Incompatibility of sulfate compounds and soluble bicarbonate salts in the Rio Cruces waters: an answer to the disappearance of Egeria densa and black-necked swans in a RAMSAR sanctuary

Abstract

The Carlos Anwandter Sanctuary, a RAMSAR site, is situated downstream from the junc- tion of the Rio Cruces and Rio Calle-Calle near Valdivia in Southern Chile. The Rio Cruces was a bicarbonate-rich aquatic ecosystem until January 2004, when a pulp mill began pouring >40 t of sulphate (SO4) and 6 to 9 t of H2SO4 each day into the River. Soon after, black-necked swans, which take refuge in the Sanctuary, began to die and emigrate. Previous studies showed that the food of the birds, the submerged vascular plant Egeria densa, had been eliminated from areas of the Sanctuary af- fected by the mill's effluent. Here we describe the cause of this loss of plants and birds. E. densa is a C4 plant that uses calcium bicarbonate to compensate for low concentrations of CO2 in its local environ- ment. Without calcium bicarbonate in the water, the plant photorespires, loses turgor, turns brown and dies. Here we demonstrate that the sulphate and acid dumped into the river by the pulp mill caused an anionic/cationic disequilibrium resulting in the precipitation of both Mg 2+ and Ca 2+ oxides — as brucite and portlandite — and other metallic (Fe, Al, Cu) oxides which enhanced the concentration of H + , thus perpetuating the ionic disequilibrium. Further, aqueous sulphate under light acidic conditions could precipitate gypsum, thus further removing calcium from the water. An ecologically and statistically significant loss of calcium bicarbonate (ANOVA, p < 0.05) is shown: samples of water affected by the mill's effluent contained 37% less (HCO3) - than those collected outside the area affected by the effluent. In a microcosm experiment, E. densa samples were exposed to no sulphate (4 replicates: control), 2.5 g l -1 of K2SO4 (4 replicates: Treatment 1), 4.9 g l -1 of K2SO4 (4 replicates: Treatment 2), and 9.8 g l -1 of K2SO4 (4 replicates: Treatment 3). Plants in the control microcosms produced oxygen through photosynthesis at a rate of 0.24 ml O2 g -1 h -1 . Plants in Treatment 1 produced oxygen at half the rate of the controls: 0.11 ml O2 g -1 h -1 . Plants in Treatment 2 produced oxygen at a rate of 0.003 ml O2 g -1 h -1 or 2 orders of magnitude lower than plants in the control microcosms. After only 8 h of exposure to the experimental conditions of Treatment 3, the plants produced oxygen at a rate of only 0.0001 ml O2 g -1 h -1 or 3 orders of magnitude lower than controls. Differences in oxygen production among the controls and treatments were statistically significant (ANOVA, p < 0.05).