Gelatinous zooplankton abundance and distribution in the Baltic Sea during the Alkor cruise AL544, September 2020

The biodiversity and distribution of gelatinous macrozooplankton was assessed in the Baltic Sea during September 2020. The dataset includes 236,329 species-specific gelatinous macrozooplankton records across 73 stations in the south-western, central and northern Baltic Sea. Focus was devoted to changes in depth distributions in relation to salinity and an extended oxygen depletion event of the south-western Baltic Sea. In total, 56 Multinet-midi casts (5 depth-strata), 4 Multinet-maxi (9 depth-strata), as well as 17 bongo and 52 WP2 casts were performed from the surface to >3m above the bottom. Additionally, CTD data described the physical environment. Depth resolved information on the abundance (m-3 and m-2) and size structure of i) the non-indigenous ctenophore Mnemiopsis leidyi including larvae (1-3mm), transitional (4-5mm), young adult (6-9mm) and adult (≥10mm) ctenophores, as well as the native scyphozoan jellyfish species ii) Aurelia aurita and iii) Cyanea capillata are provided. In total 40,601 individual gelatinous macrozooplankton specimens from samples and sub-samples were analyzed (raw-counts) with 39,771 Mnemiopsis leidyi (corrected-count 235,499), 744 Aurelia aurita and 86 Cyanea capillata records.

At 89% of the stations adult M. leidyi were caught with an average density of 1.27±0.97 ind m-3 station-1 (max 8.4 ind m-3, Flensburg Fjord, 10-7.5m). Young M. leidyi adults were caught at 78% of the stations with 3±2.4 ind m-3 station-1 (max 29.4 ind m-3, Eckernförde Bight, south of Flensburg Fjord, 0-2.5m), while being absent in the northern Arkona Basin and east of Bornholm. At 71% of the stations, transitional M. leidyi were caught with an average density of 19.2±21.8 ind m-3 station-1 (max 129.2 ind m-3, WP2, Kiel Bight), while being absent also from the central Arkona Basin. Larvae M. leidyi were present at 68% of all stations, with the maximum found in Kiel Bight with 642 ind m-3 (WP2) and generally high larvae densities with >500 ind m-3 found at 4 stations in the south-western Kiel Bight. Average larvae density during September was 257 ± 188 ind m-3 station-1. The scyphozoan jellyfish species were found at much lower densities, (range: 0.003 - 1.7 ind m-3), with the maximum abundances observed in the Arkona Basin (n=79 A. aurita: 4 to 6m). A. aurita was primarily found in the upper 30m, while C. capillata was primarily present in waters >30 m (at average densities of 0.002 to 0.07 C. capillata m-3).The here presented data are essential to further investigate responses of jellyfish and ctenophores to climate change, especially considering salinity and low oxygen conditions, important global change pressure which are of special concern for the Baltic Sea.