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Mortalität von Zooplankton in Seenökosysteme und seine Rolle im vertikalen Kohlenstofffluss (ZooFlux)

Fachliche Zuordnung Hydrogeologie, Hydrologie, Limnologie, Siedlungswasserwirtschaft, Wasserchemie, Integrierte Wasserressourcen-Bewirtschaftung
Mikrobielle Ökologie und Angewandte Mikrobiologie
Förderung Förderung von 2016 bis 2021
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 319532628
 
Erstellungsjahr 2022

Zusammenfassung der Projektergebnisse

The objective of the proposed study was to quantifying the role of dead zooplankton (carcasses) in the carbon cycle of aquatic ecosystems. Thereby, we have successfully tested the hypothesis that microbial degradation of carcasses during their sinking facilitates the degradation of more refractory organic matter (interactive effect). The project was part of the joint effort of a research team from Germany and Russia. In order to estimate the contribution of zooplankton carcasses to the carbon cycle, we have combined field observations in both Germany and Russia, mesocosm and laboratory experiments as well as modeling. In-situ estimations of the zooplankton mortality and sinking rates by sediment traps in the lakes have been accompanied by measurements of turbulent mixing conditions across the stratified water column using modern hydrodynamic field techniques. Laboratory experiments have quantified microbial colonization and degradation rates of zooplankton carcasses by using advanced 13C-labelling approaches. Both field and laboratory data have been successfully used for an advanced model predicting retention time of carcasses in the water column and their contribution to the carbon cycle. Various model scenarios were used to evaluate the role of environmental factors for settling and degradation of dead zooplankton and on potential changes due to climate-driven hypolimnion warming in lakes. Using abundance data of zooplankton in water column and dead zooplankton in sediment traps, e.g. in Lake Stechlin, an ecological-mathematical model was developed to simulate the abundance and sinking of zooplankton carcasses and predict the related release of labile organic matter (LOM) into the water column. We found species-specific differences in mortality rate of the dominant zooplankton: Daphnia cucullata, Bosmina coregoni and Diaphanosoma brachyurum (0.008, 0.129 and 0.020 day^−1, respectively) and differences in their carcass sinking velocities in the metalimnion (and hypolimnion): 2.1 (7.64), 14.0 (19.5) and 1.1 (5.9) m day^−1, respectively. Our model simulating formation and degradation processes of dead zooplankton predicted a bimodal distribution of the released LOM: epilimnic and metalimnic peaks of comparable intensity, ca. 1mg DW m^−3 day^−1. Maximum degradation of carcasses up to ca.1.7mg DW m^−3 day^−1 occurred in the density gradient zone of the metalimnion. LOM released from zooplankton carcasses into the surrounding water may thus stimulate microbial activity and facilitate microbial degradation of more refractory organic matter (ROM); therefore, dead zooplankton are expected to be an integral part of water column carbon source/sink dynamics in stratified lakes. This has been verified by our lab studies revealing a tight interaction between chemical and microbial diversity. This finding should be considered in the context of overall C turnover in aquatic ecosystems, since long-term changes and seasonal pulses of allochthonous and autochthonous OM input can affect both aquatic chemo- and microbial diversity, impacting net remineralisation rates in response. Zooplankton carcasses are an important and up to now largely neglected piece of the puzzle with regard to OM degradation in both marine and freshwater ecosystems, and thus should be considered in more detail in future studies on C cycling. The combination of laboratory and field experiments, as we have done in other studies relevant to the project, shows the special role of terrigenous ROM input as well as fungi for overall OM degradation - compared to bacteria. These additional studies of the project have opened the door for new research directions and concepts.

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