NITROGEN TRANFORMATIONS AND THE NITROGEN BUDGET OF A HYPERTROPHIC IMPOUNDMENT (HARTBEESPOORT DAM,SOUTH AFRICA)

SUMMARY

The major components of the nitrogen balance of hypertrophic Hartbeespoort Dam were studied at weekly intervals from October 1980 to September 1984. The Crocodile River contributed over 80Z of the hydraulic load and over 97X of the annual total nitrogen load to the impoundment. More than 802 of this nitrogen load was composed of nitrate. Monthly measurements of nitrification and denitrification rates carried out within the impoundment over one annual cycle revealed that the rates of both processes were highest at the ammonia-oxygen chemocline during summer stratification and at the sediment-water interface during winter isothermy. Mass balance calculations showed that between 35 and 49% of the annual total nitrogen load were lost via denitrification each year; the data are compared with those in the literature. Conditions in the lake will favour a change in phytoplankton species composition from Microcystis to a nitrogen-fixing species if the present trend of decreasing inorganic nitrogen concentrations and N: P ratios is maintained.