Phytoplankton dynamics throughout a yearly cycle in a freshwater lagoon of Patagonia: Physiological responses to solar radiation

S BARBIERI, ELENA; J GONCALVES, RODRIGO; VIRGINIA ESTELA VILLAFAÑE; WALTER HELBLING, E

Puerto Rico

Congreso; 33rd Annual Meeting of the American Society for Photobiology; 2006

A time series study was carried out in the temperate Chiquichano lagoon in Patagonia, Argentina (43o14S, 65o 18W) from February 2005 to February 2006. The aim of the study was to determine thedynamics of phytoplankton as well as the impact of solar radiation on photosynthetic efficiency (whichwas determined using a pulse amplitude modulated Water-PAM fluorometer) and pigments contents.Samples for experimentation and other determinations (e.g., CDOM, nutrients, species composition)were collected every two weeks, the evening before experimentation, pre-filtered to removezooplankton (<100 um), and kept in dark (18 oC) throughout the night until the following morning.Then, three radiation treatments (duplicates) were implemented: a) PAB (280-700 nm), uncoveredquartz tubes; b) PA (320-700 nm), tubes covered with a cut-off filter at 320 nm, and c) P (395-700nm), tubes covered with cut-off filter at 395 nm. The samples were exposed to solar radiation from 9AM to 5 PM, inside a water-bath (for temperature control), and every hour photosynthetic parameterswere measured; also samples for dark recovery were taken at noon and at the end of the experimentand measured every hour until 8 PM, being the last measurement the following morning.Phytoplankton dynamics and concentration thoroughout the year was strongly controlled byzooplankton (i.e., Daphnia menucoensis) grazing so that Chl-a concentration varied from > 800 ugL-1 during periods when its abundance was low, to about 4 ug L-1 when D. menucoensis dominatedthe zooplankton population. There was a significant impact of UVR on photosynthesis throughout theyear, but samples collected during fall and winter (i.e., when KdPAR was low) were more sensitivethan in other seasons. A significant recovery was observed in darkness for all radiation treatmentsand samples, but it was not complete even after 20 hours. Chronic damage varied between 2 and85% of the original yield value, and this variation was a function of the previous exposure to solarradiation.