Dynamics of dissolved inorganic carbon and aquatic metabolism in the Tana River basin, Kenya

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, reservoirs (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 mineralisation
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.