Drivers of the carbonate system variability in the southern North Sea : River input, anaerobic alkalinity generation in the Wadden Sea and internal processes

The presented work examines the carbonate system in the southern North Sea and its sensitivity to river input, anaerobic total alkalinity (TA) generation in the Wadden Sea and internal processes by using the ecosystem model ECOHAM. Furthermore, it is aimed to reproduce observations of high TA concentrations in the German Bight that could not be reproduced in former model studies. The study consists of three main parts that examine the TA production in the southern North Sea, the impact of riverine inputs on TA in the southern North Sea and the impact of TA and DIC exported from the Wadden Sea. The TA production in the southern North Sea was examined in the first main chapter. A prognostic treatment of TA was implemented into ECOHAM that enables the calculation of TA concentration changes due to the uptake and release of nutrients into the water column as well as calcification and decalcification. It was shown that the internal processes that produced TA irreversibly were mainly driven by the uptake of allochthonous nitrate and its subsequent denitrification. In the year 2008, about 76 Gmol TA yr-1 (228 mmol TA m-2 yr-1) was produced in the entire model domain (332,050 km²). Thereof, 13 Gmol TA yr-1 (221 mmol m-2 yr-1) were produced in the validation area (59,338 km²). TA production in shelf seas on annual scales was also derived from denitrification rates in former studies. Therefore, the internal turnover of TA calculated in the study at hand was compared to simulated denitrification in the validation area and in the whole model domain in 2008. A total amount of 80 Gmol N yr-1 (241 mmol N m-2 yr-1) was denitrified in the whole model domain, whereas 22 Gmol N yr-1 (370 mmol N m-2 yr-1) was denitrified in the southern North Sea. The deviation of denitrification from TA production was also examined for the years 1977 – 2009. Denitrification exceeded the TA production in the whole model domain / southern North Sea by 13 Gmol yr-1 / 11 Gmol yr-1 on average. Furthermore, it was shown that TA production correlates with nutrient supply from rivers in the southern North Sea but observed high TA concentrations could even not be reproduced in years with high river loads of TA. In the second main chapter it was examined whether observed high TA concentrations in the German Bight could originate from rivers. River loads of TA and DIC used in former studies were based on daily observations of freshwater discharge and on one concentration for TA and DIC for each river. Thus, these data lacked seasonal variability that can occur due to changes in riverine concentrations. Therefore, new data of river input of TA, DIC and nitrate were introduced for the main continental rivers. The level of TA concentration in the German Bight could be increased due to the increased loads of the river Rhine, but it was not possible to reproduce the observed high TA concentrations in the German Bight. Furthermore, the effective river load (Riveff) was introduced in this chapter in order to take freshwater discharges of rivers into account and to obtain a quantity that enables a comparison with TA production in the North Sea. As a result it was shown that concentration changes caused by river inputs in the German Bight were comparable with a TA consumption of 3 Gmol yr-1 in 2008. Thus, the effect of dilution due to freshwater discharge dominated there. This was in contrast to the Riveff of the river Rhine, which was comparable with a TA production of 24 Gmol TA yr-1. In the third main chapter the impact of Wadden Sea exchange rates of TA and DIC on concentrations in the German Bight was examined. Therefore, sources and sinks of TA and DIC were implemented into the model that indentified the dynamic behaviour of the Wadden Sea as an area of effective production and decomposition of organic material. The respective exchange rates were calculated by using measured pelagic DIC and TA concentrations in the Wadden Sea and modelled tidal water mass exchange. It was possible to bring the simulations significantly closer to observations in summer due to the implementation of the Wadden Sea. About 40 Gmol TA yr-1 were exported from the Wadden Sea into the North Sea, which was lower than the first estimate by Thomas et al. (2009) who calculated about 73 Gmol TA yr-1 originating from the Wadden Sea. Furthermore, the interannual variabilities of TA and DIC concentrations were examined for the years 2001 – 2009, which was mainly driven by hydrodynamic conditions. It was shown that the occurrence of weak meteorological blocking situations can lead to enhanced accumulation of simulated TA in the German Bight. In summary, it was found that the Wadden Sea was an important driver of the carbonate system variability in the southern North Sea. 68% of all TA concentration changes in the German Bight were caused by Wadden Sea export of TA, 23% were caused by the internal production of TA in the model and 9% caused by effective river loads.