Biological pump

In oceanic biogeochemistry, the biological pump is the sum of a suite of biologically-mediated processes that transport carbon from the surface euphotic zone to the ocean's interior.

Overview

The organic carbon that forms the biological pump is transported primarily by sinking particulate material, for example dead organisms (including algal mats) or faecal pellets. However, some carbon reaches the deep ocean as dissolved organic carbon (DOC) by physical transport processes such as downwelling rather than sinking.

Carbon reaching the deep ocean by these means is either organic carbon or particulate inorganic carbon such as calcium carbonate (CaCO₃). The former is a component of all organisms, the latter only of calcifying organisms, for example coccolithophores, foraminiferans or pteropods. In reference to the different use of these materials in organisms, the organic carbon portion of this transport is known as the soft tissues pump, while the inorganic carbon portion is known as the hard tissues pump.

In the case of organic material, remineralisation (or decomposition) processes such as bacterial respiration, return the organic carbon to dissolved carbon dioxide. Calcium carbonate dissolves at a rate dependent upon local carbonate chemistry. As these processes are generally slower than synthesis processes, and because the particulate material is sinking, the biological pump transports material from the surface of the ocean to its depths. For an overview, see Raven & Falkowski (1999).

The biological pump has a physico-chemical counterpart known as the solubility pump.

Anthropogenic changes

Land-use changes, the combustion of fossil fuels, and the production of cement have led to a flux of CO₂ to the atmosphere. Presently, about one third (approximately 2 Gt C y^-1; Takahashi et al., 2002; Orr et al., 1997) of anthropogenic emissions of CO₂ are believed to be entering the ocean. However, the biological pump is not believed to play a role in this flux. This is because the biological pump is ultimately limited by the availability of light and nutrients, and not by carbon. The extra carbon provided by anthropogenic activities does not lead to an increase in biological productivity in the oceans. This is in contrast to the situation on land, where elevated atmospheric concentrations of CO₂ increase primary production. This occurs because plants on land are able to improve their water-use efficiency (= decrease transpiration) when CO₂ is easier to obtain.

However, climate change may affect the biological pump in the future by warming and stratifying the surface ocean. It is believed that this could decrease the supply of nutrients to the euphotic zone, reducing primary production there. Also, changes in the ecological success of calcifying organisms caused by ocean acidification may affect the biological pump by altering the strength of the hard tissues pump (Orr et al., 2005). This may then have a "knock-on" effect on the soft tissues pump because calcium carbonate acts to ballast sinking organic material.

References

Orr, J. C., E. Maier-Reimer, U. Mikolajewicz, P. Monfray, J. L. Sarmiento, J. R. Toggweiler, N. K. Taylor, J. Palmer, N. Gruber, C. L. Sabine, C. Le Quéré, R. M. Key and J. Boutin (2001). Estimates of anthropogenic carbon uptake from four three-dimensional global ocean models. Global Biogeochem. Cycles 15, 43-60.

Orr, J. C. et al. (2005). Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437, 681-686.

Raven, J. A. and P. G. Falkowski (1999). Oceanic sinks for atmospheric CO₂. Plant Cell Environ. 22, 741-755.

Takahashi, T., S. C. Sutherland, C. Sweeney, A. Poisson, N. Metzl, B. Tilbrook, N. Bates, R. Wanninkhof, R. A. Feely, C. Sabine, J. Olafsson and Y. C. Nojiri (2002). Global sea-air CO₂ flux based on climatological surface ocean pCO₂, and seasonal biological and temperature effects. Deep-Sea Res. Pt. II 49, 1601-1622.

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Overview

Anthropogenic changes

References

See also