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| Titel |
Biogeochemistry of a large, meromictic tropical lake (Lake Kivu, East Africa): insights from a stable isotope study covering an annual cycle |
| VerfasserIn |
Cedric Morana, François Darchambeau, Fabrice Muvundja, Fleur Roland, Zita Kelemen, Marc-Vincent Commarieu, Bruno Leporcq, Georges Alunga, Pascal Masilya, Jean-Pierre Descy, Alberto V. Borges, Steven Bouillon |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250090827
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| Publikation (Nr.) |
 EGU/EGU2014-5086.pdf |
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| Zusammenfassung |
| Lake Kivu (East Africa) is a large (2370 km2) and deep (maximum depth of 485 m)
meromictic lake. Its vertical structure consists of an oxic and nutrient-poor mixed layer down
to 70 m maximum, and a permanently anoxic monimolimnion rich in dissolved
gases (methane and carbon dioxide) and nutrients. Seasonal variation of the vertical
position of the oxic-anoxic interface is driven by contrasting air humidity and wind
speed regimes between rainy (October-May) and dry (June-September) seasons.
The latter is characterized by a deepening of the oxic zone, and an increased input
of dissolved gases and inorganic nutrients. The mean annual photic depth is 18
m, but water transparency slightly decreases during the dry season. In this study,
we present a comprehensive data set covering a full annual cycle at a fortnightly
resolution, which combine hydrochemical data, δ13C and δ15N measurements of
particulate organic carbon and nitrogen (POC, PN) and zooplankton, δ13C of dissolved
organic and inorganic carbon (DOC, DIC), nutrients and gases (CH4) concentrations,
phytoplankton biomass and composition. In the euphotic zone, phytoplankton biomass was
constant during the rainy season, but doubled during the dry season. In contrast,
δ13C-DIC increased linearly with time during the rainy season, deviating from
the values expected at isotopic equilibrium with the atmosphere, then suddenly
decreased in the dry season due to the vertical mixing with 13C-depleted DIC. Results
of mass-balance calculations indicate that the δ13C-DIC increase reflects the net
autotrophic status of the mixed layer. Irrespective of the season, the δ13C-POC signatures
were constant from the surface to the oxic-anoxic interface, then showed a local
and abrupt excursion to values as low as -40o reflecting the incorporation of a
13C-depleted source in the POC. While the large pool of DIC is the main carbon
source for POC in surface waters, CH4 contributes significantly to C fixation at the
oxic-anoxic interface all year round. The δ13C signature of the DOC pool shows very
little variation in the mixolimnion and thus appears to be uncoupled from the POC
pool, suggesting that old and refractory compounds constitute the major part of the
DOC pool. Also the more labile and freshly produced DOC that reflects the δ13C
signature of the POC, is rapidly mineralized. Finally, we noticed a shift toward higher
values in the δ15N-PN during the dry season (from 0.5oto 4.0) and δ15N-PN was
significantly related to the proportion of cyanobacteria in the euphotic zone. Hence
the variation of δ15N-PN in surface waters could either reflects a change in the
dominant phytoplankton taxa or in the biogeochemical processes controlling the upward
nitrate and ammonium fluxes. Zooplankton δ15N signatures mirrored the seasonal
changes in δ15N-PN and were significantly correlated to phytoplankton biomass,
highlighting their dependence on autochtonous sources of organic matter in this large lake. |
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