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On September 16th starting at 16:00 doctoral student Hares Khan will be defending his thesis „Pelagic calcite precipitation in lakes: from a global to a local perspective on its drivers and implications." It is joint defense with the University of Barcelona.

Inland waters are relevant components of the global carbon cycle acting as active sites for biogeochemical processes. Lakes and reservoirs store and emit carbon in large quantities, comparable in magnitude to other major global carbon fluxes. This is of great relevance within the context of climate change. It is therefore important to quantify carbon fluxes from lakes for managing greenhouse gas emissions and climate change mitigation programs. Identifying the environmental conditions that drive carbon fluxes in lakes is needed for understanding the processes that influence the carbon cycle in lacustrine ecosystems. This knowledge is essential for improving Earth System Models and carbon budgets.

Carbon emissions and storage in lakes have widely been attributed to organic processes, whereby organic carbon is either stored in the sediments or degraded through metabolic processes that release greenhouse gases to the atmosphere, such as carbon dioxide or methane. However, the inorganic process known as calcite precipitation, that describes the formation of calcium carbonate from water, can also play an important role in this regard. However, it has never been quantified at a global scale and the environmental conditions for its occurrence are not well understood. This thesis offers a first global estimation of pelagic calcite precipitation in lakes and its associated carbon emissions. Furthermore, it identifies the main drivers that control this process and its implications at a global and local scale. The main findings suggest that pelagic calcite precipitation is potentially a widespread process that can occur in most of the world’s lakes. Globally, its relevance is similar in magnitude to organic carbon burial in the sediments. However, its contribution to carbon dioxide emissions is negligeable. Primary production was identified as a main driver of pelagic calcification. However, the efficiency of primary production to induce calcification depends on the geochemistry of a lake, especially its level of alkalinity and saturation in calcite. This relationship can be used to account for this process in lake carbon models. This thesis stresses the need to adjust our current understanding of lake carbon cycling by accounting for organic processes along with inorganic processes such as calcite precipitation.

Supervisors are Dr. Biel Obrador (University of Barcelona, Spain) and Dr. Alo Laas (Estonian University of Life Sciences). Opponent is Prof. Marie Elodie Perga (Université de Lausanne). The defense will take place via virtual webroom but is located in University of Barcelona in Aula Magna. 

Abstract is available in Library of Estonian University of Life Sciences DSpace archive.