Modelling work in LOCATE WP3 brought together terrestrial and marine approaches to studying how dissolved organic carbon, that originates on land (terrigenous DOC), is carried by rivers into the sea and transported throughout the global ocean.

LOCATE developed a new unified terrestrial-marine model to better understand the breakdown and fate of this terrigenous DOC to shed light on this important but uncertain component of the global carbon budget. The new model is applied directly coupled at two scales of riverine and marine coupled hydrodynamic-biogeochemical models that explore the interactions of terrigenous DOC with the wider aquatic ecosystem and its transport. The two scales are: 1) the estuarine and catchment scale to explore the detailed interactions within these complex environments; and 2) the UK national scale to investigate transport pathways and transformations of DOC into the sea and across the continental shelf.

LOCATE also conducted preliminary investigations of terrigenous DOC transport at a global scale using Lagrangian particle tracking approaches driven by high resolution global model simulations. These consider the 100 largest rivers around the world, with the aim of providing a first look at the long-term fate of terrigenous DOC, and specifically whether or not it becomes isolated from atmosphere exchange on climate relevant time scales.

Composite image of research areas

The models developed in LOCATE

UniDOM

The transport of dissolved organic matter (DOM) across the land-ocean-aquatic-continuum (LOAC), from freshwater to the ocean, is an important yet poorly understood component of the global carbon budget. Exploring and quantifying this flux is a significant challenge given the complexities of DOM cycling across these contrasting environments.

We developed a new model, UniDOM, that unifies concepts, state variables and parameterisations of DOM turnover across the LOAC. Terrigenous DOM is divided into two pools, T1 (strongly-UV-absorbing) and T2 (non- or weakly-UV-absorbing), that exhibit contrasting responses to microbial consumption, photooxidation and flocculation.

Data are presented to show that these pools are amenable to routine measurement based on specific UV absorbance (SUVA). In addition, an autochtonous DOM pool was defined to account for aquatic DOM production. A novel aspect of UniDOM was that rates of photooxidation and microbial turnover are parameterised as an inverse function of DOM age.

Model results, which indicated that ~ 5% of the DOM originating in streams may penetrate into the open ocean, are sensitive to this parameterisation, as well as rates assigned to turnover of freshly-produced DOM. The predicted contribution of flocculation to DOM turnover was remarkably low, although a mechanistic representation of this process in UniDOM was considered unachievable because of the complexities involved.

Our work highlighted the need for ongoing research into the mechanistic understanding and rates of photooxidation, microbial consumption and flocculation of DOM across the different environments of the LOAC, along with the development of models based on unified concepts and parameterisations.

Read the full paper at https://link.springer.com/article/10.1007/s10533-019-00621-1

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