Long-term changes in inorganic carbon in the Elbe estuary

Overall, estuaries are net CO₂ sources to the atmosphere, releasing an estimated 0.25 Pg C yr^-1, which could counterbalance the shelf uptake of approximately 0.25 Pg C yr^-1. River discharge can influence both, the CO₂ flux from estuary to the atmosphere, as well as the magnitude of dissolved inorganic carbon (DIC) exported to coastal waters. In Europe, climate change is expected to cause an increased precipitation in winter and longer periods of drought in summer. The goal of this study is to elucidate the influence of climate-change-induced hydrological changes on an estuarine carbonate system.

The Elbe River is one largest river basins in central Europe, where over 24 million people live in the catchment area. Since 2014, annual Elbe river discharge has been relatively low at 492.95 m³ s^-1, compared to the mean river discharge from 2008 to 2018 at 652.95 m³ s^-1. 2018 was especially dry, with a discharge of 441 m³ s^-1, the lowest annual mean river discharge since 1992. The Elbe estuary has been extensively sampled by the Flussgebietsgemeinschaft (FGG) Elbe (Elbe River Basin Community), qualifying the region as a suitable site to study the natural and anthropogenic impacts on estuarine systems.

Preliminary results of the 1985-2018 FGG dataset indicate a major shift in the carbonate system dynamics in the Elbe estuary. From assessing the behaviour of DIC and other ecosystem parameters along the estuary over time, the region can be separated into three ecosystem states. During the time of high pollution, from 1985 to 1990, the estuary exhibited high levels of DIC. Between 1991 and 1996 is the transitional period. After 1997, the ecosystem parameters appear to be exhibiting similar patterns throughout each year with similar levels and therefore this period can be classified as the current ecosystem state. Since 1997, DIC exhibits a drawdown in spring and summer months in the upper region, coinciding with the increase in dissolved oxygen saturation and pH, which can indicate that this region is net autotrophic. Further downstream, DIC then increases along the estuary, and often peaks in the maximum turbidity zone.

For this study, we apply multiple linear regression to determine the relative importance of ecosystem variables that contribute to annual and monthly DIC variability in the recent ecosystem state. Key ecosystem variables include particulate and dissolved organic carbon, pH, dissolved oxygen and river discharge.