BOREAL ENVIRONMENT RESEARCH 21: 528–540 © 2016 ISSN 1239-6095 (print) ISSN 1797-2469 (online) Helsinki 24 February 2016 Moon jellyfish, Aurelia aurita, in the Gulf of Gdansk: threatening predator or not? Dominika Brulińska1)*, Michał Olenycz1)2), Marcelina Ziółkowska1), Stella Mudrak-Cegiołka3) and Maciej Wołowicz1) 1) Department of Marine Ecosystem Functioning, Institute of Oceanography, University of Gdansk, Al. Marszałka Piłsudskiego 46, PL-81-378 Gdynia, Poland (*corresponding author’s e-mail: dominika. [email protected]) 2) Department of Ecology, Maritime Institute in Gdańsk, ul. Długi Targ 41/42, PL-80-830 Gdańsk, Poland 3) Department of Marine Plankton Research, Institute of Oceanography, University of Gdansk, Al. Marszałka Piłsudskiego 46, PL-81-378 Gdynia, Poland Received 6 Jan. 2015, final version received 20 Jan. 2016, accepted 30 Oct. 2015 Brulińska D., Olenycz M., Ziółkowska M., Mudrak-Cegiołka S. & Wołowicz M. 2016: Moon jellyfish, Aurelia aurita, in the Gulf of Gdansk: threatening predator or not? Boreal Env. Res. 21: 528–540. The seasonal population dynamics and feeding preferences of the moon jellyfish, Aurelia aurita, in the Gulf of Gdansk (southern Baltic Sea) were investigated. Medusae were pre- sent in the water column from June to November, with maximum occurrence in August and September. The medusa bell diameter and weight increased during the study period reached maximum values in October. The relationship between bell diameter and wet weight was strong. No ephyrae were observed during the study period. Gastric content analysis revealed that the medusae fed mainly on copepods and cladocerans. Rotifers that dominated the water column throughout the study period were not found in the jellyfish guts, but the stable isotope signature indicated that they could have been a significant source of derived carbon. Low numbers of plankton prey and the lack of fish larvae in A. aurita guts suggest that the jellyfish is of minor relevance as a predator and competitor in the Gulf of Gdansk. Introduction jellyfish cause problems for fisheries by clogging trawl nets, competing for prey, preying on fish, Jellyfish occur in great abundance in many blocking water intakes to power plant cooling marine coastal waters, and this is a natural systems, killing fish species reared in aquacul- phenomenon in such ecosystems (Graham et ture, and reducing the attractiveness of seaside al. 2001). More frequent blooms are, however, resorts for tourists (Möller 1980, Mills 2001, reported worldwide (Boero et al. 2008). Obser- Lynam et al. 2006, Purcell et al. 2007, Baumann vations suggest that mass jellyfish outbreaks are and Schernewski 2010). caused by overfishing, eutrophication, climate Species of the genera Aurelia have a global change, translocations, and habitat modification distribution and often form aggregations (Pur- (Richardson et al. 2009). The mass occurrence cell 2003). One of the most extensively studied of medusae is of commercial consequence since species is the moon jellyfish, Aurelia aurita Editor in charge of this article: Johanna Mattila BOREAL ENV. RES. Vol. 21 • Moon jellyfish in the Gulf of Gdansk: threatening predator or not? 529 Fig. 1. Location of sam- pling sites in the Gulf of Gdansk. (Scyphozoa), which is widespread in coastal the medusae or their significance in the pelagic waters between 40°S and 70°N (Möller 1980). food web. Therefore, the aim of this study was The moon jellyfish is a predator that feeds on to investigate the population size of A. aurita in a wide variety of planktonic organisms such as the Gulf of Gdansk and to determine their prey fish larvae, ciliates, diatoms, rotifers, copepods, structure to define their role in the regulation of cladocerans, and barnacle larvae (reviewed in the mesozooplankton community in this area. Arai 1997). It can affect the structure of plankton The role of the moon jellyfish in the food web communities, and reduce standing fish stocks by and as a zooplankton predator are also discussed. competing with planktivorous fish for zooplank- ton prey and by preying on fish larvae (Möller 1980, Schneider 1989, Bamstedt et al. 2001, Material and methods Barz et al. 2006). Finally, it can also modify the food web structure. Sampling and analysis of medusae gut The moon jellyfish occurs in the Baltic Sea content (Barz and Hirche 2005), and is the dominant scyphomedusa species in the Gulf of Gdansk Aurelia aurita and mesozooplankton samples (southern Baltic Sea) that is an important tourism were collected every two weeks: during the first and fishing region of Poland (Schernewski and and the third weeks of a month from April to Schiewer 2002). In the biological context, it is a December 2008 at 11 sites located in the Gulf diverse ecosystem providing nursery grounds for of Gdansk (Fig. 1). Oblique hauls with a plank- many species of commercial and non-commer- ton net (0.75 m diameter, 100 µm mesh size) cial fishes (Lizińska 2002). Although A. aurita equipped with a flow meter were conducted from is common in the Polish fisheries zone of the the r/v Oceanograf 2 owned by the Institute Baltic Sea (Janas and Witek 1993), there are no of Oceanography at the University of Gdansk. data on the size (bell diameter), weight, or diet of Surface water salinity and temperature measure- 530 Brulińska et al. • BOREAL ENV. RES. Vol. 21 ments were taken concurrently with a WTW kept frozen until analysis. Pelagic food web Multiline P4 analyser equipped with the Tetra- components were collected vertically with two Con 352 probe. WP2 nets, phytoplankton (25–100 µm size) was All medusae examined were blotted, then collected with nets with 25 µm mesh and 57 cm measured (bell diameter) on board the vessel, diameter, while mesozooplankton, jellyfish and and then dissected to collect prey organisms fish (200–500 µm size) were sampled with nets from the canals, stomachs, and gastric pouches. with 200 µm mesh and a diameter of 57 cm. Zooplankton prey was preserved in 4% formal- The plankton was then sieved gently through dehyde, and later analyzed in the laboratory 1 mm mesh (mesozooplankton) and 125 µm under a Nikon SMZ800 zoom stereomicroscope. mesh (micro-zooplankton) to remove larger, The organisms were counted and identified to the free-floating items. Sampling was repeated sev- genus or species. The medusae were frozen, and eral times to collect sufficient quantities for later weighed in the laboratory. stable isotope analysis. The plankton collected was then placed separately in water collected in situ, transported to laboratory, and left overnight Mesozooplankton sampling and analysis to clear their guts. After this, the phytoplank- ton and mesozooplankton samples were frac- Mesozooplankton samples were collected simul- tionated through 25–100 µm and 200–500 µm taneously with the medusae, preserved in 4% sieves using the modified method by Rolf and formaldehyde, and analysed in the laboratory Elmgren (2000). This method is based on a under a stereomicroscope. The organisms were vacuum set of polyethylene bottles with differ- identified to the genus or species. ent mesh sized sieves and facilitates segregating planktonic fractions by size (Table 1). To ensure that samples of all zooplankton types were well Stable isotope analysis separated and dead organisms were not ana- lysed, the light trapping method was used. In a Pelagic food web components such as suspended dark room, each segregated fraction was placed organic matter, phytoplankton, zooplankton, jel- at one end of a long container, and then a light lyfish, and planktivorous fish were sampled in source was placed at the opposite end of the August 2011 to assess the trophic position of jel- container. After 15 minutes, live zooplankton lyfish in the pelagic food web structure by means that swam towards the light was collected, while of determining their δ13C and δ15N contents. dead individuals remained in the second, dark Additionally, δ13C and δ15N of zooplankton, jel- part of a container. This separation method relies lyfish and fish samples were used to assess on the positive phototaxis of some zooplankton the assimilation of different sources by Aurelia orders (cladocerans and copepods). After sepa- aurita by using Isotopic Mixing Model (see sec- ration, mesozooplankton was filtered thought tion ‘Isotopic mixing model’). Whatman GF/C and stored in vials. Subsamples Suspended particulate organic matter sam- of each zooplankton fraction were preserved in ples were obtained by filtering sea water through formaldehyde and taxonomically analysed. The grade GF/F Whatman glass microfiber filters remaining parts of the samples were stored at immediately after sampling. The samples were –20 °C until stable isotope analysis. Afterwards Table 1. Classification of plankton by size. Symbol Size (µm) Collected plankton type zoo200–500 200–500 zooplankton zoo100–200 100–200 zooplankton zoo50–100/phyto50–100 050–100 phytoplankton/zooplankton phyto25–50 025–500 phytoplankton BOREAL ENV. RES. Vol. 21 • Moon jellyfish in the Gulf of Gdansk: threatening predator or not? 531 phytoplankton fractions were filtered through analysis. Kruskal-Wallis one-way ANOVA was Whatman GF/C filters, washed by Milli-Q water used compare jellyfish size (bell diameter, wet and stored at –20 °C. weight) and prey composition among sampling After that all plankton fractions, jellyfish and sites and sampling periods, as well as δ13C and fish (muscle) were ground to a fine powder using δ15N among food web components. a Mixer Mill mm 200 homogenizer. Selectivity of A. aurita for prey taxa was esti- The stable isotope ratio of C and N was mated using Pearre’s selectivity index calculated measured using an Isoprime Micromass IRMS- on pooled data (Wintzer et al. 2011). Differences EA (MicroMass CHN analyser coupled with between species composition in jellyfish guts MICROMASS mass spectrometer) that provides and water were evaluated with a χ2-test. simultaneous data on carbon and nitrogen con- Data analyses were performed in STATIS- tent. Isotope composition was expressed as stand- TICA ver.