10 May 2023
 | 10 May 2023
Status: this preprint is open for discussion.

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

Louise C. V. Rewrie, Burkard Baschek, Justus E. E. Beusekom, Arne Körtzinger, Gregor Ollesch, and Yoana G. Voynova

Abstract. Estuaries are an important contributor to the global carbon budget, facilitating carbon removal, transfer and transformation between land and coastal ocean. Estuaries are also susceptible to global climate change and anthropogenic perturbations. We find that a long-term significant increase in dissolved inorganic carbon (DIC) of 6–21 µmol kg-1 yr-1 (1997–2020) in a temperate estuary in Germany (Elbe Estuary), was driven by an increase in upper estuary particulate organic carbon (POC) content of 8–14 µmol kg-1 yr-1. The temporal POC increase was due to an overall improvement in water quality observed in the form of high rates of primary production and a significant drop in biological oxygen demand. The magnitude of mid-estuary DIC gain was equivalent to the increased POC production in the upper estuary, suggesting that POC is efficiently remineralized and retained as DIC in the mid-estuary, with the estuary acting as an efficient natural filter for POC. In the context of the significant DIC increase, the impact of a recent extensive drought period (2014–2020) significantly lowered the annual mean river discharge (468 ± 234 m3 s-1) compared to the long-term mean (690 ± 441 m3 s-1, 1960–2020). During the drought period, the late spring internal DIC load in the estuary doubled. This suggests that the drought induced a longer dry season, starting in May (earlier than normal), increased the residence time in the estuary and allowed for a longer remineralization period for POC. Annually, 77–94 % of the total DIC export was laterally transported to coastal water, reaching 89 ± 4.8 Gmol C yr-1, and thus only a maximum of 23 %, at 10 Gmol C yr-1, was released via carbon dioxide (CO2) evasion, between 1997 and 2020. Export of DIC to coastal waters decreased significantly during the drought (2014–2020: 38 ± 5.4 Gmol C yr-1), on average by 24 % compared to the non-drought period. In addition, we have identified that seasonal changes in DIC processing in an estuary require consideration in order to understand both the long-term and future changes in air-water CO2 flux, DIC export to coastal waters, as well as the impacts of prolonged droughts on the land-ocean carbonate system.

Louise C. V. Rewrie et al.

Status: open (until 21 Jun 2023)

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  • RC1: 'Comment on egusphere-2023-961', Anonymous Referee #1, 26 May 2023 reply

Louise C. V. Rewrie et al.

Louise C. V. Rewrie et al.


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Short summary
Post a 1980’s heavy pollution, a long-term inorganic carbon increase in the Elbe Estuary (1997–2020) was fuelled by phytoplankton and organic carbon production in the upper estuary, due to improved water quality. A recent drought (2014–2020) modulated the trend, extending water residence time, dry summer season into May, and enhancing production of inorganic carbon in the estuary. However, the annual inorganic carbon export to coastal waters decreased in the drought period.