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From November 2002 to June 2003, we carried out experiments to determine the temporal variability of P vs. E curves and the effects of solar ultraviolet radiation (UVR, 280-400 nm) on photosynthetic parameters of natural phytoplankton assemblages from Patagonia. Samples were collected at 2 sites, Bahia Nueva and Bahia Camarones (Chubut, Argentina), and exposed to solar radiation for 4 to 6 h under 3 quality radiation treatments (i.e. PAR + UVR, 280-700 nm; PAR + UV-A, 320-700 nm; and PAR only, 400-700 nm), and under 6 to 8 levels of ambient irradiance (i.e. by using 0 to 5-7 layers of neutral density screens). Samples collected at Bahia Nueva had relatively high Pmax values (~3-4 µg C (µg chl a) -1 h -1 ) during the pre-bloom period (i.e. late summer to late fall) and relatively low values (~1.5-2.5 µg C (µg chl a) -1 h -1 ) during the post-bloom period (i.e. late spring to early summer); similar results were observed in samples collected at Bahia Camarones. The light saturation parameter Ek, on the other hand, did not show a clear pattern and values ranging from 50 to 400 µmol m -2 s -1 were observed throughout the study period. High Pmax values were significantly correlated with the concentration of large diatoms (R 2 = 0.6, p < 0.05), the taxonomic group that characterized the pre-bloom period. UVR significantly reduced Pmax values (p < 0.05) during the pre- bloom but not during the post-bloom period. UVR also significantly affected Ek (p < 0.05) in all Bahia Camarones samples but only in some from Bahia Nueva. UV-A was responsible for the bulk of photosynthetic inhibition throughout the study period; the highest UV-A-induced integrated inhibi- tion values in the water column were ~16.5 and 7% for Bahia Nueva and Bahia Camarones, respec- tively. On the other hand, UV-B-induced photosynthetic inhibition reached maximum values of 2.3 and 3.9% for Bahia Nueva and Bahia Camarones, respectively. Since under certain environmental conditions P vs. E parameters can be significantly reduced by UVR, we suggest that remote sensing algorithms using these parameters should also consider the impact of UVR in their estimates of primary production.

Photosynthesis versus irradiance characteristics in phytoplankton assemblages off Patagonia (Argentina): temporal variability and solar UVR effects