Pelagic calcite precipitation in lakes: From a global to a local perspective on its drivers and implications
- Autores: Hares Khan
- Directores de la Tesis: Alo Laas (dir. tes.), Biel Obrador Sala (dir. tes.)
- Lectura: En la Universitat de Barcelona ( España ) en 2021
- Idioma: inglés
- Tribunal Calificador de la Tesis: Marie-Elodie Perga (presid.), Daniel von Schiller Calle (secret.), Lluís Gomez Gener (voc.)
- Programa de doctorado: Programa de Doctorado en Ecología, Ciencias Ambientales y Fisiología Vegetal por la Universidad de Barcelona
- Materias:
- Enlaces
- Tesis en acceso abierto en: TESEO
- Resumen
Inland waters are relevant components of the global carbon cycle acting as hot spots for biogeochemical processes by which carbon is stored, transformed, and outgassed to the atmosphere. Therefore, less than half of the carbon that is exported from terrestrial ecosystems to inland waters finally reaches the oceans. Lakes and reservoirs have the ability to store and emit carbon in large quantities, comparable in magnitude to other major global carbon fluxes such as fossil fuel combustion, the oceanic carbon dioxide (CO2) sink and the continental carbon sink. Substantial scientific studies in the last two decades have attempted to quantify these fluxes and identify their drivers so that inland waters are accounted for in carbon budgets and Earth System models. Much of these studies have given prevalence to organic processes in determining carbon fluxes in lakes. Organic carbon burial has been described as the main form of the sedimentary carbon sink of lakes. Similarly, mineralization of organic carbon resulting from aquatic net heterotrophy has been identified as the main driver controlling CO2 emissions to the atmosphere. Hence, global carbon budgets and lake models have mostly accounted for the organic components of the carbon cycle in lakes, assuming that inorganic processes such as dissolved inorganic carbon (DIC) reactions or calcium carbonate precipitation play a negligible role. However, geological evidence from sedimentary cores shows that sedimentary carbonates act as a long-term carbon sink in lakes. Furthermore, recent studies suggest that calcite precipitation may be a relevant source of the CO2 emissions from lakes and that calcite precipitation can have important effects on the DIC equilibria. Yet, calcification, its resulting storage in the sediments, and its associated CO2 emissions have never been quantified at a global scale. Therefore, the relative contributions of organic to inorganic processes in the carbon budget of lakes is unclear. This stresses the need for a global estimation of this process. Calcite precipitation is acknowledged to be promoted by primary production through several mechanisms. However, the relationship between primary production and calcification has not been clearly described in freshwaters. Hence, the drivers of calcite precipitation are still unclear. Understanding the drivers of calcite precipitation is needed to be able to parameterize this process for a proper inclusion of inland waters’ carbon cycle in Earth System Models. This is particularly relevant in lake models because calcification can cause discrepancies between empirical data and theoretical models that rely solely on metabolic processes for assessing lake carbon fluxes. The main objective of this thesis is to understand the relevance of calcite precipitation, its drivers, and implications from a global to a local scale on lakes. The main findings are divided into three chapters, each corresponding to a published journal article. The first journal article aims at assessing the global relevance of pelagic calcification and its associated CO2 emissions in lakes by providing a first estimate of these fluxes at a global scale. This estimate is based on the relationship between calcification rates, reported in the literature, and water alkalinity. The main findings suggest that pelagic calcification in lakes is a globally relevant process, similar in magnitude to organic carbon burial in lake sediments. Its associated CO2 emissions, however, are minor due to the buffering effect of the carbonate equilibria. Furthermore, pelagic calcification can potentially occur in lakes of alkalinity above 1 meq L-1, representing more than half of the surface area covered by lakes and reservoirs globally. The importance of lake pelagic calcification stresses the need to understand its drivers so that this process can be parameterized and accounted for in lake models and carbon budgets of inland waters. This is addressed in the second journal article that combines in situ incubation experiments with high frequency data, obtained from automatic monitoring stations in several European lakes, to understand the drivers of pelagic calcification. A strong relationship between calcification, Net Ecosystem Production (NEP) and calcite saturation was identified, whereby the calcification to NEP ratio is strongly determined by calcite saturation in water. The effect of NEP on calcification acts at a short temporal scale, whereby an increase in NEP induces calcification. Yet, the efficiency of NEP to induce calcification at a short temporal scale depends on calcite saturation which follows variations at a longer temporal scale, usually reaching its highest values during the summer season. The relationship between the calcification to NEP ratio and calcite saturation can be used as a parameterization for calcite precipitation in lakes and reservoirs. Furthermore, this relationship can also explain the imbalances between inorganic carbon dynamics and lake metabolism often observed in lakes. The third journal article identifies such imbalances in eight Estonian lakes, where large deviations from the metabolic 1:1 stoichiometry between dissolved oxygen (DO) and DIC can occur in lakes of higher alkalinity. The main findings in this article suggest that these deviations are caused by calcification and can be modelled using the calcification to NEP ratio. This thesis provides a first global estimate of pelagic calcite precipitation and its associated CO2 release and offers a first parameterization of calcification that can be used to account for calcification in lake carbon budgets. These results should be considered as a stepping stone to be further built upon. Furthermore, this work 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 in the carbon budgets and models of lacustrine ecosystems.
