Quantification of dissolved organic matter and the metabolic balance in river networks: mechanisms and model simulations of CO2 emissions
We want to discover how land and rivers are connected to better understand the global carbon cycle and river food webs north of the Arctic circle. Rivers are the 'blood vessels' draining the 'body of the Earth’ and export vast amount of carbon to the sea and the atmosphere. A large fraction of this carbon is from dissolved organic matter leaching out of soils and giving the water a yellow brown colour. On its journey to the sea, bacteria on the riverbed use oxygen to transform this terrestrial carbon into CO2. The increase in bacterial activities may suppress the growth of green algae by competing for limiting nutrients, thus changing the metabolic balance between photosynthesis and respiration (increasing CO2 emissions) and the quality of the food available for consumers.
We have deployed sensors throughout the river Tana network to estimate daily and annual carbon and O2 pulses of the aquatic ecosystem. We concomitantly estimated the changes in terrestrial land vegetation activity using satellite imageries at the daily time scale. We described and sampled riparian soils in four sub-catchments with varying land cover to assess soil carbon pool and transformation through the soil horizons. We carried out a laboratory experiment to explore the fate of dissolved organic carbon from mire onto the metabolic balance (photosynthesis, respiration and associated CO2 emissions) of aquatic microcosms. We are using state-of-the-art methods to identify and describe the transformation of thousands of molecules (metabolites) at the soil – water interfaces and during their journey along the river throughout the growing season.
Finally, we are building a mathematical model to represent, in the most parsimonious way, our understanding of carbon processes in large northern latitude river basins with heterogeneous landscape (rocks, tundra, pine and birch forest and peat plateau with discontinuous permafrost - frozen mounts thawing rapidly with warming). We are using the River Tana (Norway) flowing north of the arctic circle as a model river basin to collect data and run scenarios of climate and land use change.
The annual global export of carbon to rivers is higher than land carbon sequestration. Stream and rivers link the land to the oceans and play a significant role in greenhouse gas emissions contributing 85% of CO2 emissions from inland waters. A large proportion of these CO2 emissions, in the northern hemisphere, come from in-stream processes: the metabolic balance between photosynthesis and respiration. Warming is not thought to alter much the metabolic balance of streams, but changes in dissolved organic matter (DOM) supply, e.g. through thawing permafrost or changes in precipitation could greatly increase respiration activity and associated CO2 emissions, amplifying warming.
QUANTOM wants to discover why and how DOM is leached from soils and transformed in the river. The primary difficulty is that DOM is made of 10,000+ different molecules with different reactivities to light, microbes, nutrients (N, P), temperature and their interactions. QUANTOM will directly estimate DOM transformation using state-of-the-art molecular characterisation of DOM telling us something about the nature of the reactions, the underlying mechanisms. QUANTOM will couple land vegetation growth with in-stream DOM transformation and associated metabolic balance using satellite observation and in-situ sensors at daily time steps. QUANTOM will test how DOM retention and transformation is affect by temperature and nutrient availability through whole ecosystem and lab experiments with biofilms (metabolic reactors), rather than bottle incubations. QUANTOM will deliver a flexible mathematical framework to model and simulate DOM transformations under global climate and land cover change, test it in the largest natural river of Scandinavia draining into the Arctic ocean, with a vision to make the modelling approach applicable globally in natural northern rivers.