Sea water chemistry

Sea water chemistry of the North Sea shows large spatial and temporal variability, interlinked with climate and biology. Detecting trends in times of ongoing anthropogenic impact is challenging against this highly dynamic background.

Whereas ocean acidification (OA) in the open ocean is relatively easy to predict as a function of future atmospheric CO2 levels and based on inorganic carbon chemistry, coastal areas are impacted by varying amounts of terrestrial/ riverine input and strong differences in seasonal biological activity. Coastal seas are characterized by high primary productivity and thus increased atmosphere-sea CO2 fluxes through the ‘continental shelf pump’. High primary productivity enables CO2 drawdown, increasing the (sub)surface pCO2 which contributes to the intermediate layers of the open ocean by open ocean-shelf sea exchange.

In the North Sea, more than 99% of the present carbon is exported to the North Atlantic Ocean, whereas less than 1% is buried in sediments. Of all the exported carbon, approximately 97% is in the form of DIC, with the increase in North Sea DIC being caused by uptake of atmospheric CO2 and by local conversion of DOC into DIC. Within the North Sea, there is a distinction between the northern and central parts which act as a CO2-sink all year round and the southern part, which is characterized by a high seasonal (spring) primary production, and thereby a temporal high CO2 uptake. At an annual scale, differences in temperature and biological activities result in major changes, with either of the two processes being dominant. The southern part is dominated by temperature effects, with an increase of pCO2 in summer, while the northern part is clearly dominated by biological processes with a summer decrease of pCO2 typical for mid and high latitude waters. The highest seasonal amplitude in pCO2 is observed in the central part as a consequence of early stratification and high biological activity.