Primary production of different phytoplankton size fractions in Andean oligotrophic shallow lakes: the influence of light gradients

MODENUTTI, B.; M. BASTIDAS NAVARRO; C. CALLIERI; E. BALSEIRO; R. BERTONI

Congreso; Shallow Lake Conference; 2008

We studied lake production and the contribution of different phytoplanktonic size fractions (picophytoplankton < 2 μm, and phytoplankton > 2 μm) in three shallow oligotrophic Andean lakes (Lakes Escondido, Morenito and El Trébol). In these environments dissolved substances are the major light-absorbing components. Since dissolved organic matter interacts with light climate, we aimed to study how the different phytoplanktonic size fractions were affected. We carried out field and laboratory experiments. In the field we carried out in-lake experiments in which we incubated lake water (quartz tubes at three different depths (0 m, 3 m and 6 m in Lake Escondido and 0 m, 4 m and 8 m in lakes Morenito and El Trébol) for Primary Production estimations (H14CO3 uptake and in situ incubation). In the laboratory we incubated lake water under ultraviolet light (UVA-340 fluorescent tubes Q-Panel) or kept in darkness for 4 hours. In this case we measured electron transfer rate with a Water Pam. The phytoplankton > 2 μm was composed mainly by mixotrophic species of Chrysophyceae and Cryptophyceae as dominant groups, although in Lake El Trébol the fraction was dominated by Bacillariophyceae. Picophytoplankton was composed by Synechococcus like cells with a dominance of phycoerithin-rich cells, nevertheless a small increase of phycocyanin-rich cells was observedin LakeEscondido. The decrease in the phycoerithrin-phycocyanin ratio observed in the latter would be related with the underwater light sprectrum affected by the abundant dissolved organic substances acting as an absorption filter relatively transparent to the red edge of the spectrum. Our in-lake experimentsindicated that PAR was sufficient to support photosynthesis at any depth, almost up to lake bottom since both phytoplanktonic size fractions were able to produce at all incubation depths. Picophytoplankton was photosynthetically more efficient than the larger autotrophs in all lakes. However, we observed a decreased in the photosynthetic efficiency of both fraction at surface incubations, probably as a consequence of the high irradiances (PAR and UVR) recorded during the 4 hours of incubation. Laboratory experiments also showedthat ultraviolet radiation has a strong influence in the electron transfer efficiency (ETR) of mixotrophic algae.