AL 544 Cruise Report week 1

Goal of the cruise is to investigate biological oceanography of the Baltic Sea along the extended salinity gradient from intermediate saline SW Baltic Sea towards low saline northern Gotland Basin of the Baltic Sea. Apart from teaching a diverse set of biological oceanographic sampling techniques, the aim is to characterize the macro‐zooplankton and ichthyo‐plankton community. We observed large accumulations of native and invasive macrozooplankton species in different areas of the Baltic Sea as well as extended anoxia in bottom waters, especially in NE waters. Due to unstable weather conditions, we had to modify the cruise plan along the way. In short, we cancelled northern most investigation area and eastern waters to avoid being trapped by an approaching low pressure. Instead we included an in depth investigation of the SW Baltic Sea, where the invasive comb jelly Mnemiopsis leidyi is currently forming local blooms with very high abundance patches. We did not observe M. leidyi at stations north of the Bornholm Basin. In summary, we managed to sample large parts of the salinity gradient with very interesting results, especially concerning anoxic conditions in bottom waters and added a high resolution investigation of the SW Baltic Sea in order to understand population dynamics of the invasive comb jelly Mnemiopsis leidyi.