Dominant contribution of nitrogen compounds in precipitation chemistry in the Lake Victoria catchment (East Africa)

Adama Bakayoko,Frédéric Julien,Véronique Yoboué,Jonathan E Hickman,Eric Gardrat,Frank Roux,Corinne Galy-Lacaux,Claire Delon

doi:10.1088/1748-9326/abe25c

Abstract

This work provides a complete chemical characterization of rains collected in the tropical rural site of Mbita (Kenya) on the shores of Lake Victoria (annual rainfall 1259.3 mm). We present a wet nitrogen deposition budget including inorganic and organic dissolved nitrogen in relation with atmospheric sources of gases and particles, precipitation rate and air mass transport. A unique 2 yr monitoring data set (2017–2019), providing 183 rain samples was collected and analyzed according to international standards (WMO/GAW). Considering that precipitation represents the largest contributor of water to the Lake Victoria (80%), this study gives new insights in the seasonality of nutrients wet deposition (WD) inputs in the unique natural resource represented by Lake Victoria and its catchment. Four main contributions to the chemical composition of precipitation, were identified: (a) a 28% terrigenous contribution related to crustal and biomass sources, (b) a 14% marine contribution related to Indian ocean air masses intrusion, (c) a 15% organic contribution due to volatile organic carbon emissions from biomass burning and vegetation and (d) a predominant nitrogenous contribution of 39% due to livestock and fertilizers, biomass burning and neighboring agricultural fires. Ammonium and nitrate volume weighed mean concentrations are 36.75 and 8.88 μeq l−1, respectively. Rain in Mbita is alkaline (pH = 5.8) highlighting neutralization by heterogeneous chemistry. Total nitrogen WD is 8.54 kgN ha−1 yr−1, 58 760 tN yr−1 for the entire lake, with 26% attributed to dissolved organic nitrogen. A total atmospheric deposition of 15 kgN ha−1 yr−1 is estimated taking into account dry deposition estimate from literature, showing that the Lake Victoria ecosystem is exposed to eutrophication. An extensive and regular monitoring of wet and dry nitrogen deposition is highly recommended both in-shore and off-shore to help improving the efficiency of nitrogen use in agricultural areas and reduce nitrogen losses around Lake Victoria.

Highlights

Determining atmospheric budgets of key chemical compounds is crucial to understand the functioning of ecosystems and biogeochemical cycles
This study presents for the first time a complete characterization of the precipitation chemistry in a tropical rural site in Kenya on the shores of the Lake Victoria, based on the monitoring of quality cri pt controlled in situ measurements from May 2017 to April 2019
In the Lake Victoria basin, four main contributions to the chemical composition of precipitation were related to potential atmospheric sources of gases and particles of importance

Summary

Introduction

Determining atmospheric budgets of key chemical compounds is crucial to understand the functioning of ecosystems and biogeochemical cycles. In these budgets, the importance of the chemical us composition of wet deposition as a source of nutrients, e.g, nitrogen (N), sulphur (S) or carbon (C) is widely recognized (Duce et al 2009), and shows a high spatial and temporal variation. The importance of the chemical us composition of wet deposition as a source of nutrients, e.g, nitrogen (N), sulphur (S) or carbon (C) is widely recognized (Duce et al 2009), and shows a high spatial and temporal variation This composition reflects various interacting physical and chemical mechanisms in the atmosphere an including biogenic and anthropogenic sources of pollutants, atmospheric transport and chemical transformation processes as well as removal processes (Galy-Lacaux et al 2009).

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Conclusion