Abstract. Within the context of the predicted and observed increase in droughts and floods with climate change, large summer floods are likely to become more frequent. These extreme events can alter typical biogeochemical patterns in coastal systems. The extreme Elbe River flood in June 2013 not only caused major damages in several European countries but also generated large-scale biogeochemical changes in the Elbe estuary and the adjacent German Bight. The high-frequency monitoring network within the Coastal Observing System for Northern and Arctic Seas (COSYNA) captured the flood influence on the German Bight. Data from a FerryBox station in the Elbe estuary (Cuxhaven) and from a FerryBox platform aboard the M/V Funny Girl ferry (traveling between Büsum and Helgoland) documented the salinity changes in the German Bight, which persisted for about 2 months after the peak discharge. The Elbe flood generated a large influx of nutrients and dissolved and particulate organic carbon on the coast. These conditions subsequently led to the onset of a phytoplankton bloom, observed by dissolved oxygen supersaturation, and higher than usual pH in surface coastal waters. The prolonged stratification also led to widespread bottom water dissolved oxygen depletion, unusual for the southeastern German Bight in the summer.

The hypothesis of a recent reversal in the eutrophication of the Wadden Sea and the potential of inshore waters in denitrification is explored. Salinity, temperature and nitrate concentrations in the List Tidal Basin (Northern Wadden Sea) have been measured about twice weekly since 1984. Salinity has a clear seasonal cycle with lowest salinities of about 27 in late winter and highest salinities of about 31 in summer. Mean annual deviations from the long-term mean salinity correlate significantly with riverine freshwater discharge. Winter nitrate concentrations are generally high (about 50 lM on average). The major part of the variability is related to salinity (*35%). Temperature had a minor impact (*1%). Superimposed on this, a long-term decrease of about 1 lM per year was found. Together, these processes account for about 45% of the nitrate variability. The long-term decrease of about 2% per year is similar to continental riverine trend in total nitrogen loads. In contrast to the List Tidal Basin, salinity explained more than 90% of nitrate variability in the off-shore German Bight. Salinity (30) normalised winter nitrate data of the German Bight also show a long-term decreasing trend. Most of the List Tidal Basin data are either on or below the nitrate-salinity relation found in the German Bight. This observation suggests that denitrification has a major impact on the winter nitrate concentrations in the Northern Wadden Sea compared to the German Bight. It is hypothesised that a large part of the unexplained variability is related to weather-dependent changes in residence time of tidal water masses in the Wadden Sea and circulation patterns within the German Bight.

<p><strong>Abstract.</strong> Within the context of predicted and observed increase in droughts and floods with climate change, large summer floods are likely to become more frequent. These extreme events can alter typical biogeochemical patterns in coastal systems. The extreme Elbe River flood in June, 2013 not only caused major damages in several European countries, but also generated large scale biogeochemical changes in the Elbe Estuary and the adjacent German Bight. Due to a number of well documented and unusual atmospheric conditions, the early summer of 2013 in Central and Eastern Europe was colder and wetter than usual, with saturated soils, and higher than average cumulative precipitation. Additional precipitation at the end of May, and beginning of June, 2013, caused widespread floods within the Danube and Elbe Rivers, as well as billions of euros in damages. The floods generated the largest summer discharge on record within the last 140 years. The high-frequency monitoring network in the German Bight available within the Coastal Observing System for Northern and Arctic Seas (COSYNA) captured the flood influence on the German Bight. Monitoring data from a FerryBox station in the Elbe Estuary (Cuxhaven) and from a FerryBox platform aboard the M/V <i>Funny Girl Ferry</i> (traveling between Büsum and Helgoland) documented the salinity changes on the German Bight, which persisted for about 2 months after the peak discharge. The flood generated a large influx of nutrients, dissolved and particulate organic carbon on the coast. These conditions subsequently led to the onset of a chlorophyll bloom within the German Bight, observed by dissolved oxygen supersaturation, and higher than usual pH in surface coastal waters. The prolonged stratification also led to widespread bottom water dissolved oxygen depletion, unusual for the south eastern German Bight in the summer.</p>

Fig. S1. Correction of pH data measured at HPA Pile in the Elbe Estuary. Original hourly pH (black) was plotted along with a moving average (red line), and the corrected pH (blue), obtained by subtracting the moving average from the original dataset, and then adding the average of the first 1000 samples, where drift is minimal.