A research group led by the South Korean Pohang University has measured the effects of atmospheric and fluvial nitrogen deposition [through nitrate] in the coastal seas around China, Korea and Japan.

In their Science publication they conclude nitrogen levels have been significantly raised over large parts of the Northwest Pacific Ocean since 1980. Atmospheric deposition proved to be of larger importance than nitrogen runoff from industry and agriculture through river systems.

Over the same period phosphorus concentrations had not increased. As both are growth-limiting nutrients in coastal seas, the nitrogen pollution may have ecological ramifications, favouring organisms that require a lot of nitrogen and little phosphorous.

Climate relevance?

A growth-limiting nutrient for an organism or ecosystem is the bottleneck that slows down biology. Replenish that one element and the whole cycle of life speeds up. As with plants this means more photosynthesis it means more biomass production and a higher CO2 uptake. We know forests grow faster and denser when nitrogen supply is high [and that on land more nitrogen could mean more nitrous oxide – bad news for the climate].

In an estimated 40 percent of the oceans iron is the most important growth limiter. Iron fertilisation could help create an algae bloom, it is theorised. In the rest of the oceans quite often it is nitrogen and phosphorus that are what most organisms have a craving for.

More biology, more carbon?

‘More biology’ however is not always beneficial for the climate. If the larger biomass production corresponds with an equal increase in biomass oxidation the only gain for the carbon cycle may be a slight increase in carbon stored in living biomass [the main reason why it is better to have a lot of forest than to have little forest].

True carbon gains would come when somehow part of the biomass would be sequestered deep and safe. In the oceans this could happen when dead organic matter accumulates at the ocean floor. Unfortunately we can’t count on all plankton corpses to neatly pile up down deep – there is biology there too, recycling elements, including carbon.

Algae are bright green

If coastal nitrogen pollution would lead to local algae blooms that could have another climate effect though. Algae colour the water, from deep and dark blue, to light and reflective green. It is much like those satellite pictures that show the sediment load of rivers entering the clear dark waters of the deep ocean.

So if we continue to emit large quantities of nitrate and try and add as much phosphorus [use more washing powder!] as we can we may be able to influence albedo on an ocean scale – and hopefully create a little bit of global cooling. Wouldn’t that be great news for the environment!

© Rolf Schuttenhelm | www.bitsofscience.org