Effects of hydroclimatic anomalies on bacteriological quality of the Middle Paraná River (Santa Fe, Argentina).

Lenitic and lotic aquatic environments were studied in the Middle Paraná River basin, one of the largest rivers of the world (basin: 1,510,000 km2, length: 2,570 km). The Paraná River (S 31 degrees 42' 04"; W 60 degrees 29' 39"), during El Niño-Southern Oscillation (ENSO) (1997-1998), registered maximum flows of 29,962 m3 s(-1) (more than twice the mean flow) and also positive pluvial anomalies (A) (from +120 mm to +161 mm). However, concentrations of Escherichia coli remained low, with values similar to the mean (p > 0.05) (1.3802 log10cfu/100 ml). Only a maximum peak (1.903 log10cfu/100 ml), significantly higher than the mean (p < 0.0001), was detected during the ascending phase of the flood pulse. During La Niña (September 1999-March 2000; Q = 11,255 m3 s(-1); A = -78 mm to -84 mm), concentrations of E. coli were significantly lower than the mean (p < 0.0001). During the post-Niña period, with normal flows (Q = 14,900 m3 s(-1)) but with strong positive pluvial anomalies (A = +118 mm to +377 mm), the fecal indicator density reached maximum values (up to 2.699 log10cfu/100 ml, p < 0.0001). The multiple regression model used with all the results obtained in the Paraná River showed that variations in concentrations of E. coli are mainly explained by pluvial precipitations and hydrometric level (p = 0.001). Temporal patterns of concentrations of E. coli in a small, secondary course (Q = 101 m3 s(-1)) followed those of the Paraná River, but concentrations and peaks of E. coli produced by exceptional rains showed a higher magnitude (maximum: 3.325 log10cfu/100 ml). In the floodplain lakes, away from urban centers, concentrations of E. coli were not associated to hydroclimatic anomalies, but directly to vegetable cover: maximum concentrations were always registered during periods that lakes remained covered by floating macrophytes (mainly Eichhornia crassipes); on the contrary, minimum concentrations were registered during periods without vegetable cover.