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| Titel |
Gaseous carbon balance for a small boreal lake following artificially increased DOC loading |
| VerfasserIn |
Hannu Nykänen, Sari Peura, Paula Kankaala, Roger Jones |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250047229
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| Zusammenfassung |
| Lakes constitute a globally significant source of greenhouse gases carbon dioxide (CO2) and
methane (CH4) to the atmosphere. Most lakes are now known to be net sources of CO2 to the
atmosphere due to mineralization of allochthonous organic carbon from the catchment
outweighing net autochtonous carbon production and retention in the lake. Predicted changes
in precipitation and hydrological patterns may change carbon input from catchment area. In
this study possible effects of artificially increased organic carbon input on a lake carbon cycle
were evaluated.
The studied lake, Alinen Mustajärvi, is a small, oligotrophic, mesohumic headwater lake
located in a boreal coniferous forest area in southern Finland. Because of its sheltered
position and high concentration of allochthonous humic substances, the lake is normally
steeply stratified with respect to temperature and oxygen. The lake is covered by ice for 5–6
months each year and is partially meromictic. The natural epilimnetic DOC concentration of
the lake is around 11 mg C l-1. Dissolved organic carbon concentration in the
epilimnion was increased by adding carbon as cane sugar. The goal was to increase the
amount of labile DOC available for heterotrophic bacteria without affecting the light
climate for autotrophic phytoplankton. Cane sugar was chosen because, as well as
increasing DOC availability, its isotopic signature is typical of C4 plant and thus
markedly different from that of local C3 plants, allowing the added DOC to act as an
isotopic tracer in the lake carbon fluxes. The year 2007 served as a control year,
while during the ice-free periods (May to October) of 2008 and 2009 66 kg of cane
sugar was added monthly, thus adding 22 g C m-2to the epilimnion annually. This
amount was calculated to produce an approximately 2 mg l-1 rise in the DOC
concentration in the epilimnion, intended to raise labile DOC concentration from around the
first quartile level for boreal lakes to that in lakes around the third quartile level
(Henriksen et al. 1998). During 2010 sugar addition ceased and recovery of the lake was
monitored.
CH4 and DIC concentrations and their stable isotopic composition were measured at the
deepest point of the lake from 1 m intervals. Fluxes of CH4 and CO2 from the lake surface to
the atmosphere were estimated with boundary layer diffusion equations presented by Phelps
et al. 1998 and Cole and Caraco 1998. Amount of CH4 oxidized was estimated from vertical
CH4 and DIC stable isotope measurements and by comparing differences between observed
concentrations and CH4potentially transported by turbulent diffusion between different
vertical layers in the lake (Kankaala et al. 2006). Community respiration and primary and
bacterial production were measured from the euphotic zone. Concentrations of nutrients,
Chlorophyll a, DOC and POC were measured from composite samples from the
meta-, epi- and hypolimnion as well as the biomass of bacteria, phytoplankton and
zooplankton. δ13C and δ15N of dissolved and particulate organic matter (DOM, POM)
and zooplankton were analyzed from composite samples from the meta-, epi- and
hypolimnion.
Results from stable isotope analyses indicate that the added cane sugar was
effectively used in specific lake processes, while changes in estimated CH4 and
CO2 fluxes suggest that the additional carbon loading had an effect on overall lake
metabolism.
References
Cole, J. J., and Caraco N. F. 1998. Limnol. Oceanogr., 43, 647-656.
Henriksen A., Skjelvåle B.L., Mannio J., Wilander A., Harriman R., Curtis C., Jensen
J.P. Fjeld E. & Moiseenko T. 1998. Ambio 27: 80–91.
Kankaala P., Huotari J., Peltomaa, E., Saloranta T. & Ojala, A. 2006. Limnol. Oceanogr.,
51, 1195–1204.
Phelps, A. R., Peterson K. M., and Jeffries M. O. 1998. J. Geophys. Res., 103, 29029 –
29036. |
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