Effects of Heavy Metal Contamination (Cr, Cu, Pb, Cd) and Eutrophication on Zooplankton in the Lower Basin of the Salado River (Argentina)

The effects of heavy metal contamination (Cr, Cu, Pb, Cd) in the lower basin of the Salado River (Argentina) were studied on the zooplanktonic community. The determination of heavy metals in water and sediments was carried out in a previous study. Zooplankton was analyzed quali- and quantitatively. Total density, by-group density (Copepoda, Cladocera and Rotifera), micro and mesozooplankton density, biomass, species richness (S), and species diversity (H) were studied. The results showed that total density of zooplankton was significantly higher in the river than in the channels and streams (p < 0.001), with dominance of rotifers but a higher copepod biomass. Calanoida dominated over Ciclopoidea and Harpacticoida. Total species richness was 74, showing the highest values (59 and 56) at the points corresponding to the Salado River at localities Manucho and San Justo (MSR, SJSR) and the lowest ones in North and South channels (NCH, SCH), with 16 and 17 species, respectively), and in the two sampling stations of Las Prusianas stream (LP1, LP2), between 13 and 38 species. The species diversity showed low values (1.8 to 2.3) in channels and streams, and higher values (3.0) in the Salado River, at Manucho and San Justo. Absolute biomass varied in the order SJSR > MSR > LP1 > NCH > SCH > LP2, similarly to absolute density, which varied in the order SJSR > MSR > LP1 > NCH > SCH > LP2. The comparison of the content of heavy metals in water between the control site (SJSR) and the most contaminated sites showed significant differences with the North and Las Prusianas 1 and 2 channels (ANOVA p = 0.001; 0.012 and 0.011, respectively) and non-significant differences, although close to the significance level, with the South Channel and Manucho (p = 0.08; p = 0.059). The following positive correlations were found: depth with mesozooplankton density, H and S (p < 0.001); temperature with microzooplankton density, H and S (p < 0.004), and dissolved oxygen with mesozooplankton density, H and S (p < 0.01), but not with microzooplankton, indicating a higher tolerance of the organisms belonging to this fraction. A negative correlation was found between biomass of copepods and concentration of Pb and Cu (p < 0.05 and p = 0.01, respectively). Rotifers were the most tolerant to heavy metal contamination, followed by copepods and cladocerans. Diversity (H) and richness (S) were good indicators of stress of contaminated systems. The clustering of biological variables and the concentration of heavy metals in water and sediments showed three groups of environments: the first one was the main course of the river, with lower contamination by heavy metals and higher density, biomass, H and S, which separated clearly from the other two groups of the tributaries, composed by channels and streams. In the tributaries, r strategists and a few tolerant species, such as Eucyclops neumani, proliferated. The results of this study show that zooplankton responds as good descriptor of water quality, constituting an efficient tool to assess heavy metal contamination.