Impact of El Niño-Southern Oscillation on plankton ciliates from a saline lowland river in South America

Abstract

We analyzed the structure and dynamics of plankton ciliates from the Salado River, Buenos Aires (Argentina), during El Niño-Southern Oscillation events, to evaluate their response to alterations of certain environmental variables during these climatic phenomena. The basin exhibits intense land use at the headwaters and fluctuating salinity along the main channel, canals and tributaries. During El Niño (1997-1998), heavy rainfall caused high discharge, and low mean conductivity values were recorded along with low mean abundances in the entire basin. During neutral (2004-2005) and La Niña (2007-2009) events, the discharge was reduced, and mean conductivity was double the values recorded during El Niño, with maxima of mean abundances and diversity at the headwaters and the lower basin. The most abundant ciliates were tintinnids, heterotrichs and peritrichs. Species replacement was detected along the river system, mainly during the neutral event. At the headwaters and canals, abundance peaks of certain species were attributed to the strong human impact (nutrients and sewage inputs) and increased salinity, allowing the establishment of euryhaline ciliates. In the middle basin, freshwater species predominated, although less abundantly due to higher water flow. The mouth sector showed abundance peaks of an estuarine tintinnid. Temperature, conductivity and suspended solids were the most important variables driving ciliate structure. High discharge as a result of heavy rainfall during El Niño negatively influenced ciliate assemblages. In contrast, low water conditions during the other events allowed the development of diverse assemblages, increasing species number and abundance of particular species.