2. Study area
The northeastern German coast of the Baltic Sea is basically a flooded glacial moraine landscape. It is characterized by extensive bays, shallow lagoons (the boddens) and marine inlets, which form the mixing zone between rivers and the marine realm (Correns, 1977; Schwarzer, Ricklefs, Bartholomä, & Zeiler, 2008). These zones are characterized by hydrologic events which in turn affect biochemical characteristics of the ecosystem. For example they react sensitively both to discharge pulses from rivers and inflow events from the marine side after storms (Gocke, Rheinheimer, & Schramm, 2003). Further, a dense net of water management structures, such as water retention flaps, partially attenuate freshwater flow from creeks into the lagoons. Here, automatic ones will release water from creeks when the lagoons are low while some manual flaps exist, which are operated by hand (Funkel, 2004) .
For this study, water samples were taken along three transects, encompassing salinity gradients (Figs. 1 and 2; Supplementary Tables S1 and S2):