The impact of water hyacinth (Eichhornia crassipes) in a eutrophic subtropical impoundment (Lake Chivero, Zimbabwe).: I.: Water quality

We assessed the impact of water hyacinth on the water quality of Lake Chivero, a highly eutrophic man-made reservoir near Harare (Zimbabwe). We compared water quality among limnetic (always without macrophytes) and littoral zones with and without hyacinths in randomly selected sites and along a transect from inflow to outflow of the lake. In addition, water hyacinth uptake rates of ammonium, nitrate and phosphate were measured with a short-term experiment. Along the lake transect, the average ammonium and phosphate concentrations as well as transparency of the water were significantly lower in the vegetated than in the unvegetated zones. The randomly selected sites with hyacinth were also more turbid and had higher levels of chlorophyll-a, lower oxygen, pH and nitrate values and higher levels of COD than at sampling sites in the unvegetated pelagic and littoral zones. The unvegetated sites could be discriminated by the increased levels of nutrients in the pelagic in comparison with the littoral sites. The uptake rate of ammonium (2.6 mg N h(-1) kg fresh weight(-1)) was higher than that of nitrate (1.3 mg N h(-1) kg fresh weight(-1)). Combining uptake rates and total hyacinth standing crop, it follows that under the prevailing winter and nutrient conditions, the current hyacinth standing crop had a maximum daily removal capacity of 1.5% of the ammonium load. This value may be higher in periods of intensive biomass increase under favourable summer conditions. Similarly, maximum values of 0.1% and 0.025% were taken up daily by water hyacinth from the total nitrate and phosphate stocks, respectively. Our results indicated a strong local influence of water hyacinth on nutrient concentrations in the vegetated parts of the lake but due to the limited hyacinth cover (3.2% or 83 ha) of the lake and the prevailing winter conditions, the over-all effects on nutrient levels in the lake were likely to be low.