The Land Ocean Aquatic Continuum (LOAC)

With a first order approach, the global carbon cycle can be conceptualized using only three large reservoirs: the atmosphere (which receives the anthropic emissions of CO₂ resulting from fossil fuel consumption and land-use change), the terrestrial domain and the oceanic domain. This approach was used in the previous article of this MOOC describing the notions of natural, present-day and anthropogenic carbon cycles. However, each of these large compartments can be subdivided into smaller reservoirs to provide a more detailed representation of the Earth System.

Until 20 years ago, regardless of the complexity of the representation of the oceanic and continental parts of the global carbon cycle, the connection between these two domains was either ignored or reduced to its simplest expression: a lateral flux representing the discharge of carbon from rivers into the ocean. However, this representation remains a oversimplification of role of the ecosystems in charge of this connection.

On its way to the ocean from terrestrial soils, carbon must journey through several systems as diverse as headwaters, rivers, lakes, estuaries, wetlands and coastal oceanic waters. This cascade of inter-connected ecosystems is called the Land-Ocean Aquatic Continuum (LOAC). In 2007, Cole et al., acknowledged that carbon fluxes were significantly altered during their transit along the LOAC through CO₂ exchange with the atmosphere and burial of particulate organic in aquatic sediments. This means that the LOAC is more than just a passive pipe transferring carbon from the continents to the ocean and cannot be reduced to a single flux. Indeed, at least two additional fluxes connecting the LOAC to the atmosphere and aquatic sediments should be considered to better represent to the role of the LOAC in the global carbon cycle. 

In 2013, Regnier et al. quantified, for the first time, the magnitude of the carbon fluxes transiting through the LOAC for present days as well as for the pre-anthropogenic period allowing, by difference, to evaluate the contribution of the anthropic perturbation on the LOAC. This confirmed that already in its natural state, a large fraction of the carbon entering the LOAC is either re-emitted into the atmosphere under the form of CO₂ or buried in aquatic sediments as organic matter before reaching the ocean. Furthermore, an additional complexity stems from the fact that all compartments of the LOAC do not exhibit the same behaviour with respect to their processing of carbon. 

For instance, inland waters as a whole emit CO₂ into the atmosphere while estuaries, in average take up CO₂ from the atmosphere. Finally, this study also suggested that a fraction of the terrestrial anthropogenic carbon sink is actually transferred to and through the LOAC, eventually reaching the ocean. In recent years, a lot of additional research has been carried out to better understand and quantify the contributions of each of the compartment of the LOAC to the global carbon cycle and improve its representation in regional and global models used to investigate climate dynamics (Earth System Models).

Suggested further reading:

Cole, J.J., Prairie, Y.T., Caraco, N.F. et al. Plumbing the Global Carbon Cycle: Integrating Inland Waters into the Terrestrial Carbon Budget. Ecosystems 10, 172–185 (2007). https://doi.org/10.1007/s10021-006-9013-8

Regnier P., Friedlingstein P., Ciais P. et al. Anthropogenic perturbation of the carbon fluxes from land to ocean. Nature Geoscience, 6, 597-607 (2013). doi:10.1038/ngeo1830.