0 100 200 300 400 500 600 700 800 F re s h w a te r d is c h a rg e ( m 3s -1) Meuse Ourthe Geer Blc. Grav. 0 20 40 60 80 100 120 140 % O2 (% ) Meuse Ourthe Geer Blc. Grav. 0 20 40 60 80 100 120 140 160 180 C h lo ro p h y ll -a ( µ g L -1) Meuse Ourthe Geer Blc. Grav. 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 p C O2 (p p m ) Meuse Ourthe Geer Blc. Grav. 0 2000 4000 6000 8000 10000 12000 14000 0 100 200 300 400 500 600 700 % N2 O ( % ) e x c e p t G e e r % N2 O ( % ) e x c e p t G e e r Meuse Ourthe Blc. Grav. Geer 0 100,000 200,000 300,000 400,000 500,000 600,000 0 10,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90,000 100,000 % C H4 (% ) o n ly G e e r % C H4 (% ) e x c e p t G e e r Meuse Ourthe Blc. Grav. Geer 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 0 200 400 600 800 p C O2 (p p m ) Discharge (m3s-1) 0 100 200 300 400 500 600 700 0 200 400 600 800 % N2 O ( % ) Discharge (m3s-1) 0 10000 20000 30000 40000 50000 60000 0 200 400 600 800 % C H4 (% ) Discharge (m3s-1) 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 0 5 10 15 20 25 p C O2 (p p m ) Température ( C) 0 100 200 300 400 500 600 700 0 5 10 15 20 25 % N2 O ( % ) Température ( C) 0 10000 20000 30000 40000 50000 60000 0 5 10 15 20 25 % C H4 (% ) Température ( C) 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 25 50 75 100 125 p C O2 (p p m ) %O2(%) 0 100 200 300 400 500 600 700 25 50 75 100 125 % N2 O ( % ) %O2(%) 0 10000 20000 30000 40000 50000 60000 25 50 75 100 125 % C H4 (% ) %O2(%)
CO2, CH4 and N2O dynamics in Belgian rivers across a gradient of anthropogenic disturbance
Borges A.V.
1,*, F. Darchambeau
1, and A. Beulen
1 1University of Liège (Belgium) * alberto.borges@ulg.ac.be
Forest (%) Grassland (%) Cropland (%) Catchment (km²) Pop. density (habit. km-²) Meuse 64 14 22 36000 255 Ourthe 47 32 21 1837 71 Geer <1 7 93 463 194 Blanc Gravier ~100 ~0 ~0 ~20 ~0
Two rivers and two streams close to the city of Liège in Belgium (Meuse, Ourthe, Geer and Blanc Gravier) were sampled to describe the dynamics of CO2, CH4and N2O (for the first time in Belgium for freshwaters). The four systems were chosen to cover a gradient of size (stream to river) and of human influence (mainly forested to mainly agricultural watersheds). The study covers the period from February 2011 to March 2013 with weekly sampling in surface waters. The variables were very contrasted in the four systems, the Geer showing a strong enrichment in nitrogen NH4+et NO
2-) and phosphorous in relation to the other three systems. The
O2concentrations were much lower, and the concentration of CH4, N2O and pCO2were much higher in the Geer than in other three systems. The concentrations in CH4, N2O and pCO2were higher in the Ourthe than in
the Meuse and than in the Blanc Gravier. Marked seasonal variations were observed in the 4 systems. In general the concentration of CH4, N2O and pCO2were higher in summer than in winter. This is related on one hand to the increase of temperature in summer that stimulates bacterial activity. Also in summer, the availability of organic matter for bacterial activity is higher after the spring phytoplankton blooms and also from allochthnous inputs from the watersheds. The increase of temperature and bacterial consumption of O2in the water column leads to a lesser O2penetration in the sediments that could stimulate benthic anaerobic processes among which methanogenesis and denitrification, leading to an increase of CH4and N2O in the water column. Also, the production of N2O by denitrification strongly increases at low O2. During low water, the increase of residence time of the water mass and the decrease of current (decrease of degasing) allow an accumulation of CO2, CH4and N2O in the water column. On the contrary during high water, dilution and
increase of current (increase of degasing) lead to a decrease of concentrations. The four systems were over-saturated in CH4, N2O and CO2, excepted during spring phytoplankton blooms when an under-saturation of CO2 was observed in the Ourthe. Hence, the four systems were sources of CH4, N2O and CO2 to the atmosphere. Diffusive CO2fluxes varied from 24 to 607 mol m-2yr-1(Ourthe and Geer, respectively). Diffusive CH
4
fluxes varied from 28 to 8199 mmol m-2an-1(Blanc Gravier and Geer, respectively). Diffusive N
2O fluxes varied from 2 to 201 mmol m-2an-1(Ourthe and Geer, respectively).
O ( % ) e x c e p t G e e r
