Dynamic seasonal nitrogen cycling in response to anthropogenic N-loading in a tropical catchment, Athi–Galana–Sabaki River, Kenya

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, waste water 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 (67–36%), high ammonium (NH4+) concentrations (1193–123 μmol L−1), and high dissolved methane (CH4) concentrations (6729–3765 nmol L−1). Total dissolved inorganic nitrogen (DIN) concentrations entering the study area were highest during the dry season (1195 μmol L−1), while total 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 instream N-cycling prior to discharge to the ocean. Conversely, increased residence time during the dry season creates two differences comparative to wet season conditions, where (1) intense cycling and removal of DIN in the upper- to mid-catchment leads to significantly less DIN export during the dry season, and (2) as a result of the intense DIN cycling, dry season particulate N export is significantly enriched in the N stable isotope ratio (δ15NPN), strongly reflecting the dominance of organic matter as the prevailing source of riverine nitrogen. The rapid removal of NH4+ in the upper study area 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 in the drainage network. The intense upper- to mid-catchment N-cycling leads to a significantly enriched δ15NPN 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 found between seasonal δ15NPN and oxygen stable isotope ratios (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 in the impacts of future land management decisions.