Solar radiation-induced photochemical reactions transform vast quantities of carbon and nitrogen in the Baltic Sea

A study published on 19 Jan 2012 in the web page of Journal of Geophysical Research quantified how much organic carbon and nitrogen is transformed by solar radiation-induced photochemical reactions over the entire Baltic Sea during one year. The photochemical reactions transform approximately 3 325 000 tons of carbon and 43 500 tons of nitrogen.

The magnitude of photochemical transformation of nitrogen is 30 times larger than the annual discharge of nitrogen by the city of Helsinki, the capitol and the largest city of Finland. The nitrogen transformation concerns the conversion of poorly bioavailable dissolved organic nitrogen to ammonium, the most preferred form of nitrogen for phytoplankton. Because nitrogen limits the primary production of phytoplankton in the Baltic Sea, the photochemical production of ammonium is expected to stimulate the production of phytoplankton in particular during the summer with highest need for nitrogen.

The magnitude of carbon transformed by photochemical reactions is similar to the annual amount of organic carbon transported by rivers to the Baltic Sea. Thus, solar radiation-induced photochemical reactions form a potential sink for organic carbon loading from the catchment to the Baltic Sea. The photochemical transformation of carbon consist of the mineralization of organic carbon to carbon dioxide (1 905 000 tons per year) and the transformation of poorly bioavailable dissolved organic carbon to forms preferred by plankton (1 420 000 tons per year). The latter amount is quickly consumed by the smallest organisms of plankton, bacteria. Most of the carbon consumed by bacteria is respired to carbon dioxide but about a quarter of it builds up bacterial biomass. This carbon bound in bacterial biomass is transferred through the food webs of the Baltic Sea and may enter up to fish in the upper level of food web. The annual fish catch in the Baltic Sea is circa 100 000 tons of carbon and only 7 % of the magnitude of bioavailable carbon produced by photoreactions. This comparison indicates that only a small proportion of phototransformed dissolved organic carbon ends up to fish, and most of the bioavailable forms of organic carbon produced by solar radiation is respired to carbon dioxide in the food web. Thus, most of the photochemically transformed organic carbon is eventually converted to carbon dioxide, a green house gas or an acidifying agent in sea.

Aarnos, H., Ylöstalo, P., Vähätalo, A. V. Seasonal phototransformation of dissolved organic matter to ammonium, dissolved inorganic carbon and labile substrates supporting bacterial biomass across the Baltic Sea. J. Geophys. Res., 117, G01004, doi:10.1029/2010JG001633.

http://www.agu.org/pubs/crossref/2012/2010JG001633.shtml

More information: Anssi.Vahatalo@novia.fi