CO
2, CH
4and N
2O dynamics and fluxes in the brackish Lake Grevelingen (The Netherlands)
Borges A.V.
1,*, Filip Meysman
2, and Jérôme Harlay
1 1ULg (BE)
2NIOZ * alberto.borges@ulg.ac.be
-30 -20 -10 0 D e p th ( m ) 2 9 .0 2 9 .4 2 9 .8 3 0 .2 3 0 .6 3 1 .0 3 1 .4 3 1 .8 3 2 .2 3 2 .6 3 3 .0 J F M A M J J A S O N D Salinity -30 -20 -10 0 D e p th ( m ) 0 1 2 3 4 5 6 7 8 9 1 0 11 12 13 14 15 61 17 18 19 20 21 22 J F M A M J J A S O N D Temp. (°C) -30 -20 -10 0 D e p th ( m ) 0 2 0 40 60 80 100 021 140 160 180 002 220 240 260 082 300 320 340 360 J F M A M J J A S O N D O (µM) 2 -30 -20 -10 0 D e p th ( m ) 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0 7 0 0 7 5 0 8 0 0 8 5 0 9 0 0 9 5 0 1 0 0 0 1 0 5 0 1 1 0 0 1 1 5 0 1 2 0 0 1 2 5 0 1 3 0 0 1 3 5 0 J F M A M J J A S O N D pCO (ppm)2 -30 -20 -10 0 D e p th ( m ) 0 50 10 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 5 0 6 0 0 6 5 0 7 0 0 J F M A M J J A S O N D CH (nM) 4 -30 -20 -10 0 D e p th ( m ) 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 J F M A M J J A S O N D N O (nM) 2 0 5 10 15 20 25 30 35 0 100 200 300 400 d e p th ( m ) CH4(nM) Jan. May Aug. Oct. 0 5 10 15 20 25 30 35 0 5 10 15 20 25 d e p th ( m ) N2O (nM) Jan. May Aug. Oct. 0 5 10 15 20 25 30 35 200 400 600 800 1000 12001400 1600 d e p th ( m ) pCO2(ppm) Jan. May Aug. Oct. 0 5 10 15 20 25 30 35 0 50 100 150 200 250 300 350 d e p th ( m ) O2 (µM) Jan. May Aug. Oct. 0 5 10 15 20 25 30 35 29 30 31 32 33 d e p th ( m ) salinity Jan. May Aug. Oct. 0 5 10 15 20 25 30 35 0 5 10 15 20 25 d e p th ( m ) Temperature (°C) Jan. May Aug. Oct.
Lake Grevelingen in the South West Netherlands is a former estuary locked off from the sea by two dikes and a brackish lake since 1971 (salinities from 29 to 33 during our sampling). It is connected with the North Sea by sluices, has a surface area of 108 km2, a mean depth of 5.3 m, a maximum depth of 48 m, and about 60% of the area the depth is less than 5 m. From January
2012 to December 2012, a biogeochemical survey was conducted at monthly interval at a fixed station (35 m depth) at Den Osse. Here, we focus on the analysis of partial pressure of CO2(pCO2), and concentrations of CH4and N2O obtained throughout the water column. The water column was isothermal in winter, stratification settled in spring, was maximal in summer (August), and
vertical mixing occurred in fall. Overall, salinity increased from surface to depth, ranged from 29.57 to 31.57 in surface waters and from 30.55 to 32.74 in bottom waters, and was minimal in winter and maximal in summer. O2in surface waters from 367 to 361 µM, and was always lower in bottom waters than in surface water waters. This pattern was very faint in Winter and very
marked in August when anoxia occurred below 22 m. pCO2in surface waters ranged from 270 to 650 ppm, and followed a typical seasonal cycle for temperate coastal environments shifting
from CO2over-saturation in winter to spring CO2under-saturation due to the spring phytoplankton bloom, and shifting back to over-saturation in fall. Unlike the adjacent Southern Bight of the North Sea and the adjacent Oosterschelde, CO2under-saturation prevailed in summer in Lake Grevelingen due to a summer-time bloom, as also evidenced by O2. pCO2was vertically virtually
homogeneous in winter and fall, and showed the strongest vertical gradient during the anoxic event in August. CH4values were minimal in winter (~20 nM) and as stratification developed
during spring and summer a distinct maximum of CH4(up to 730 nM) developed at the pycnocline (5 to 10 m). N2O showed little seasonal variations and only a very faint increase with depth,
except in August when bottom waters became anoxic. At this time, N2O shown a maximum (~22 nM) at the oxycline (probably related to enhanced N2O production by nitrification at low O2
