Recent inorganic carbon increase in a temperate estuary driven by water quality improvement and enhanced by droughts

Estuaries are an important component of the global carbon budget as sites of removal and transformation for carbon between land and coastal ocean. Drought conditions can lengthen river and estuarine water residence time, which can extend the retention and alter the cycling of organic carbon and nutrients. To better understand the functioning of an estuary under the current threat of climate change related droughts, we use the Elbe Estuary as an example, examining a period since 1997, when annual mean DIC in the mid to lower Elbe Estuary increased significantly, and with focus on the drought conditions since 2014. 

The recent (1997-2020) significant DIC increase by 6 to 15 µmol L^-1 yr^-1 we found is due to increase in upper estuary POC content of 8-14 µmol L^-1 yr^-1 in late spring and summer (May-August). The significant increase in POC was associated with dominating autotrophy (with negative AOU and pH > 9), and an overall improvement in water quality shown in significant (> 50%) decrease in BOD₇ since 1997. We found that microbial respiration of organic matter from upstream regions accounted for most of the DIC produced in the mid-estuary, therefore, the increased POC is efficiently remineralized to DIC by the mid-estuary region.

The Elbe River and estuary was subject to significantly lower river discharge between 2014 and 2020 (468 ± 234 m³ s^-1), nearly 40% of the long-term average (1960-2020, 690 ± 441 m³ s^-1). In addition, May was the only month with a significant negative trend in mean monthly river discharge from 1997, and down to 264 ± 19 m³ s^-1 by 2020, a discharge usually observed during summer and early autumn. During the recent drought period (2014-2020), the internal gain in the carbon load as DIC in the mid to lower estuary was significantly higher, by up to 3 times, compared to the non-drought period (1997-2013). This suggests that the drought in the Elbe watershed caused a significant reduction in the average river discharge in May, likely increasing the residence time in the estuary, subsequently permitting a longer period for remineralisation of POC and greatest production of DIC in the mid-lower Elbe Estuary.