Climate regulation – and much more

By absorbing carbon dioxide from the atmosphere, the ocean mitigates global warming. This invaluable service is based on chemical and biological processes in the seawater. The cycling of elements also secures many other important ecosystem services. Climate change may disturb their balance.

Life-giving elemental cycles

Carbon, nitrogen and phosphorus: three interacting elemental cycles are essential for life in the ocean. The most important of them is the carbon cycle which also plays a key role in the climate system. Other ocean ecosystem services will be affected by changes in the cycling and exchange of elements as well.

By absorbing approximately 30 per cent of the human-induced carbon dioxide (CO₂) emitted every year, the ocean slows down the increase of the greenhouse gas in the atmosphere and thereby mitigates global warming. But the more carbon dioxide is dissolved in seawater, the lower the ocean’s buffer capacity becomes, limiting its ability to take up more. Warming further increases the problem: with rising temperatures, seawater holds less and less CO₂.

The biological carbon pump

Apart from chemical reactions, biological processes control the ocean’s CO₂ uptake via the so-called biological carbon pump. In the surface layer, phytoplankton uses carbon dioxide and sunlight to produce organic matter. Some of these particles sink towards deeper layers and are degraded in the ocean interior. In this way, the biological pump tends to lower the carbon dioxide concentration at the surface of the ocean and thus propels the uptake from the atmosphere.

Ocean acidification and rising temperatures lower the efficiency of the biological carbon pump. Surface layer warming increases stratification of the water column. This reduces the nutrient supply into the sunlit upper layer. A decrease in nutrients leads to less organic matter produced at the surface, reducing the amount of carbon transported to the deep.

Ocean acidification weakens the pump

Ocean acidification has been found to cause a shift in the phytoplankton community towards smaller organisms. Since smaller plankton sinks more slowly, the pumping diminishes.In addition, it also becomes harder for calcifying plankton to produce calcium carbonate structures in a more acidified ocean. For example, the single-celled alga Emiliania huxleyi wraps itself with chalky platelets. These particles serve as ballast and accelerate the transport of organic matter towards the deep ocean. Hence, less production of calcium carbonate can weaken the biological pump.

These changes to how carbon sinks to the ocean depths will be partially compensated by the increased fluidity of warmer surface waters which make particles sink faster. The complex interaction of these processes makes it extremely difficult to estimate their ultimate effects on the biological carbon pump.

The nitrogen and the phosphorus cycles

Apart from the cycling of carbon, two other nurient cycles need to be in balance to ensure the ocean system functions. The nitrogen and the phosphorus cycles sustain marine life and these will be affected by climate change as well. Nitrogen gas, not accessible to phytoplankton, is fixed by specialised organisms in warm surface waters and made available for the food web. Ocean acidification boosts this important nitrogen source. This could benefit the production of organic matter and hence CO₂ pumping unless limited supplies of other nutrients hamper this effect. At the seafloor, where oxygen is sparse, microbes process oxygenated forms of nitrogen, transforming them back to nitrogen gas which is lost to the atmosphere. As the ocean’s oxygen levels decline, the loss of bioavailable nitrogen accelerates, while the release of phosphorus from deep sea sediments increases. Only if the elemental cycles remain roughly in balance, however, will the ocean’s productivity be sustained in the future.

Combined, ocean acidification and warming are perturbing marine elemental cycles. In addition to slowing down CO₂ uptake, this is bound to affect life-sustaining fluxes of essential nutrients, impacting ocean productivity, marine food webs and the services they provide.