In situ responses of phytoplankton from the subtropical Lake La Angostura (Tucumán, Argentina) in relation to solar ultraviolet radiation exposure and mixing conditions

WALTER HELBLING, E; EUGENIA FARIAS, MARIA; FERNANDEZ ZENOFF, VERONICA; VIRGINIA ESTELA VILLAFAÑE

HYDROBIOLOGIA

Año: 2006 vol. 559 p. 123 - 134

0018-8158

In situ experiments were conducted at various depths in the water column to determine the effects of solarultraviolet radiation (UVR, 280–400 nm) on photosynthesis of natural phytoplankton assemblages fromthe subtropical Lake La Angostura (Argentina, 2645¢ S; 6537 W, 1980 m asl.). Water samples weretaken daily and incubated under three radiation treatments: (a) Samples exposed to UVR + PhotosyntheticAvailable Radiation (PAR) – PAB treatment (280–700 nm); (b) Samples exposed to ultraviolet-Aradiation (UV-A) + PAR – PA treatment (320–700 nm), and, (c) Samples exposed to PAR only – Ptreatment (400–700 nm). Additionally, depth profiles were done to determine different physical (i.e.,temperature and underwater radiation field) and biological characteristics of the water column – photosyntheticpigments, UV-absorbing compounds, cell concentration, deoxyribonucleic acid (DNA) andcyclobutane pyrimidine dimers (CPDs). The effects of UVR on natural phytoplankton assemblages weresignificant only in the first 50 cm of the water column, causing a decrease in photosynthetic rates of 36 and20% due to UV-A and ultraviolet-B radiation (UV-B), respectively; below this depth, however, there wereno significant differences between radiation treatments. Concentration of CPDs per mega base of DNA innatural phytoplankton was low, <27 CPDs MB)1 between 0 and 4 m. Data on net DNA damage, togetherwith that on mixing conditions of the water column, suggest that mixing can favour phytoplankton byallowing cells to be transported to depths where active repair can take place. This mechanism to reduceUVR-induced DNA damage would be of great advantage for these assemblages dominated by smallcyanobacteria and chlorophytes where UV-absorbing compounds that could act as sunscreens are virtuallyabsent.