Hydrography of intertidal environments in Schleswig-Holstein, Germany

Abstract. The current status of intertidal waters in the wake of ongoing global change was assessed in a baseline study with a 36 month time series of water level, temperature, and salinity measurements from Bottsand lagoon on the Baltic Sea coast, and on the mudflats off Schobüll at the North Sea coast of Schleswig-Holstein, Germany. Extreme events, storm surges and heat waves were also recorded in a temporal resolution of 20 minutes. At Bottsand lagoon, the temperatures followed the air temperatures in winter, and were higher than the air temperatures in spring and summer. The annual averages varied from 12.1 to 12.6 °C, the air temperatures varied from 11.1 to 11.2 °C. The salinities showed one or two months periods of consistently higher or lower values in winter and spring. The annual averages ranged from 14.7 to 16.9 units. The lagoon showed a different variability than that of the open Baltic surface waters, where the temperatures and salinities were lower in summer and higher in winter. The seasonal salinity differences were less developed in the mid 1960s, when the connectivity of the lagoon with the Baltic Sea was less restricted, and a sandy shoal in the lagoon was not present. In Husum Bight off Schobüll, water temperatures were lower than the air temperatures in winter and higher in spring and summer. The annual average water temperatures ranged from 10.8 to 11.4 °C, and the air temperatures from 9.9 to 10.2 °C. High waters were warmer during the day than at night-time in spring and early summer only. The annual average salinities off Schobüll ranged from 24.0 to 27.2 units. The values were higher in summer and lower in winter. This seasonal cycle was related to variations in the Elbe river runoff, which largely influences the salinity in the south-eastern German Bight. The same seasonal cycle was recorded in the Sylt Roads time series. Cross-correlations of the records revealed that it takes seven weeks for an Elbe river freshwater pulse to reach Schobüll, and three weeks more to proceed to Sylt. On average, the salinities were 2.7 units lower off Schobüll than off Sylt, which mirrors a pervasive gradient of landward decreasing salinities in the Wadden Sea. They were induced by a local, low-salinity lens on top of tidal waters, fed by groundwater seepage or by freshwater runoff. A cross correlation with the precipitation record revealed salinity decreases about one week after high precipitation. The cumulative salt marsh submergence times per period of observation, i.e. inundation frequencies, were very variable at the lower boundaries of the lower and upper salt marsh vegetation zones. The inundation frequencies were consistently higher at Bottsand than at Schobüll, where the same halophyte assemblages prevailed. As the average salinity was 10 units higher at Schobüll, the differences of inundation frequencies suggest that a certain salinity has to be maintained in the soils to sustain specific halophyte assemblages. A mass occurrence of small Pacific oyster shells was observed before the vegetation boundary off Schobüll in spring 2024. The data suggested an oyster spatfall triggered by the North Sea heat waves in summer 2023, with temperatures exceeding 23 °C, and a subsequent wipe-out during a period of salinities lower than 18 units after an Elbe river discharge event in January 2024. The biotic responses to environmental extremes highlighted the vulnerability of Wadden Sea ecosystems at times of Global Change.