Changes in phytoplankton morpho-functional groups induced by extreme hydroclimatic events in the Middle Paraná River

DEVERCELLI, M.

Israel

Workshop;  15th Workshop of the International Association of Phytoplankton Taxonomy and Ecology (IAP); 2008

International Association of Phytoplankton Taxonomy and Ecology

Extreme hydrological fluctuations in the South American Paraná River are unequivocally coupled with ENSO´s global climatic phenomenon. In order to find out changes on phytoplankton community triggered by extraordinary flood (November 1997-1998) and drought (November 1999-2000), the main channel of the Middle stretch was monthly analysed, and compared with normal hydrological period (November 1977-1978). The multivariate RDA showed that water discharge and its physical related factors play a stirring role on phytoplankton dynamics, and light availability acts as a selective force (p<0.01). Phytoplankton density and biovolume were lower during El Niño and higher during La Niña (p<0.01). A great development of small unicellular flagellated belonging to morpho-functional groups Y (Cryptomonas) and X2 (Chlamydomonas and Plagioselmis nannoplanktica) was observed during both periods. They were accompanied by small and weakly silicified centric diatoms (D). The high reproduction rates allow them to counteract the dilution effect, and exploit the better conditions of transparency and hydraulic stability during drought. This scenario differs significantly from that encountered during normal conditions where group P (Aulacoseira granulata) dominated. The absence of those large filaments during peaks of flood and drought could be explained by sedimentation processes, organisms’ retention by obstacles with water displacement in the plain, and a need of inocula from other environments to the main flow. Hydroclimatic anomalies provoke changes in phytoplankton structure of the Paraná River, favoring C- and CRS-strategists instead of the typical R-strategists. The community could be re-structured rapidly when the events end, but given variations persist it could be hypothesized that the magnitude and duration of extreme discharges may determine the intensity and persistence of changes. The interruption of the constant feed back between the main channel and floodplain waterbodies due to both low connectivity and inundation of alluvial valley, may govern phytoplankton dynamics in this large river.