Pelagic oxygen consumption rates in the Elbe estuary: Proxies and spatial patterns.

The Elbe estuary is strongly impacted by human activities including dredging, land reclamation and eutrophication. Since about 30 years, water quality improved leading to major phytoplankton blooms in the Elbe river. When these blooms enter the upper estuary including the Hamburg port area, they collapse leading  to low oxygen conditions. During a cruise in September 2020 we measured oxygen consumption rates in water samples of the Elbe Estuary between the coastal North Sea (Wadden Sea) and  the weir in Geesthacht including a large freshwater part of the Elbe estuary. In addition, suspended matter samples were taken and analysed for chlorophyll, particulate C (PC) and particulate N (PN). Oxygen consumption rates reached maximum values at both the marine side (~0.3 µmol/(l*h)) and the freshwater  side of the estuary (~0.65 µmol/(l*h)) and a distinct minimum near the Estuarine Turbidity Maximum near the onset of the salinity gradient. Based on specific weights of phytoplankton and PC content, we estimated the contribution of newly formed organic matter. This estimate correlated significantly with the observed oxygen consumption rates. We suggest that most of the riverine organic matter is degraded within the freshwater part of the Elbe estuary before reaching the salinity gradient. This is in line with significant amounts of nitrate being released within the freshwater part of the estuary.