The Effect of Suspended Solids Loading from the Linthipe River on Light in Lake Malawi Greg K. McCullough * and David Barber Centre for Earth Observations Science Department of Environment and Geography University of Manitoba Winnipeg R3T 2N2 ABSTRACT. 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 depen- dence of transparency on total loading. INDEX WORDS: Fluvial loading, suspended sediment, turbidity, light extinction, transparency, tropi- cal lakes, Lake Malawi. J. Great Lakes Res. 33:466–482 Internat. Assoc. Great Lakes Res., 2007 INTRODUCTION The fishes of Lake Malawi (known as Lake Nyasa in Tanzania, and Lake Niassa in Mozam- bique) are a major source of protein for the people of Malawi and because of their beauty and diver- sity, a significant component of the tourist econ- omy. There is a growing concern that increasing nutrient and sediment yields associated with devel- opment in the watershed may have an adverse im- pact on this valuable resource. Lake Malawi’s great biodiversity includes an estimated 800 species of cichlids (Snoeks 2000). Seehausen et al. (1997) showed in Lake Victoria that by constraining color vision, turbidity interferes with mate selection among cichlids, thereby blocking the mechanism of reproductive isolation necessary to maintain biodi- versity. In Lake Malawi, Duponchelle et al. (2000) * Corresponding author. E-mail: gmccullo@cc.umanitoba.ca 466 observed that cichlids did not feed, defend territory, or perform courtship rituals when Secchi trans- parencies were less than 2 m. In Lake Victoria, See- hausen et al. (1997) documented lower biodiversity in areas that had become turbid due to eutrophica- tion. In Lake Malawi near the mouth of the Linthipe River, where sub-meter Secchi transparencies are common during the rainy season, Sululu (2000) has likewise shown that cichlid species richness and abundance are inversely correlated with suspended sediment concentration. In this paper, we investigate the distribution and effect of light attenuation by suspended material carried by the Linthipe River into southern Lake Malawi. Flow and sediment discharge into Lake Malawi follow two distinct paths. Most plunges down to the metalimnion; a smaller fraction spreads buoyantly as surface plumes. The process of plung- ing in the delta region was described by McCul-