Phytoplankton dynamics throughout a yearly cycle in a freshwater lagoon of Patagonia: Physiological

BARBIERI, E.S.; GONÇALVES, R.J.; VILLAFAÑE, V. E.; HELBLING, E.W.

Río Grande, Puerto Rico

Congreso; ASP 33rd Biennial Meeting; 2006

A time series study was carried out in the temperate Chiquichano lagoon in Patagonia, Argentina (43o 14S, 65o 18W) from February 2005 to February 2006. The aim of the study was to determine the dynamics of phytoplankton as well as the impact of solar radiation on photosynthetic efficiency (which was 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 remove zooplankton (<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), uncovered quartz tubes; b) PA (320-700 nm), tubes covered with a cut-off filter at 320 nm, and c) P (395-700 nm), tubes covered with cut-off filter at 395 nm. The samples were exposed to solar radiation from 9 AM to 5 PM, inside a water-bath (for temperature control), and every hour photosynthetic parameters were measured; also samples for dark recovery were taken at noon and at the end of the experiment and measured every hour until 8 PM, being the last measurement the following morning. Phytoplankton dynamics and concentration thoroughout the year was strongly controlled by zooplankton (i.e., Daphnia menucoensis) grazing so that Chl-a concentration varied from > 800 ug L-1 during periods when its abundance was low, to about 4 ug L-1 when D. menucoensis dominated the zooplankton population. There was a significant impact of UVR on photosynthesis throughout the year, but samples collected during fall and winter (i.e., when KdPAR was low) were more sensitive than in other seasons. A significant recovery was observed in darkness for all radiation treatments and samples, but it was not complete even after 20 hours. Chronic damage varied between 2 and 85% of the original yield value, and this variation was a function of the previous exposure to solar radiation.