The compound impacts of heatwaves and droughts can increase the risk of hypoxia in estuaries already exposed to anthropogenic stressors, such as pollution and altered geomorphology. The Elbe Estuary hosts the third largest port in Europe and in summer this region is frequently characterized by an oxygen deficit in the water column (Schöl et al., 2014). Surprisingly, the low oxygen zone extended above the Port region in 2018 and 2022. This study aims to assess the state of hypoxia in the upper estuary (2014 – 2024) in relation to the compound influence of a multi-year drought (2014–2023), superimposed on the stressor of heatwaves and temperature extremes in this temperate estuary.

We found that the dissolved oxygen (DO) concentration in the upper estuary fell below hypoxic levels (DO < 93.8 µmol L^-1), predominantly during heatwave events at times with low river discharge (< 276 m³ s^-1). The hypoxia in the shallow upper estuary was driven by intensified remineralization of organic matter (OM), with negative summer net ecosystem metabolism (NEM)  down to -717 ± 189 mg C m^-2 d^-1 in July 2019. The unexpected summer net heterotrophy were observed since 2015, one year after the onset of the prolonged drought. The high spring and summer gross primary production rates up to 14140 ± 1451 mg C m^-2 d^-1 (June 2024) and net autotrophy in the lower river, driven by excessive nutrient loads. provided high particulate organic carbon load to the estuary. This was indicated by high sumer POC concentrations with a June mean of 435 ± 189 µmol L^-1 (2014 – 2024). The microbial remineralization of this labile organic matter in the upper estuary was likely intensified by heatwave events and prolonged by low summer river discharge consuming oxygen. This ultimately resulted in hypoxia extending further upstream than normal into the shallow upper estuary. To combat the hypoxic events in the estuary we propose further reductions in nutrient inputs from the watershed, which can help to decrease river primary production and to lower the amount of labile organic matter available for remineralization in the estuary.

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