Hydrography of intertidal environments in Schleswig-Holstein, Germany

Abstract. The current status of intertidal waters in the wake of global change was assessed in a baseline study with a 36 month time series of water level, temperature, and salinity measurements from Bottsand lagoon, Baltic Sea, and from the mudflats off Schobüll, North Sea coast of Schleswig-Holstein, Germany. At Bottsand lagoon, annual average water temperatures varied from 12.1 to 12.6 °C, the air temperatures varied from 11.1 to 11.2 °C. The water temperatures followed the air temperatures in winter, and were higher than the air temperatures in spring and summer. The annual average salinities ranged from 14.7 to 16.9 units. The lagoon showed a different variability than 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. In Husum Bight off Schobüll, annual average water temperatures ranged from 10.8 to 11.4 °C, and the air temperatures ranged from 9.9 to 10.2 °C. The water temperatures were lower than air temperatures in winter and higher in spring and summer, when the high waters were warmer during the day than at night-time. The annual average salinities off Schobüll ranged from 24.0 to 27.2 units. They were higher in summer and lower in winter, when the Elbe river runoff was generally enhanced. The same seasonal cycle was recorded in the Sylt Roads time series. Cross-correlations revealed that it takes seven weeks for an Elbe river freshwater pulse to reach Schobüll, and three weeks more to proceed to Sylt. The average salinities were 2.7 units lower off Schobüll than off Sylt. This pervasive gradient of landward decreasing salinities was 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 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 suggested that a certain salinity has to be maintained in the soils to sustain specific halophyte assemblages. A mass occurrence of small Austrominius modestus plates was observed before the vegetation boundary off Schobüll in spring 2024. The data suggested a wipe-out of juvenile barnacles during a short period of strong frost in late November 2023, when daily mean temperatures of up to −10.2 °C were recorded. This biotic response to environmental extremes highlighted the vulnerability of Wadden Sea ecosystems at times of Global Change.