In situ variability of photosynthetic quantum yield in phytoplankton assemblages from a freshwater lagoon of Southern China

VIRGINIA ESTELA VILLAFAÑE; KUNSHAN GAO,; PING LI,; GANG LI,; WALTER HELBLING, E

Aix-les-Bains, Francia

Congreso; 11th European Society for Photobiology Congress; 2005

During March-April 2005 we carried out studies to determine the daily variability in photosynthetic parameters in phytoplankton assemblages from a freshwater tropical lagoon in Southern China (23.3˚ N, 116.6˚ E).  Water samples were taken every hour within the upper mixed layer at 0, 0.5 and 1 m depth.  Additionally, a surface sample (taken at 8:30 am) was exposed to surface irradiance inside quartz tubes (i.e., fixed samples) under three radiation treatments (i.e., PAB: 280-700 nm; PA: 320-700 nm; and P: 400-400 nm).  Photosynthetic parameters were measured hourly in the lagoon and in the fixed samples using a Water-PAM fluorometer (Walz).  Solar radiation (UV-B, UV-A, and PAR) was continuously monitored with a broad-band filter radiometer (ELDONET), and attenuation in the water column was obtained with an ELDONET submersible radiometer; temperature was monitored hourly with a YSI 600XL probe.  PAR irradiance at noon varied between 250 and 450 W m-2, and the attenuation coefficient (kPAR) was 1.2 m-1.  During the study period, chl-a concentration varied between 4 and 27 µg chl-a l-1 and the community was dominated by of chlorophytes and cyanophytes.  The initial quantum yield (Y) at the surface was variable between 0.3 and 0.6, mainly due to changes in the radiation conditions (i.e., sunny vs. cloudy days).  There was a significant increase of Y with depth, suggesting that mixing was not strong and some differential acclimation occurred in the water column.  A significant decrease in Y was determined at noon in all samples, but they fully or partially recovered in the afternoon.  The fixed samples had the same daily pattern as those collected at different depths, and they were inhibited by UVR up to 75%.  No significant differences were found between fixed samples exposed to PAR and the surface samples in the lagoon throughout the day, suggesting that the slow (but significant) natural mixing was important to dilute out the impact of UVR observed in the samples.