Susceptibility of bacterioplankton in ultraoligotrophic Patagonian lakes to nutrient and substrate addition.

ROBERTO BERTONI,; CRISTIANA CALLIERI,; E BALSEIRO,; BEATRIZ ESTELA MODENUTTI O BEATRIZ MODENUTTI O B. MODENUTTI

Palermo Italia

Congreso; SEFS5; 2007

SEF

We measured bacterioplankton activity in four North Patagonian lakes during austral summer 2006. These lakes are ultraoligotrophic with TDP concentration ranging 1.46-2.27 µg L-1. They are deep (90 - 460 m max depth range), clear (kd PAR 0.09 ? 0.16 m-1) with  a pronounced DCM. The trophic chain is mainly made up by large mixotrophic ciliates, nanoflagellates and picoplankton. For this reason the bacterioplankton predation constitute a large portion of the energy tranfer within the trophic chain. The range of leucine uptake was 10-50 pmoles Leu L-1 h-1 showing a reduced metabolic activity also in productive season. We made laboratory experiments with natural samples from Lake Gutierrez, limited by phosphorus and organic carbon. We amended natural samples with ortophosphate (P), organic carbon as glucose (C) and both (PC), keeping the natural C:P ratio. We  followed  in the short time the growth and bacterial activity variation to undestrand the importance of  P and C addition as heterotrophic activity enhancer. The experiments were performed in spring (November) and summer (February) at different temperatures (12°C and 17°C respectively) and different bacterial metabolic status ( 23 ? 64 pmoles Leu L-1 h-1 respectively). In the spring experiment we did not observe an increase in growth rate after 24 hours for all the treatments but the leucine uptake  increased significantly in the PC treatment (from 23 to 136 pmoles leu L-1 h-1). In the summer experiment an increase in growth was observed already after 4 hours. After 24 hours a noticeable increase in leucine uptake became evident in all the treatments (from 64 to 1084 (P), 1071 (C), 1364 (PC) pmoles Leu L-1 h-1). These results indicate the presence, in summer, of a bacterial population with low metabolic rate but potentially able to respond quickly to nutrient/substrate addition. On the other hand, the spring population were more abundant and more resilient to medium alteration. The low activity of  bacterioplankton  of andine ultraoligotrophic lakes is not an inherent property of such population but it can be altered in the short-term when nutrien/substrate pulse is applied in summer condition, i.e when they are metabolically ready to react.