Significance of terrestrial inflows to carbon and nitrogen distribution in the Lake Victoria surface water

Abstract

This was an investigation to characterize seasonal and spatial distribution of particulate organic matter (POM) in the surface water of Lake Victoria at Speke Gulf, by using carbon-13 and nitrogen-15 isotopic signatures. Speke Gulf is under the influence of terrestrial inflow through river Simiyu. The POM from the lake surface water had higher C:N ratio values decreasing towards the deeper lake water, an indication of decreasing nitrogen content relative to carbonic materials. Samples away from the river mouth provided C:N ratios within the Redfield ratio range (C:N:P; 106:16:1) indicating materials of phytoplanktonic origin. The POM isotopes composition indicated a maximum δ 13 C value of −21.2‰ close to the river mouth and lowest value (~ −25‰) away from the river influence. The δ 15 N maximum (14.7‰) was observed near the river mouth with lower values (5.8‰) found in deeper waters away from the river mouth. Overall, there was a general decrease of 13 C and δ 15 N values from shallow water to deeper water. There was higher δ 15 N value dispersion (std. Dev. ±3.4‰) compared to δ 13 C values with std. Dev.±1.2‰. These values also varied with seasons, a phenomenon showing significant influence of land run off on the lake elemental distribution, carbon and nitrogen in particular.