Using modified multiple phosphorus sensitivity indices for mitigation and management of phosphorus loads on a catchment level

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 similar environmental drivers and catchment characteristics.