Abstract. The coastal ocean is strongly affected by ocean acidification because of its
shallow water depths, low volume, and the closeness to terrestrial dynamics.
Earlier observations of dissolved inorganic carbon (DIC) and total
alkalinity (TA) in the southern part of the North Sea, a northwest European
shelf sea, revealed lower acidification effects than expected. It has been
assumed that anaerobic degradation and subsequent TA release in the adjacent
back-barrier tidal areas (Wadden Sea) in summertime is responsible for
this phenomenon. In this study the exchange rates of TA and DIC between the
Wadden Sea tidal basins and the North Sea and the consequences for the
carbonate system in the German Bight are estimated using a 3D ecosystem
model. The aim of this study is to differentiate the various sources
contributing to observed high summer TA in the southern North Sea. Measured
TA and DIC in the Wadden Sea are considered as model boundary conditions.
This procedure acknowledges the dynamic behaviour of the Wadden Sea as an
area of effective production and decomposition of organic material.
According to the modelling results, 39 Gmol TA yr−1 were exported from
the Wadden Sea into the North Sea, which is less than a previous estimate
but within a comparable range. The interannual variabilities in TA and DIC,
mainly driven by hydrodynamic conditions, were examined for the years 2001–2009. Dynamics in the carbonate system are found to be related to specific
weather conditions. The results suggest that the Wadden Sea is an important
driver for the carbonate system in the southern North Sea. On average
41 % of TA inventory changes in the German Bight were caused by riverine
input, 37 % by net transport from adjacent North Sea sectors, 16 % by
Wadden Sea export, and 6 % were caused by internal net production of TA.
The dominant role of river input for the TA inventory disappears when
focusing on TA concentration changes due to the corresponding freshwater
fluxes diluting the marine TA concentrations. The ratio of exported TA
versus DIC reflects the dominant underlying biogeochemical processes in the
Wadden Sea. Whereas aerobic degradation of organic matter played a key role
in the North Frisian Wadden Sea during all seasons of the year, anaerobic
degradation of organic matter dominated in the East Frisian Wadden Sea.
Despite the scarcity of high-resolution field data, it is shown that
anaerobic degradation in the Wadden Sea is one of the main contributors of
elevated summer TA values in the southern North Sea.
The impact of intertidal areas on the carbonate system of the southern North Sea
