Cruise report Alkor AL544

Goal of the cruise was to investigate the biological oceanography of the Baltic Sea along the extended salinity gradient from intermediate saline SW Baltic Sea waters towards low saline north eastern stations in the Gotland Basin. Apart from teaching a diverse set of biological oceanographic sampling techniques, the aim was 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. Due to unstable weather conditions, we had to modify the cruise plan along the way and instead of investigating the northern and eastern Gotland Basin, we focused on an intensified station grid in the SW Baltic Sea. In short, we conducted a station grid covering 75 stations of which >60 were investigated with a depth stratified sampling design using the hydrobios multinets midi and maxi with 5 and 9 discrete net samplings per station, respectively. Additionally, Bongo and WP2 nets were employed to characterize the plankton community. In the SW Baltic Sea we observed large quantities of the invasive comb jelly Mnemiopsis leidyi, while at northern stations we observed the native moon jellyfish Aurelia aurita. We did not observe M. leidyi at stations north of the Bornholm Basin.

In summary, large parts of the salinity gradient of the Baltic Sea have been investigated, with very interesting results, especially concerning anoxic conditions in bottom waters. Also, the high resolution investigation of the SW Baltic Sea has to date not been conducted before, where we investigated stations with a depth stratified sampling design. The students received a sophisticated training to conduct biological oceanographic investigations, covering the physical and chemical oceanography as well as biological aspects, especially focusing on life analyses of plankton samples.