Benthic carbon dioxide flux

What is the benthic carbon dioxide flux?

The benthic carbon dioxide flux refers to the amount of carbon dioxide (CO₂) released from a unit area of sediment over a specific time interval, during the decomposition of organic matter.

Figure 1. Box and whisker diagram showing median, ranges and 25^th and 75^th percentiles for benthic CO₂ fluxes for some coastal waterways from around Australia. The data was extracted from the OzCoast and OzEstuaries database. Abbreviated names are as follows: Hinchinbrook Channel (HC); Myall Lake (MYL); Swan River (SWR); Wallis Lake (WL); Port Phillip Bay (PPB); Wilson Inlet (WI); Durras Lake (DL); and Moreton Bay (MB). Note that most waterways have median values that are in the oligotrophic range, based on the criteria of Eyre (2002) [1].

What causes the benthic carbon dioxide flux to change?

• Organic matter loadings to sediment (a.k.a. trophic status) are usually the main control on the magnitudes of the benthic CO₂ fluxes.
• Part of the benthic CO₂ flux may also result from the dissolution of calcium carbonate: e.g. CaCO₃ + H₂O + CO₂ = Ca²⁺ + 2HCO₃^-.

Significance of the benthic carbon dioxide flux

Managers need reliable criteria to define acceptable nutrient loads to coastal waterways in order to effectively manage eutrophication [1]. Such criteria are best based on carbon loadings (rather than nutrient loadings) because excess nutrients alter the delicate balance between carbon production and metabolism [1,2]. The benthic CO₂ flux is a proxy of organic carbon loading to sediment, and is inversely correlated to denitrification efficiency and benthic p/r ratio in some wave-dominated estuaries in southeastern Australia [1]. Fluxes of <48 mmol m^-2 d^-1, 49-96 mmol m^-2 d^-1, 97-144 mmol m^-2 d^-1 and >144 mmol m^-2 d^-1, in these systems, are indicative of oligotrophic, mesotrophic, eutrophic and hypertrophic conditions, respectively [1] (Figure 1).

The efflux of carbon dioxide from the sediment contributes to water column PCO₂, which strongly influences whether a waterway is net autotrophic or net heterotrophic. It also affects water column pH and hardness because changes in dissolved CO₂ concentrations influence the saturation state of calcium carbonate.

Considerations For Measurement and Interpretation

The efflux of CO₂ from sediment is determined using benthic chambers. Benthic chambers are open-bottom containers (usually constructed out of perspex) that enclose an area of sediment and overlying water. The chambers are deployed on the sediments/substratum, to capture gas and solute movement between the sediment and the water column. Total CO₂ is usually calculated from pH and alkalinity [3].

Existing Information And Data

Benthic CO₂ flux data for a small number of Australian coastal waterways were compiled during the National Land and Water Resources Audit. This information is available from the OzEstuaries website.

More information on organic matter (changed from natural).

References

1. Eyre, B. and A.J.P. Ferguson 2002. Sediment biogeochemical indicators for defining sustainable nutrient loads to coastal ecosystems, Proceedings of Coast to Coast 2002 - "Source to Sea", Tweed Heads, pp. 101-104.
2. Nixon, S.W. (1995). Coastal marine eutrophication: A definition, social causes, and future concerns. Ophelia 41, 199-219.
3. Grasshoff, K., Ehrnhardt, M., Kremling, K. (1983). Methods of seawater analysis . Verlag Chemie, Weinheim.