Wet deposition of atmospheric nitrogen contributes to nitrogen loading in the surface waters of Lake Tanganyika, East Africa: a case study of the Kigoma region

Abstract

Lake Tanganyika, an African Great Lake, is a complex tropical ecosystem that has been subjected to extreme climate-related changes in the last century, including seasonal changes in temperature and rainfall, decreased overall annual rainfall, and greater frequency of rainstorms. Atmospheric nitrogen (N) is an important component of the lake's N loading, but how long-term and seasonal changes in precipitation affect this loading still needs clarification. This study aimed to improve our understanding of the seasonal features of N deposition in the lake, by monitoring atmospheric N deposition concentrations and fluxes from March 2013 to February 2014. There was a significant temporal variation in wet N depositions in the study area. The distribution of the annual rainfall into major (March-May 299.8 mm) and minor (October-December 343.2 mm) rainy seasons translated into 20 and 30% of N deposition. In September and January-February, there was 10 and 12% precipitation, representing 43 and 7% of N deposition in the lake. Nitrogen deposition was highest in September due to farmlands' burning during the dry season (June-August), leading to N accumulation in the atmosphere. In conclusion, the pattern of N deposition appears to be driven by the unique climatic characteristics of the lake basin and to be closely associated with local anthropogenic activities.

Depth-discrete metagenomics reveals the roles of microbes in biogeochemical cycling in the tropical freshwater Lake Tanganyika

“…Nitrogen cycling has been well-studied in LT, mostly focused on nitrogen inputs and outputs, and from a chemical and biogeochemical perspective. For example, atmospheric deposition and nitrogen from river inflow contribute much of the nitrogen to LT [ 56 ]. In addition, it is shown that incoming nitrogen in the form of nitrate is quickly used by organisms in the surface waters, and the upper water column is often nitrate depleted.…”

Depth-discrete metagenomics reveals the roles of microbes in biogeochemical cycling in the tropical freshwater Lake Tanganyika

“…Nitrogen cycling has been well studied in LT, mostly focused on nitrogen inputs and outputs, and from a chemical and biogeochemical perspective. For example, atmospheric deposition and nitrogen from river inflow contribute much of the nitrogen to LT[57]. Additionally, it is shown that incoming nitrogen in the form of nitrate is quickly used by organisms in the surface waters, and the upper water column is often nitrate depleted.…”

Status and changes of water quality in typical near-city zones of three East African Great Lakes in Tanzania

“…Kigoma Bay is next to the city of Kigoma in Tanzania, which is the largest city on the eastern shore of Lake Tanganyika and a transportation hub in Western Tanzania that has experienced rapid development (Zhang et al 2020). Anthropogenic N pollution in combination with the effects of climate change is a growing threat to the water quality of the lake in the Kigoma region (Gao et al 2018). The city and related agricultural areas are the most important potential sources of pollution to the lake waters of Kigoma Bay.…”

“…Seasonal differences of the NH 4 + -N distribution were also found in Mwanza Gulf, i.e., the NH 4 + -N concentrations in the surface waters were higher during the rainy season than in the dry season. Increased ammonification in the water bodies during the blooming period and input of external nutrients from the lake basin via river runoff and the atmosphere (Gao et al 2018) could be the main reasons for this seasonality. Relatively high NH 4 + -N concentrations found in central and southern parts of the gulf could be related to external inputs from agricultural sources.…”