DISSOLVED OXYGEN DEPLETION AND NUTRIENT UPTAKE IN AN IMPOUNDMENT INFESTED WITH EICHHORNIA CRASSIPES (MART.) SOLMS

Abstract

Bon Accord Dam, near Pretoria, had 95% of its surface area covered by water hyacinth (Eichhornia crassipes (Mart) Solms). The mean surface dissolved oxygen concentration was 1,9 mg O₂ 2⁻¹ which is below the minimum concentration required by most angling fish. Nutrient uptake was considerable with nitrate (NO3-N) and orthophosphate (P0i,-P) uptake being 98% and 53% in the summer and 66% and 47% in the winter, respectively. Nutrient uptake of Na, C1, Ca, Mg, SO₄, and K was not large and these nutrients exhibited a net export from the impoundment during summer. Nutrient uptake values in Bon Accord Dam were not in agreement with values recorded in the literature. After eradication of the water hyacinth by herbicides, the dissolved oxygen concentration increased until supersaturation caused by algal blooms. Decay and mineralisation of water hyacinth and/or lack of efficient uptake by algae caused increases in nutrient concentrations of ammonia, ortho and total phosphorus in the outflow water.