Abstract
Abstract. As part of a broader study on the riverine biogeochemistry in the Athi–Galana–Sabaki (A-G-S) River catchment (Kenya), we present data constraining the sources, transit and transformation of multiple nitrogen (N) species as they flow through the A-G-S catchment (~47 000 km2). The data set was obtained in August–September 2011, November 2011, and April–May 2012, covering the dry season, short rain season and long rain season respectively. Release of (largely untreated) wastewater from the city of Nairobi had a profound impact on the biogeochemistry of the upper Athi River, leading to low dissolved oxygen (DO) saturation levels (36–67%), high ammonium (NH4+) concentrations (123–1193 μmol L−1), and high dissolved methane (CH4) concentrations (3765–6729 nmol L−1). Riverine dissolved inorganic nitrogen (DIN; sum of NH4+ and nitrate (NO3−); nitrite was not measured) concentration at the most upstream site on the Athi River was highest during the dry season (1195 μmol L−1), while DIN concentration was an order of magnitude lower during the short and long rain seasons (212 and 193 μmol L−1, respectively). During the rain seasons, low water residence time led to relatively minimal in-stream N cycling prior to discharge to the ocean, whereas during the dry season we speculate that prolonged residence time creates two differences comparative to wet season, where (1) intense N cycling and removal of DIN is possible in the upper to mid-catchment and leads to significantly lower concentrations at the outlet during the dry season, and (2) as a result this leads to the progressive enrichment of 15N in the particulate N (PN) pool, highlighting the dominance of untreated wastewater as the prevailing source of riverine DIN. The rapid removal of NH4+ in the upper reaches during the dry season was accompanied by a quantitatively similar production of NO3− and nitrous oxide (N2O) downstream, pointing towards strong nitrification over this reach during the dry season. Nitrous oxide produced was rapidly degassed downstream, while the elevated NO3− concentrations steadily decreased to levels observed elsewhere in more pristine African river networks. Low pelagic primary production rates over the same reach suggest that benthic denitrification was the dominant process controlling the removal of NO3−, although large cyanobacterial blooms further downstream highlight the significant role of DIN assimilation by primary producers also. Consequently, the intense nitrification and uptake of N by algae leads to significant enrichment of 15N in the PN pool during the dry season (mean: +16.5 ± 8.2‰ but reaching as high as +31.5‰) compared to the short (+7.3 ± 2.6‰) and long (+7.6 ± 5.9‰) rain seasons. A strong correlation between the seasonal N stable isotope ratios of PN (δ15NPN) and oxygen stable isotope ratios of river water (δ18OH2O; as a proxy of freshwater discharge) presents the possibility of employing a combination of proxies – such as δ15NPN of sediments, bivalves and near-shore corals – to reconstruct how historical land use changes have influenced nitrogen cycling within the catchment, whilst potentially providing foresight on the impacts of future land management decisions.
Highlights
Human activities over the last two centuries have drastically influenced regional and global nitrogen (N) cycles (Galloway et al, 1995; Howarth et al, 1996; Galloway and Cowling, 2002)
As part of a broader study on the riverine biogeochemistry in the A-G-S catchment, we present data collected during three climatic seasons and a one-year bi-weekly sampling regime to constrain the sources, transformations and transit of multiple N species as they pass through the A-G-S River, stretching from downstream of urban Nairobi to the outlet at the Indian Ocean
Total dissolved inorganic nitrogen (DIN) concentrations at S1 were highest during the dry season (1195 μmol L−1), and almost completely in the form of NH+4 (99.8 %)
Summary
Human activities over the last two centuries have drastically influenced regional and global nitrogen (N) cycles (Galloway et al, 1995; Howarth et al, 1996; Galloway and Cowling, 2002). A network of microbial metabolic pathways and abiotic reactions are involved in the transformation and removal of N from aquatic systems, including N2 fixation, ammonification, anaerobic NH+4 oxidation (anammox), nitrification, denitrification, dissimilatory reduction of NO−3 to NH+4 (DNRA), assimilation of DIN into biomass, NH3 volatilization, and NH+4 adsorption and desorption. These pathways may be (chemo)autotrophic or heterotrophic, with the prevailing pathway dependant on the presence or absence of oxygen and organic carbon (OC) within the local environment (Trimmer et al, 2012).
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