External Nutrient Inputs into Lake Kivu: Rivers and Atmospheric Depositions Measured in Kibuye

Abstract

Quantifying the external nutrients inputs is a key factor for understanding the formation of methane in Lake Kivu. This tectonic lake located between Rwanda and DRC contains a big quantity of dissolved gases predominated by carbon dioxide, methane and sulphide. The CH4 is most probably produced in the lake, mainly in the sediments, from decomposing organic material and by reduction of CO2. The sediments are carried out into the Lake Kivu which consequently may leads to the high production of methane from the decomposition of organic matter contained in sediments and biomass. For quantifying the external nutrient inputs into Lake Kivu, rivers in Kibuye catchment and atmospheric deposition were analyzed for phosphorus, nitrogen and Silica. The results found show that a total budget of 276 tN.yr^-1 or an areal specific load of 401 kgN.km^-2.yr^-1 of DIN (ammonia nitrogen and nitrate nitrogen), 5 tP.yr^-1 or 11 kgP.km^-2.yr^-1 of SRP, 59 tP.yr^-1 or 87 kgP.km^-2.yr^-1 of TP, 1122 tSi.yr^-1 or 2570 kgSi.km^-2.yr^-1 and 31620 t TSS.yr-1 or 41 tTSS.km^-2.yr^-1 are deposited into Lake Kivu through rivers in Kibuye Catchment. The contributions of atmospheric deposition are considerable where about 2176 tN.yr^-1 and 1638 tN.yr^-1 of DIN respectively for wet deposition and dry deposition are deposited in Lake Kivu. It was observed that nitrite nitrogen is negligible in atmospheric deposition and riverine inputs. DSi predominates in riverine inputs and is negligible in atmospheric deposition. Ammonia nitrogen comes from atmospheric deposition while nitrate nitrogen comes from riverine nutrient inputs. Considering the molar ration DSI: DIN: SRP of 10:20:1, the limiting nutrient for the primary productivity in external riverine inputs is phosphorus. Since the lake-internal nutrient recycling is about an order of magnitude larger than the external sources, the recent increase of dissolved methane in Lake Kivu is not generated only by external nutrient inputs.

Key words: nutrients, atmospheric deposition, riverine inputs, internal cycling, Lake Kivu

This paper was submitted and published without the knowledge of the third author M. Schmid