Impact of ultraviolet radiation on aquatic ecosystems of Patagonia, Argentina

WALTER HELBLING, E; S BARBIERI, ELENA; ALEJANDRA MARCOVAL, M; VIRGINIA ESTELA VILLAFAÑE

La Plata

Congreso; VIII Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2004

The Patagonia area is located in close proximity to the Antarctic continent and thus receiving enhanced UV-B radiation (280 - 315 nm) in addition to the normal UVR and PAR levels.  In marine ecosystems, it was found that normal UVR (280 - 400 nm) does affect phytoplankton assemblages during the three phases of the annual succession: a) pre-bloom season (i.e., late summer-fall), b) bloom season (winter / early spring), and c) post-bloom season (late spring-summer).  Small-size cells characterize the pre- and post-bloom communities, which have a relatively high photosynthetic inhibition due to UVR.  During the bloom, characterized by microplankton diatoms, photosynthetic inhibition is low because of the low UVR levels during winter; this community however, is more sensitive to UVR when estimated through biological weighting functions.  In situ studies have shown that UVR significantly affects not only photosynthesis but also the DNA molecule, but these negative effects are rapidly reduced in the water column.  We have also found that UVR affects photosynthesis versus irradiance (P vs E) parameters in natural phytoplankton assemblages, so that UVR reduced Pmax values during the pre- but not during the post-bloom.  However, there was a significant temporal variability of P vs. E parameters, which were greatly influenced by the nutrient status of cells and taxonomic composition, and in turn associated to the stratification conditions (e.g., wind speed and duration).  In our study area, wind speed is one of the most important variables that condition the development of the winter bloom by regulating the depth of the upper mixed layer (UML) and hence the mean irradiance received by cells.  Studies on the interactive effects of UVR and mixing highlighted the fact that phytoplankton responses to UVR vary according to the assemblage (i.e., taxonomic composition and cell structure) as well as to the depth of the UML, with cells using UVR when mixing is deep.  We also carried out long term experiments to determine the effects of solar UVR on taxonomic composition, and we have found that UVR can shape the diatom community structure in some assemblages of coastal waters.  For comparative purposes, we carried out experiments in different Andean lakes, and it was found that when exposed to UVR, microplanktonic cells had higher photosynthesis inhibition rates than pico- nanoplanktonic cells.  Overall, our studies indicate that cell size plays a very important role when estimating the impact of UVR on phytoplankton, with large cells being more sensitive when determining photosynthesis inhibition, whereas small cells are more sensitive to DNA damage.  Contribution N° 61 of EFPU.  Supported by ANPCyT PICT 2000-8184.