Balancing Nutrient Inputs to Lake Kivu

The primary production in meromictic Lake Kivu is sustained by external nutrient inputs and by internal loading due to upwelling caused by sub-aquatic sources. We present here the results of external loading of phosphorus (P), nitrogen (N) and silica (Si) by rivers and atmospheric deposition measured from 2006 to 2008. These external inputs are compared to internal loading. The input of soluble-reactive P (SRP), supplied in equal parts from rivers and atmospheric deposition, adds up to 230 t P yr-1, 20 times less than total P load. Ammonium (mainly via rainwater) and nitrate (mainly via rivers) are primary sources of the dissolved N load (5400 t N yr-1), with both species contributing ∼50%. Dissolved Si input (40,000 t Si yr-1) is unique in that only ∼60% enters by rivers, while the remaining ∼40% comes from sub-aquatic sources and atmospheric deposition is negligible. Based on the molar nutrient ratios, we identify P as the limiting factor for algae production. Despite the strong anthropogenic impact on the catchment and the high particle erosion (74 t km-2 yr-1), the area-specific nutrient mobilization is rather low. The external nutrient input is therefore not the cause for the reported increase of methane production in the last decades. External loading to the epilimnion plays a lesser role for all three nutrients (∼10% for SRP, ∼25% for dissolved N and ∼45% for dissolved Si), as compared to the lake-internal loading by upwelling (90%, 75% and 55%, respectively). Lake Kivu, therefore, is similar to other East African large lakes in that the internal loading exceeds the external loading. Despite the substantial uncertainty of the load estimates of up to 50%, we can conclude that the observed nutrient input is consistent with the primary production of 260 g C m-2 yr-1 recently measured by Sarmento et al. (2006) and also consistent with the lake-internal fluxes established by Pasche et al. (in press).