Start date : September 2019 

• PhD Thesis: Julie Rotschi
• Contact: Jean-Philippe Jenny, Isabelle Domaizon, Irene Gregory-Eaves

Description 

Inland surface waters modify and transfer large amounts of carbon (C) from watersheds, contributing significantly to the global C cycle and acting on the climate^1,2. Continental and oceanic sedimentation is the main long-term storage mechanism for C exported by inland waters. However, the storage of C in continental sediments (reservoirs, rivers, lakes) has received much less attention than that of marine sediments, and the C-sources and the dynamics of C-sequestration remain poorly understood on a large scale, in particular concerning the action of biological productivity in carbon fixation^1,3. In addition, long-term dynamics are still poorly evaluated, necessitating the use of geological archives and modelling.

The biogeochemical (pigments, DNA) and geochronological (e.g. 14C) study of lacustrine sedimentary archives enables the continuous documentation, at an annual or multi-year resolution, of the evolution of carbon sequestration by lakes and algal productivity over centuries to thousands of years⁴. The study of these sediment archives also makes it possible to document the sources of C inputs, associated to external sources that depend on C-eroded from watersheds⁵ (allochthonous C), and on lake primary production⁶ (Autochtonous C). However, it is unclear how these respective sources influence long-term C-sequestration, and it therefore seems urgent to address this issue, particularly with respect to the contribution of primary production that has received relatively little attention at the global scale over time scales multi-decadal to centennial⁷.

This doctoral research project is therefore focused on the study of control factors of carbon sequestration by lakes over long time, at regional and global scales. Through a multi-scale approach, we propose to consolidate a global and multi-proxy paleo-limnological database (ie of lacustrine sedimentary archives) in order to analyze the 300-years historical trajectories of primary production for 420 sites in the world and to determine which of the algal groups have contributed to total primary productivity over time, and what have been the effects on carbon transfer and its sequestration in sediments. This project will address for the first time on these scales of space and time the issue of the dynamics of biological communities interacting with the transfer of matter in lake -watershed systems. This project will also lay the foundations for a long-term numerical model to simulate primary production and lacustrine carbon fluxes, which will build on existing models developed by the AFB-Irstea Cluster. This project will be part of the CARRTEL research axis on carbon transfer in lakeside environment.

References

1. Regnier, P. et al. Anthropogenic perturbation of the carbon fluxes from land to ocean. Nat. Geosci. 6, 597–607 (2013).
2. Maavara, T., Lauerwald, R., Regnier, P. & Van Cappellen, P. Global perturbation of organic carbon cycling by river damming. Nat. Commun. 8, 15347 (2017).
3. Weyhenmeyer, G. Large differences in the efficiency of large lakes to transform nutrients and pollutants from the watershed. in (2018).
4. Dearing, J. A. et al. Extending the timescale and range of ecosystem services through paleoenvironmental analyses, exemplified in the lower Yangtze basin. Proc. Natl. Acad. Sci. 109, E1111–E1120 (2012).
5. Tranvik, L. J. et al. Lakes and reservoirs as regulators of carbon cycling and climate. Limnol. Oceanogr. 54, 2298–2314 (2009).
6. Sobek, S. et al. Organic carbon burial efficiency in lake sediments controlled by oxygen exposure time and sediment source. Limnol. Oceanogr. 54, 2243–2254 (2009).
7. Taranu, Z. E. et al. Acceleration of cyanobacterial dominance in north temperate-subarctic lakes during the Anthropocene. Ecol. Lett. 18, 375–384 (2015).