibition - than those from winter (dominated by large diatoms). Intensive studies carried out during summerfurther corroborated that UVR-induced photoinhibition is clearly dependant on species composition and size structure of the community.This size-dependence of UVR-induced damage was also observed in some Patagonian sites where high CPDs (cyclobutane pyrimidinedimers) levels were found in picoplankton but not in microplankton. We also evaluated the interactive effects of solar radiation and nutrientaddition during long term experiments (1-2 weeks). Nutrient addition resulted in an overall enhancement of growth rates as well as adecrease of photosynthetic inhibition. In these experiments, a shift in the dominance of species was also noticed, suggesting a selectiontowards more tolerant / less sensitive ones. In fact, at the beginning of experiments, phytoplankton communities were generally dominatedby monads and flagellates but by the end, diatoms comprised the bulk of biomass, with only one to four taxa dominating. The effects of solarradiation and nutrient addition were also assessed in diverse photosynthetic parameters of native species exposed to summer radiationconditions. It was seen that dinoflagellates were less inhibited by solar UVR than diatoms; furthermore, dinoflagellates synthesized greatamounts of UV-absorbing compounds, providing some protection to cells. Overall, we conclude that, in summer phytoplankton assemblagesof Patagonia, the responses to UVR depend on several factors but mostly on the taxonomic composition and size structure and nutrientsavailability.