The Effect of Suspended Solids Loading from the Linthipe River on Light in Lake Malawi

Increasing land use pressure has resulted in increasing suspended solids loading to many African lakes in recent decades, causing concern that littoral habitat may be degraded by sedimentation and light limitation. In a companion paper in this issue, we showed that much runoff into Lake Malawi plunges below the euphotic zone and spreads through the lake via the upper metalimnion, thus mitigating nearshore and upper water column impacts. In this study we use data from multiple surveys through two rainy seasons to describe spatio-temporal distributions of suspended solids concentration (SSC) and transparency caused by the flow of runoff through the Linthipe River delta region of Lake Malawi. SSC in interflow at 30-50 m depth was typically an order of magnitude greater than in surface plumes. Seasonal median transparency near the river mouth was inversely proportional to the suspended load in surface-buoyant inflow, but independent of total loading. This is because storm runoff was more dense (cooler and with higher SSC) than runoff during low-flow periods, so that it was more likely to plunge. The effect was to moderate inter-annual variability in light attenuation near the river mouth because transparency was lower (due to more surface-buoyant inflow of turbid water) in the year when solar irradiance was higher (less cloud cover). The effect was, however, local. Beyond a few kilometers of the river mouth upward mixing of suspended particles carried in interflow reasserted the more intuitive inverse dependence of transparency on total loading.