Gawinski, Christine (2019) Analysis of the distribution of the moon jellyfish Aurelia sp. in the Baltic Sea. (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 56, LX pp.

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Abstract

Far-reaching changes of the physical environment are expected for the Baltic Sea in the upcoming years due to climate change. Aurelia sp., a prevalent jellyfish in the Baltic Sea, is suggested to be able to cope with and to possibly benefit from ocean warming, which could lead to more frequent and severe jellyfish blooms. To study Aurelia trends, it is necessary to first decode the underlying population patterns, as different populations of the same jellyfish species might respond differently to the same environmental driver. The aim of this master thesis was to evaluate the distribution of Aurelia populations in the Baltic Sea and to provide a recent baseline of jellyfish abundance and their depth distribution. Data on abundance and depth distribution of Aurelia as well as tissue material for genetic analyses were collected on a teaching cruise with R/V ALKOR during August 2017, which covered a grid of 72 stations across the Baltic Sea. The horizontal distribution of Aurelia was assessed with Bongo nets (two 335μm, 500 μm meshed nets, Ø 0.6 m) combined with Baby-Bongos (two nets 150 μm, Ø 0.2 m). To resolve the depth distribution of Aurelia, multiple opening and closing nets were deployed (Multinet MAXI, 9 nets, mesh size 335μm, net opening 0.5 m², Multinet MIDI, 5 nets, 335μm mesh sizes, 0.25 m² opening). Potential Aurelia sub-populations and their dispersal pattern were investigated by performing molecular analysis using mitochondrial cytochrome c oxidase subunit I (COI) as genetic marker. A total of 1883 Aurelia medusae in a size range of 0.5 – 18 cm were sampled, with an average size of 7.7 ± 3.0 cm. The average abundance of Aurelia was 3.7 ± 2.5 ind. m-2 (averaged across 72 stations). The minimum abundance was 0 ind. m-2 (Kiel Bight) and the average abundances ranged between 1.4 ± 1.2 ind. m-2 (Arkona Basin ) and a max. of 4.9 ± 3.1 ind. m-2 (Gdansk Deep). The overall max. abundance per station was 11.4 ind. m-2 at a station in the Bornholm Basin and the max. abundance per depth strata was 113 ind. 100m-3 at a depth of 5 – 10 m in the Arkona Basin. Most of Aurelia were found at a depth between 0 – 30 m above the thermocline and the mean depth (Zm) ranged between a min. of 4.5 m in the Arkona Basin to a max. of 14.4 m in the Bornholm Basin. The vertical spread (Zs) of the depth distribution ranged between 0 and 8 m. A new size – wet weight correlation for Aurelia from the north-eastern Baltic Sea was described by the power function: y = 0.1401x2.465. In total, 61 medusae were sequenced throughout the Baltic Sea and the North Sea and all individuals were identified as Aurelia aurita. For genetic analysis 17 published sequences of Aurelia polyps from the North Sea and the Gullmar Fjord were included and a total of 23 haplotypes were found, of which two were shared between the North Sea and Baltic Sea, four were endemic to the North Sea and 11 to the Baltic Sea, respectively. The results indicate a significant population differentiation between the North Sea and the Baltic Sea, but not within the Baltic Sea itself. Population structure within the Baltic Sea could not be statistically resolved due to low sample size and high haplotype diversity within the Baltic Sea. Sequencing of further Aurelia individuals is needed to increase sample size and to detect a possible significant population differentiation within the central Baltic Sea. This study clearly revealed a knowledge gap about a recent baseline of jellyfish abundances in the Baltic Sea and highlights the importance of continuous long-term observations of jellyfish trends in order to conclusively address the question if jellyfish are on the rise in the Baltic Sea or not.

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