Dierking, Jan (2019) Biodiversity changes and their functional consequences in the pelagic ecosystems of the central Baltic Sea Cruise No. AL521, April 16th – May 2nd 2019, Kiel (Germany) – Kiel (Germany) BALTIC APRIL. . Alkor-Berichte, AL521 . GEOMAR Helmholtz-Zentrum für Ozeanforschung, Kiel, Germany, 21 pp. DOI 10.3289/CR-AL521.

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Abstract

This multidisciplinary cruise extended a long-term data series on ecosystem composition and functioning of the deeper basins of the Baltic Sea that has been collected in similar form since 1987. The key characteristic of the cruise was the integration of oceanographic and biological information to enhance understanding of environmental, plankton dynamics and fish population fluctuations, and ultimately, evolutionary processes in this system. The spatial focus lay on the Bornholm Basin as most important spawning area of Baltic cod, but also included the Western Baltic Sea, Arkona and Gotland Basin, Gdansk Deep, and Stolpe Trench. All objectives of the cruise were achieved, benefiting from perfect weather conditions and flawless performance of all instruments and gear.
The detailed hydrological survey (oxygen, salinity, temperature) of the entire cruise area showed higher water temperatures and an improvement of the oxygen situation in deep water layers compared to the same time period in 2018. Plankton surveys (zoo- and ichthyplankton including gelatinous plankton, with the goal to determine the composition and the abundance and vertical and horizontal distribution of species, and to take samples for later measurements of nutritional condition) showed a much higher abundance of jellyfish larvae (“ephyrae”) and small adults than in 2018. Regarding fish larvae, sprat and flounder larvae were abundant but very few cod larvae were present throughout the cruise area, both of which was consistent with the situation in previous years. Pelagic fishery hauls were carried out as planned to determine stock structure and to obtain a range of different samples (gonads, livers, stomachs, tissue samples, otoliths) for cod, whiting, sprat and herring as well as different flatfish species, and are the foundation for a range of different individual-level genetic and ecological analyses to be carried out in the laboratory on land. The abundance and distribution of fishes in the cruise area was also assessed continuously over the duration of the cruise with hydroacoustic methods. As in previous years, cod, herring, sprat and flounder dominated catches. Catches of cod increased compared to 2018, but the absence of large cod individuals in particular in Bornholm Basin was consistent with observations in previous years. Finally, the work for all additional collaborative projects, including in-depth sampling of planktonic food webs for dietary tracer work, sampling and experimental work of photosynthesis and respiration rates of different phytoplankton fractions and finally, pelagic fish stomach and plankton sampling for eDNA assessments was carried out successfully.
The resulting data- and sample sets support ongoing projects in the Research Unit Marine Evolutionary Ecology at GEOMAR, as well as the EU Horizon 2020 project GoJelly and several international collaborations.

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