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

Abstract

A basin-wide study was conducted in the Tana River basin (Kenya) in February 2008 (dry season), September–November 2009 (wet season) and June– July 2010 (end of the wet season) to assess the dynamics and sources of dissolved inorganic carbon (DIC) as well as to quantify CO2 fluxes, community respiration (R), and primary production (P). Samples were collected along the altitudinal gradient (from 3600 to 8 m) in several headwater streams, eservoirs (Kamburu and Masinga), and the Tana River mainstream. DIC concentrations ranged from 0.2 to 4.8 mmol L−1, with exceptionally high values (3.5±1.6 mmol L−1) in Nyambene hills tributaries. The wide range of _13CDIC values (−15.0 to −2.4 ‰) indicate variable sources of DIC, with headwater streams recording more positive signatures compared to the Tana River mainstream. With with only a few exceptions, the entire riverine network was supersaturated in CO2, implying the system is a net source of CO2 to the atmosphere. pCO2 values were generally higher in the lower Tana River mainstream compared to headwater tributaries, opposite to the pattern typically observed in other river networks. This was attributed to high suspended sediment in the Tana River mainstream fuelling in-stream community respiration and net heterotrophy. This was particularly evident during the 2009 wet season campaign (median pCO2 of 1432 ppm) compared to the 2010 end of the wet season (1002 ppm) and 2008 dry season (579 ppm). First-order estimates show that in-stream community respiration was responsible for the bulk of total CO2 evasion (77 to 114 %) in the Tana River mainstream, while in the tributaries, this could only account for 5 to 68% of total CO2 evasion. This suggests that CO2 evasion in the tributaries was to a substantial degree sustained by benthic ineralisation and/or lateral inputs of CO2-oversaturated groundwater. While sediment loads increased downstream and thus light availability decreased in the water column, both chlorophyll a (0.2 to 9.6 μg L−1) and primary production (0.004 to 7.38 μmol CL−1 h−1) increased consistently downstream. Diurnal fluctuations of biogeochemical processes were examined at three different sites along the river continuum (headwater, reservoir and mainstream), and were found to be substantial only in the headwater stream, moderate in the reservoir and not detectable in the Tana River mainstream. The pronounced diurnal fluctuations observed in the headwater stream were largely regulated by periphyton as deduced from the low chlorophyll a in the water column.