The Ogooué River is the fourth largest river in Africa by discharge. The Ogooué Basin mostly consists of undisturbed rainforest with some savanna grassland. Yet, there is no information on the biogeochemistry, transport fluxes and greenhouse gases in this river. Here, we report initial results of a monitoring campaign whereby 2-weekly samples were collected at Lambaréné (Gabon) [10.24°E 0.69°S] between April
2012 and March 2014 for a suite of physico-chemical and biogeochemical characteristics, including total suspended matter (TSM) concentrations, concentration and stable isotope composition of particulate organic carbon (POC and δ
13C-POC) and particulate nitrogen (PN and δ15N-PN),
chromophoric dissolved organic matter (CDOM), dissolved organic carbon (DOC and δ13C-DOC), dissolved inorganic
carbon (DIC and δ13C-DIC), concentration of greenhouse
gases (GHGs) (CO2, CH4 and N2O), as well as major elements, total alkalinity, and oxygen isotope signatures of water (δ
18O-H2O).
The water flow showed a typical bimodal structure during the study period with a first rising discharge from March to May and a second rising discharge from September to December. The mean water temperature was 27.5°C with a lowering
during the dry season. Oxygen concentrations were always near saturation in surface waters.
Low discharge is characterized by higher conductivity. The pH appeared neutral around 6.8 with small fluctuations.
Total suspended matter (TSM), particulate organic carbon (POC) and dissolved organic carbon (DOC) concentrations were positively correlated to water levels (R2 = 0.55, 0.48 and
0.79, respectively) indicating the allochtonous origin of organic matter in the Ogooué. DOC dominated over POC throughout the hydrological year. CDOM was positively correlated to water levels (R2=0.63) and DOC concentrations
(R2=0.79). 2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 c D O M ( a 3 5 0 ) (m -1 ) 5 10 15 20 25 30 35 40
Biogeochemistry, transport fluxes and emission of greenhouse gases
from the Ogooué River (west central Africa): preliminary results after two years of monitoring
F Darchambeau
1
, S Bouillon
2
, B Alhou
3
, T Lambert
1
, AV Borges
1
1
Université de Liège, Belgium
2
KULeuven, Belgium
3
Université Abdou Moumouni, Niger
2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 W a te r le v e ls ( m ) 8 10 12 14 16 2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 T e m p e ra tu re ( °C ) 24 25 26 27 28 29 30 31 2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 S p e c if ic c o n d u c ti v it y @ 2 5 °C ( µ S c m -1 ) 18 20 22 24 26 28 2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 δδδδ 1 8 O -H 2 O ( ‰ ) -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 T o ta l s u s p e n d e d m a tt e r (m g L -1 ) 0 10 20 30 40 50 60 2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 P a rt ic u la te o rg a n ic c a rb o n ( % /T S M ) 0 5 10 15 20 2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 P a rt ic u la te n it ro g e n ( m g L -1 ) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 T o ta l a lk a li n it y ( m m o l L -1 ) 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 δδδδ 1 3 C -D IC ( ‰ ) -22 -20 -18 -16 -14 -12 -10 2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 δδδδ 1 3 C -D O C ( ‰ ) -30.6 -30.4 -30.2 -30.0 -29.8 -29.6 -29.4 -29.2 -29.0 2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 δδδδ 1 3 C -P O C ( ‰ ) -34 -33 -32 -31 -30 -29 -28 2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 D is s o lv e d o rg a n ic c a rb o n ( m g L -1 ) 1 2 3 4 5 6 7 8 9 2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 C H 4 ( n M ) 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2012-04 2012-08 2012-12 2013-04 2013-08 2013-12 2014-04 N 2 O ( n M ) 4 6 8 10 12 14 16
δ13C signatures of POC and DOC showed relatively
strong seasonal variations. The δ13C-POC and DOC
values during high discharge conditions can be assumed to reflect the signature of topsoil and litter-derived terrestrial organic C. Overall, δ13 C data
in POC and DOC suggest the dominance of C3 vegetation in the Ogooué watershed.
Total alkalinity (TA) varied between 0.12 and 0.26 mmol.L-1 and displayed a decreasing pattern during
high-flow conditions. δ13C signatures of dissolved
inorganic carbon (DIC) was positively correlated to TA (R2=0.57).
Concentrations of dissolved CH4 were oversaturated throughout the hydrological year but did not present reliable seasonal patterns. N2O concentrations were slightly oversaturated (~117 %).
