Variations of estuarine metabolic alkalinity loads: Consequences for the biogeochemistry of a shelf sea (North Sea)

Recent studies have shown that anaerobic remineralisation in estuaries of the North Sea, a semi-enclosed shelf sea of the Northeast Atlantic, generates a large amount of alkalinity which is subsequently flushed into the North Sea basin. The anaerobic processes within the estuaries fed by high anthropogenic nitrate loads peaked in the 1980s. Under pristine conditions, these nutrient loads are lowered by about 90 %.

On the other hand, the goals of the 2015 Paris agreement can only be achieved with zero or even negative CO₂ emissions. Such scenarios often include the use of terrestrial bioenergy requiring an increasing usage of fertilizers. Simply by leakage, such applications induce additional nutrient (and thus alkalinity) loads into the adjacent seas.

Using a 3-D biogeochemical model for the Northwest European shelf, we investigated the North Sea – wide consequences of the different scenarios described above. Assuming only aerobic regeneration within the estuaries of the North Sea, the annual uptake of atmospheric CO₂ is reduced by about one third within a coastal band of 100 km width. More drastic changes of alkalinity discharge into the North Sea, described above, also impact areas of the central North Sea and are able to alter the annual CO₂ uptake in the order of the magnitude of the air-to-sea flux itself.