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

Abstract

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, Bahía Nueva and Bahía 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 Bahía 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 Bahía 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 (R2 = 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 Bahía Camarones samples but only in some from Bahía Nueva. UV-A was responsible for the bulk of photosynthetic inhibition throughout the study period; the highest UV-A-induced integrated inhibition values in the water column were ~16.5 and 7% for Bahía Nueva and Bahía Camarones, respectively. On the other hand, UV-B-induced photosynthetic inhibition reached maximum values of 2.3 and 3.9% for Bahía Nueva and Bahía 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.