Abstract:
The relationships between river and lake phosphorus sensitivity, environmental drivers and catchment characteristics within the upper Olifants River and Lake Loskop were studied over a period of four years to come up with mitigation and management strategies. Using modified indices it was evident that the best strategy for improving the trophic state of Lake Loskop was to drastically reduce the external nutrient loading coming from the upper Olifants River
catchment. According to the lake phosphorus sensitivity index (LPSI) developed, Lake
Loskop was phosphorus sensitive and will possibly respond to phosphorus reduction in its
upper catchment. The river phosphorus sensitivity index (RPSI) developed showed that
certain rivers and streams in the upper catchment of Lake Loskop were particularly sensitive
to increases in phosphorus (P) loads. The substrate of these rivers and streams consisted of
approximately 90% cobbles or bedrock and showed eutrophic conditions during low flow
regimes. The increase in P loads in these streams stimulates the productivity and growth of
periphyton dominated by filamentous green algae mats. On the other hand, a restriction of
light penetration into the water column by high concentrations of inorganic suspended solids
limited the growth of both benthic and planktonic algae in > 3 order streams dominated by
sand or mud bottom substrates, thus making these streams less sensitive to high P loads.
Rivers and streams in the upper catchment that required mitigation of P loads were identified
according to the RPSI and different P load management practices were outlined. The findings
of this study are important for restoration and management purposes of increased P loads in
conjunction with river characteristics and phytoplankton occurrence. The modified indices
developed for P management can be a useful tool in river basins in other parts of the world
with the same environmental drivers and catchment characteristics.
