A limnological study of an oxbow-lake covered by Eichhornia crassipes in the Paraná River

Summary

The main limnological features of an oxbow-lake of the Paraná River floodvalley (27°27′ S — 58°49′ W) have been recorded during an annual cycle. The yearly sequence of changes was strongly determined by the hydrologic regime of the river. During the low-water period (winter- early spring) the oxbow remained isolated and water hyacinth (Eichhornia crassipes) covered the water surface almost completely. Dissolved oxygen was low at the surface (0.14 mg · l⁻¹) and completely exhausted towards the bottom (2 m). Internal load brought phosphorous concentration up to 1.6 mg PPO₄. I⁻¹, ammonium up to 1.2 mg NNH₃ · 1⁻¹, and iron up to 5 mg · l⁻¹. Sulfate and nitrate became undetectable at the bottom layer. During the flood period (late spring—summer) the inflow of river water reduced the surface covered by Eichhornia crassipes, increased the oxygen content throughout the water column (surface: 1.9, bottom 5 m depth: 1.6mg.l⁻¹) and depleted nutrient concentration (0.04 mg PPO₄ · l⁻¹, 0.06 mg NNH₃ · 1⁻¹ and 0.4 mg Fe · l^−l). Phytoplanktonic primary production ranged between 40 mg C · m⁻² · d⁻¹, during the flood, and 560 mg C · m⁻³ · d⁻¹ during the low water season. Both phytoplankton density and diversity were moderate to low, Euglenophyceae and Cryptophyceae being the prevailing groups.