Ice cover determines winter oxygen depletion in a high mountain lake Granados, I.1, Toro, M.2, Sánchez-López, G.3 & Giralt, S.3
Acknowledgements The field work was funded by the Spanish Ministry of Economy and Competitiveness through the project PALEONAO (CGL2010-15767/BTE). The Servicio Territorial de Medio Ambiente de Ávila of Junta de Castilla y León gave the permissions and provided the unvaluable help of an helicopter flight with the field material.
Centro de Investigación, Seguimiento y Evaluación. Parque Nacional de la Sierra de Guadarrama.
[email protected] 2 Centro de Estudios Hidrográficos del CEDEX.
[email protected] 3 Institute of Earth Sciences Jaume Almera (CSIC).
[email protected] [email protected] 1
Study site
Methods
Cimera Lake
Sierra de Gredos, Central Range, Spain Mediterranean continental climate 40.263720N, 5.305377E 2.140 m a.s.l.
100 m 9-10m 8-9m 7-8m 6-7m 5-6m 4-5m 3-4m 2-3m 1-2m 0-1m
Buoy
Buoy
multiparameter probe
Water column
Basin: 756.000 m Lake: 44.900 m2 Volume: 216.890 m3 Max. depth: 9.4 m Max. axis: 384 m
Temperature dataloggers
2
9.5m
multiparameter probe
Residence time: 58 days Oligotrophic Discontinuous cold polymictic
Anchor
Sediment
Anchor
Buoy
● High mountain lakes are frequently characterized by water column oxygen depletion during winter ice cover. Even in oligotrophic lakes, this depletion can result in anaerobic bottom waters conditions if ice cover properties and length are appropriate. Winter ice cover represents a barrier to the gas exchange between water and the atmosphere, and also, depending of its thickness and structure, to the light penetration in the water column.
3 0 -3
ICE COVER
20
ICE COVER
Main �indings
ICE COVER
NO DATA
18
Long term monitoring
16
● Annual lake thermal regime signals: → Ice cover freeze-up: lake water stratification starts in 3-4 days. → Ice cover break-up: water column is mixed in 1-2 days → Discontinuous cold polymictic regime: very light summer stratifications are occasionally broken by heavy rainfalls.
12 10 8 6
0.5 m 1m 3m 6m 9m
4 2 0 600
NO DATA
NO DATA
NO DATA
400
200
1m 9m
100 0 32 28 24 20 16 12 8 4 0
● Dissolved oxygen: → Ice cover period: in typical winters, oxygen depletion in bottom layers is produced progressively since ice cover formation. Exceptionally winters (eg. 2011-2012 with high positive NAO index), oxygen levels are over 70% sat. → Summer period: homogeneous oxygen concentration in lake water column except very short stratification periods with very slight oxygen consumption in bottom layers.
1m 9m
NO DATA
Unusual years
● NAO index for 2011-2012 winter shows very markedly positive values (colder and dryer than usual conditions). In contrast with winters with higher snow acumulation, this 2011-2012 ice cover period was characterized by:
sep
oct
nov
dic
ene
feb
mar
abr
may
jun
2010
jul
ago
sep
oct
nov
dic
ene
feb
mar
abr
may
jun
2011
jul
ago
sep
oct
nov
dic
ene
feb
mar
abr
2012
may
jun
jul
ago
sep
2013
Key changes examples (Legend as in daily multiprobe data graphs)
110
8
100
6
90
4
80
2
Conductivity µS cm-1 25°C
10
2012 ● Ice on: After a short period with temporary ice covers, the lake surface freezes on 25-26 nov. ● After freeze-up: → An inverse thermal stratification develops in 3-4 days, which reduces vertical water movements. → As snow accumulates over ice cover it begins to block light transmission to lake, and near December 7th water column is almost dark. → There is no gas exchange with atmosphere and oxygen concentration slowly decrease near sediments, although some production remains immediately below ice cove. → Conductivity slightly increases near sediments, while remains nearly constant near ice cover.
2000
4
15
1500
14
1000
13
500
12
0
2000 1500 1000 500 0
140 120 100 80 60 40 20 0
35 30 25 20 15 10 5 0
2013 ● Ice off: ice cover melts on 15 jun (in max depth point). ● In the previous days before break up: → Higher radiation levels reach the water (patchy and thinner ice) → Inverse thermal stratification disappears, with vertical mixing of the water column. → As melted water from the catchment flow to the lake, bottom conductivity falls to the same values as surface water. On the other hand, bottom oxygen rise to saturation values. ● After brake-up: → Water column progressively warms, with incipient thermal stratification and daily cycle. → Slight oxygen rise due to photosynthesis.
120
10
110
8
100
6
90
4
80
2
-se 03 p -se 05 p -se 07 p -se 09 p -se 11 p -se 13 p -se 15 p -se 17 p -se 19 p -se p
120
16
01
0
2500
Temperature °C
0
5
Dissolved Oxygen % saturation
200
2500
PAR-2 -1 µmol m s
1
17
Conductivity µS cm-1 25°C
2
400
Avalanche
3000
03 n -ju 05 n -ju 07 n -ju 09 n -ju 11 n -ju 13 n -ju 15 n -ju 17 n -ju 19 n -ju 21 n -ju 23 n -ju 25 n -ju 27 n -ju 29 n -ju n
600
3
Dissolved Oxygen % saturation
4
Temperature °C
800
SEASONAL ICE COVER
14 12 10 8 6 4 2 0
-ju
SEASONAL ICE COVER
10 -n 12 ov -n 14 ov -n 16 ov -n 18 ov -n 20 ov -n 22 ov -n 24 ov -n 26 ov -n 28 ov -n 30 ov -n 02 ov -d 04 ic -d 06 ic -d 08 ic -d ic
Dissolved Oxygen % saturation
Temperature °C
5
Ice-free
01
Temporary ice covers
Ice-off PAR-2 -1 µmol m s
Ice-on
2011 ● Ice free period: With relatively stable conditions: → Water temperature reflects daily fluctuations, although at night is nearly homogeneous. → PAR radiation reach the sediment, so the whole water column is photic zone. → Oxygen follows also a daily cycle, although it has higher levels in surface waters. During this period, production prevails over consumption. → Conductivity in very low values.
Avalanche SEASONAL ICE COVER 10
PAR-2 -1 µmol m s
ago
3 2 1 0
0 120
10
100
8
80
6
60 40
4
20
2
0
01 -m 03 ar -m 05 ar -m 07 ar -m 09 ar -m 11 ar -m 13 ar -m 15 ar -m 17 ar -m 19 ar -m ar
jul
Conductivity µS cm-1 25°C
1m 9m
→ Thinner and relatively transparent ice cover (black ice, snow free) → Low oxygen consumption in the lake bottom ( [O2]> 70%) → Lake bottom conductivity nearly constant → Less stratified water column → In March, partial ice cover thaw along the shores. Water column homogenization (oxygen, temperature) and slight radiation entering in the lake.
Temperature °C
130 120 110 100 90 80 70 60 50 40 30 20 10 0
NO DATA
Dissolved Oxygen % saturation
Dissolved Oxygen % saturation
● Conductivity: → Surface layers: constant values throughout the year. → Bottom layers: progressive increase during ice cover period or short summer stratification periods (solutes from sediment), with sudden dilution by thawing (winter) or heavy rainfalls (summer).
300
Conductivity µS cm-1 25°C
PAR µmol m-2 s-1
500
● Photosynthetically Active Radiation (PAR) input: → Ice cover period: no PAR signal except minimal values during snow free ice cover dates. → Summer period: low PAR values reflect cloudy and stormy days (and water column mixing). Significant PAR reach lake bottom layers.
PAR-2 -1 µmol m s
Temperature °C
14
Conductivity µS cm-1 25°C
NAO Index
Daily multiprobe data
2013 ● Avalanche: On March 7th 2013 a huge snow avalanche falls over the south part of the ice cover. → Abrupt changes were recorded in water temperature, oxygen and conductivity. → Light transmission continued completely blocked. → In the following days, limnological parameters were stabilized in slightly different values than those previous to the avalanche → This changes probably implies big amounts of energy.
