Changes in phytoplankton morpho-functional groups induced by extreme hydroclimatic events in the Middle Parana River (Argentina)

The extreme hydrological fluctuations in the South American Parana River are unequivocally coupled with El Nio-Southern Oscillation's (ENSO's) global climatic phenomenon. In order to identify phytoplankton changes triggered by periods of extraordinary flood (November 1997-1998) and drought (November 1999-2000), the main channel of the Middle stretch was monthly analysed and compared with neutral hydrological conditions (November 1977-1978). These results show that water discharge plays a stirring role on phytoplankton dynamics, and that light availability acts as an important selective force. Phytoplankton transport, density, and biovolume were lower during El Nio and higher during La Nia (P < 0.01). A great development of small flagellate unicells belonging to morpho-functional groups Y (Cryptomonas spp.) and X2 (Chlamydomonas spp., Plagioselmis nannoplanctica and small Cryptophyceae) prevailed during both anomalous periods, accompanied by small centric diatoms and Skeletonema cf. potamos (D). This scenario significantly differs from those encountered during neutral conditions, where codon P (Aulacoseira granulata) dominated. The absence of these large filaments during drought and flood peaks could be explained by sedimentation processes in low waters, and by organisms' retention by obstacles after water displacement from the plain in high waters periods; however, the disruption of the inocula from floodplain areas to the main flow may be the most important factor in both periods. The hydroclimatic anomalies provoked changes in the phytoplankton structure of the Parana River, favouring X2 and Y functional groups instead of the typical P and C. The interruption of the constant feedback between the main channel and the floodplain water bodies due to low connectivity during droughts and the homogeneity of environment during floods, govern phytoplankton dynamics in this large river. The rapid occurrence of community reorganization after the conclusion of these events and the persistence of certain variations lead to the hypothesis that the magnitude and duration of extreme water discharges will determine the intensity and duration of changes.