Predatory Impact of Aurelia Aurita on the Zooplankton Community in The

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Barz and Hirche: Predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic) CM 2004/J: 12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

Not to be cited without prior reference to the authors

CM 2004/J:12

Predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic)

Kristina Barz and Hans-Jürgen Hirche

Alfred-Wegener-Institute for Polar and Marine Research Columbusstrasse, 27568 Bremerhaven, Germany tel.: +49 471 4831 1324, fax: +49 471 4831 1918 [email protected]

Abstract The annual cycle of abundance and vertical distribution of the scyphozoan medusa Aurelia aurita was studied in the Bornholm Basin (central Baltic) in 2002. Seasonal changes in prey composition and predatory impact were investigated by analysing stomach contents. The dominant prey organisms of A. aurita were several cladoceran species (Bosmina coregoni maritima, Evadne nordmanni and Podon spp.). They represented up to 93% of the food organisms. Bivalve larvae and copepods (Centropages hamatus, Temora longicornis, Acartia spp., Pseudocalanus acuspes, Eurytemora hirundoides and Oithona similis) were less frequently found. Nauplii and copepodites I-III were not eaten by the medusae neither were fish eggs and larvae used as prey. Based on average medusae and zooplankton abundance, the predatory impact was very low. In August, when mean abundance of A. aurita was highest, only 0.1% of the copepod and 0.54% of the cladoceran standing stock were eaten per day. However in regions with higher medusae or lower zooplankton abundance up to 8% of the cladoceran standing stock were eaten per day.

Keywords: Aurelia aurita, predation, predatory impact, Bornholm Basin, Bosmina coregoni maritima

1 Barz and Hirche: Predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic) CM 2004/J: 12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Introduction The central Baltic Sea is a highly stratified brackish water system, with a permanent halocline and a summer thermocline. These conditions affect the composition and distribution patterns of zooplankton, including scyphomedusae and their food. When reaching high abundance, scyphomedusae have an impact on the zooplankton communities, and can reduce the stocks of several species (e.g. Möller 1980b; Matsakis and Conover 1991; Purcell 1992; Olesen 1995; Schneider and Behrends 1998). The most important effects of predation by medusae, summarized by Purcell (1997) are: Top down control of zooplankton populations with changes in the zooplankton community, possible competition for food with fish and consumption of fish eggs and larvae. In the central Baltic Sea only two scyphozoan medusae occur. C. capillata is found only sporadically. Its abundance in the Bornholm Basin is very low, alternating with years of complete absence (Janas and Witek 1993, Lischka 1999). In contrast, A. aurita occurs continuously in the Baltic Sea, reaching high densities during several years in the western Baltic (Möller 1980a, Schneider and Behrends 1994). A. aurita is feeding on several zooplankton species, including fish larvae and eggs. When occurring in high densities a reduction in the zooplankton and herring larvae populations was observed in the western Baltic (Möller 1980b; Schneider and Behrends 1998). The aim of our study is to investigate the occurrence and abundance patterns of scyphomedusae, to describe their prey field and to estimate their effect on zooplankton and fish larvae populations under conditions provided in a highly stratified environment like the Bornholm Basin.

Material and methods Sampling location and hydrography The study area was located in the Bornholm Basin, central Baltic Sea. The station grid included 52 stations (Fig. 1) with a depth range from 30 to 90m. Samples were taken during 11 cruises from March to November 2002, with a maximum time interval of two weeks between the sampling events from July to October. Salinity, temperature and dissolved oxygen were recorded on every station with a standard CTD probe (SBE 911+, ME).

Abundance and distribution of zooplankton and scyphomedusae Zooplankton samples were collected at nine focus stations using a multinet (0.25 m² net opening, 55µm mesh size), vertically towed in 10 m intervals from near bottom to surface. Samples were preserved in a 4% borax buffered formalin-seawater solution. At all grid stations scyphomedusae were collected from oblique Bongo net hauls (0.6m diameter, 335 and 500µm). As 335 and 500µm Bongo showed no difference for the abundance of A. aurita (p>0.5-0.9; Mann-Whitney Rank Sum Test) the average values of both nets were used for the distribution and abundance of the medusae. 2 Barz and Hirche: Predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic) CM 2004/J: 12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Additional oblique hauls with a 6m² Isaaks-Kidd-Midwater-Trawl (IKMT) were used at the focus stations to validate the sampling efficiency of the Bongo net for scyphomedusae. The vertical distribution of the scyphomedusae was studied by using a trawled BIOMOC (1m² net opening) or a trawled multinet (0.25m² net opening) on 4 cruises at station 23 (90m). On each cruise three repeated day and night hauls were taken in 5 or 10m intervals from bottom to surface. All medusae collected were processed on board, identified, weighed and measured to 0.5cm below.

Prey analysis For gut content analysis single A. aurita medusae were scooped from the surface water by using a dip net (July to October 2002). Medusae were preserved immediately in a 4% borax buffered formalin- seawater solution. To investigate the predation in different depth strata medusae from BIOMOC hauls were preserved for prey analyses. In the laboratory the medusae were dissected and canals, stomach and gastric pouches were examined for prey organisms, which were counted and identified to genus or species level. Copepods were determined to stage level.

Feeding rates and predatory impact Feeding rates and predatory impact were calculated for the focus stations on four cruises from July to October when data on zooplankton abundance were available. As vertical distribution results in this study indicated the occurrence of the medusae mainly in the upper 20m (80%), medusae densities were converted accordingly. As well zooplankton data were used from the 0-20m depth level. Individual feeding rates were expressed as numbers of prey eaten medusae–1 d-1: F = Cm / D x 24 h F = no. of prey ingested medusa-1 d-1; Cm = no. of prey in the medusa; D = digestion time (h). A digestion time of 3 h for mixed prey was used from Purcell (2003) who investigated A. labiata at 14°C in Prince William Sound, Alaska. 24 hours feeding was assumed. The impact of the medusae on the zooplankton populations was calculated according to the equation: P = F x M / C x 100 P = percentage of prey standing stock consumed d-1; F = no. of copepods ingested medusa-1 d-1; M = no. of medusae m-3; C = no. of copepods m-3.

Results Hydrography The hydrography of the central Baltic is characterised by an extreme stratification. From July to September the thermocline was found between 25 and 15m, separating the surface layer from the intermediate winter water layer. Temperature in the surface layer is shown in Fig. 2. It was 16°C in the beginning of July and maximum summer temperature was reached in August (20.8°C). In November

3 Barz and Hirche: Predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic) CM 2004/J: 12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ the surface water had cooled down to 8.4°C and merged with the winter water. Below a halocline, which varied between 45m in July and 55m in November, old oxygen-depleted bottom water reached to the ground. Salinity was rather constant between the surface and the halocline (7 PSU) and increased thereafter up to 15.5 PSU. Oxygen concentration decreased rapidly below the halocline to values <2ml l-1. In September a warm water body originating from an exceptional summer inflow event migrated between the intermediate winter water and the bottom water (Feistel et al. 2003).

Zooplankton abundance During medusae occurrence, cladocerans and copepods dominated the zooplankton. Between July and September the most abundant species in the upper 10m was the cladoceran Bosmina coregoni maritima, which reached maximum densities of 267000 ind. m–3 in August. A total amount of 88% of the adult individuals was present in the upper 20m in July (Schulz and Hirche 2004). In November only 50 ind. m–3 were found. Abundance of Podon spp. and Evadne nordmanni ranged around 100 and 5000 ind. m–3. Abundant copepods were Acartia spp., Temora longicornis, Pseudocalanus acuspes and Centropages hamatus, but their maximum abundance was found in deeper water layers between 20 and 70m.

Medusae abundance and size The first Aurelia aurita medusae were observed at the end of July. In Fig. 2 mean abundance is shown for all stations per cruise, integrated over the whole water column. Highest abundance of A. aurita was found in August and beginning of September with 2.3 and 2.1 ind. 100m-³. In November still 0.9 ind. 100m-³ were observed, but in mid January no medusae were found. Mean medusa size varied only slightly, between 8.5cm in July and 13.7cm in August, no ephyrae were caught.

Spatial distribution Abundance values were integrated over the whole water column. In August 2002 some larger patches of medusae were encountered where abundance reached more than 6 ind. 100m-3 (sts. 14 and 41), but also areas with no medusae were found (sts. 40 and 43b) (Fig. 1).

Vertical distribution From July to September A. aurita had its highest abundance in the upper water layers (Fig. 3). About 80% of the animals were caught in 0 to 20m. Only 20% of the medusae were spread in the remaining water column. No statistical significant difference between day and night samples was detected (Mann-Whitney Rank Sum Test, p= 0.18-1.00).

4 Barz and Hirche: Predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic) CM 2004/J: 12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Gut content Gut content of Aurelia aurita from four cruises was analysed in specimens collected from the surface layer. The food consisted of cladocerans, copepods, bivalve and gastropod larvae, but Bosmina coregoni maritima was the most abundant prey item during the whole investigation period (Tab. 1). In July the cladocerans made up 93% of the eaten prey. The copepod fraction included copepodites IV to V and adults of Acartia spp., Pseudocalanus acuspes, Temora longicornis, Centropages hamatus and Eurytemora hirundoides and varied from 1 to 14% of the prey items. The highest value was found for Acartia spp. in October (9%). A further important group of food items were the bivalve larvae with up to 11% in September. Medusae caught by BIOMOC in deeper water layers in July and September tended to have much less food in the guts than those collected by hand.

Predatory impact Predation effects of Aurelia aurita on the zooplankton community were calculated for all focus stations. Medusae and zooplankton abundance were used from the upper 20m depth layer, according to the vertical distribution of the medusae as shown in Fig. 3. The impact was calculated from averaged medusa gut contents and digestion time of 3h for mixed prey, for all focus stations on four cruises from July to October. In Tab. 2 the impact is listed for central sta. 23, additional stations with high medusae abundance and averaged for all focus stations. According to their low importance as prey (Tab. 1), all copepod species were summarized for impact calculations. Bosmina coregoni maritima, Evadne nordmanni and Podon spp were analysed together as cladocerans. From July to September, not more than 0.28% d-1of the copepod standing stock was eaten by A. aurita. The mean values for these cruises reached a max. of 0.10% d-1 in August. In October medusae fed more copepods and the predatory effects reached a maximum of 1.15% d-1of the standing stock, but the mean value was only 0.41% d-1. The impact on the cladoceran standing stock was comparably low, but some stations were stressed by a higher predation effect. In August, 7.87% of the cladoceran standing stock were eaten d-1 on sta. 21, whereas the bigger patch of medusae caught at sta. 41 ingested only 0.9% of the cladocerans d-1, due to the extreme high prey abundance. The highest averaged impact value was found in October (1.09%) when abundance of cladocerans in the upper 20m was lowest.

Discussion Aurelia aurita was caught from July to November 2002. Janas and Witek (1993) observed A. aurita in the Polish fisheries zone (southern / central Baltic) in the years 1983 to 1991 usually from July to January, with highest numbers between August and November. During the investigations of Lischka (1999) from May to August 1998 in the Bornholm Basin, their first appearance was also in July. In the western Baltic A. aurita appeared earlier. In Kiel Fjord it was observed all year round, with first ephyrae appearing in November 1987 (Möller 1980a), entering medusae stage (>1cm) already in late 5 Barz and Hirche: Predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic) CM 2004/J: 12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ April. In Gulmar Fjord (western Sweden) ephyrae occurred from October 1983 on, and already in May all specimens had developed into medusae (Hernroth and Gröndahl 1983; Gröndahl 1988). Olesen et al. (1994) found A. aurita ephyrae in Kertinge Nor and Kerteminde Fjord (Denmark) in February 1992. Mean abundance of A. aurita in the Bornholm Basin varied from 0.25 ind. 100m-3 in July to 2.3 in August 2002. Average numbers of A. aurita found by Lischka (1999) in 1998 in the same area showed similar values for July with 0.31 ind. 100m-3 and August with 2.18. In contrast Margonski and Horbowa (1995) caught only about 0.8 ind. 100m-3 in the Bornholm Basin in August 1994 (abundance estimated from their Fig. 2). Average biomass values for A. aurita caught by Janas and Witek (1993) during the years 1983-1991 were only 50% of our values, indicating large interannual variability. In the western Baltic often much higher numbers of medusae were found. Thus in Kiel Fjord abundance of A. aurita medusae reached a maximum of 12.1 ind. 100m-3 in September 1978 and 79 (Möller 1980a). In Kiel Bight and Eckernförde Bay medusae abundance showed also a high interannual variability (Schneider 1989; Schneider and Behrends 1994). Years with high numbers alternated with years of lower concentrations comparable to our study, especially in August 1983 in Eckernförde Bay (Schneider 1989). Aurelia aurita gut content consisted of cladocerans, copepods and mollusc larvae. The guts of medusae caught by dip net contained much more prey items than medusae caught by BIOMOC in deeper water layers. To exclude the possibility, that medusae in deeper water feed less, the total prey of hand collected specimens were compared to animals from 0 to 5m depth caught by BIOMOC. As shown in Tab. 1, the medusae seemed to lose prey items during the haul, caused by impact pressure or other physical factors. Therefore gut content from BIOMOC caught medusae is only listed for information, but notice the absence of fish eggs and larvae. The prey of A. aurita varied only slightly in species composition and abundance during the study period. The gut content reflected food availability in the upper 20m during the investigated season. However, some interesting highlights in prey composition were found. Refuge from predation by small size as suggested by Suchman and Sullivan (1998) seemed to work very well in the Bornholm Basin. The small nauplii and copepodites I- III were not ingested by A. aurita, although they were available in the water column with abundances reaching 29000 nauplii m-3 and 8600 copepodites I-III m-3 in the upper 20m in October. In August and September bivalve larvae were the second abundant prey items in the medusae guts, although their abundance in the water column was lower than in other months. However, predatory impact on the bivalve population may be negligible as they were shown to avoid digestion and survive gut passage (Purcell et al. 1991). The high numbers of Acartia spp. in the guts in October were caused by the extremely increased availability of all Acartia species in the upper 10m from ~1500 ind. m-3 in July to September to 14300 in October. A. aurita could not be verified as predator on cod eggs as described by Margonski and Horbowa (1995) for the Bornholm Basin, or on fish larvae as found by Möller (1980b, 1984) and several other authors (reviewed in Purcell 1985, van der Veer 1985). Cod eggs are restricted to a salinity ≥11 PSU, 6 Barz and Hirche: Predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic) CM 2004/J: 12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ therefore in the Bornholm Basin they were distributed in ≥60m depth (Nissling et al 1994), and also sprat eggs were distributed in deeper waters around 60m (Wieland 1989). According to this, the main numbers of medusae and fish eggs vertically mismatched, and no predation occurred. Loosing prey during the haul could cause the absence of fish eggs in medusae caught by BIOMOC in deeper layers. Cod and sprat larvae were present in the upper water layers and their absence in the medusae guts could be explained by their high escape velocities (Costello and Colin 1994). While copepods were described as the main food of A. aurita in other regions (Möller 1980b, Hamner et al. 1982, Matsakis and Conover 1991, Sullivan et al. 1994, Graham and Kroutil 2001), Bosmina coregoni maritima was the most abundant prey item from July-October 2002 in the Bornholm Basin, due to its high abundance in the upper 20m during that time. The zooplankton community in the central Baltic in summer is affected by low salinity and warm surface layers (Postel 1996). The resulting predominance of the brackish species B. coregoni maritima represents a special food supply, which was utilized by the medusae. Scyphomedusae may have a strong impact on zooplankton standing stocks. Purcell (1992) estimated for Chrysaora quinquecirrha a maximum consumption of 94% d-1of the copepod population in Chesapeake Bay (USA). Schneider and Behrends (1998) correlated zooplankton abundance to medusae densities and concluded that the stocks of most zooplankton species were negative affected in A. aurita bloom years in Kiel Bight, western Baltic. Möller (1980b) also suggested, that A. aurita is the dominant factor in regulating plankton dynamics in Kiel Bight during summer. In contrast Purcell (1997) suggested, that predation on copepods usually is inadequate to reduce populations. For Aurelia labiata she found only low predation rates (max 0.08% of copepod standing stock d-1) in Prince William Sound (Alaska, USA) (Purcell 2003). In the Bornholm Basin in 2002 copepods were only of secondary importance as food in general and only a maximum of 1.15% of the standing stock were eaten d-1 (Tab. 2). The main consumed prey species Bosmina coregoni maritima was very abundant and only the combination of high medusae densities and lower prey abundance at some stations resulted in a higher impact, as found for sta. 21 in August (7.87% of the standing stock d-1). In addition, B. coregoni maritima has a very rapid parthenogenetic reproduction and can thus numerically outgrow their predators (Viitasalo et al. 2001). In consequence we suggest that predation by A. aurita does not regulate the zooplankton community in the Bornholm Basin.

Acknowledgements This study would not have been possible without the support of colleagues and students in collecting and analysing medusae. We thank the crew of RV Alkor, Heincke and A. v. Humboldt. This research was funded by GLOBEC Germany, BMBF 03F0320D.

7 Barz and Hirche: Predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic) CM 2004/J: 12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ References Costello JH, Colin SP (1994) Morphology, fluid motion and predation by the scyphomedusa Aurelia aurita. Mar Biol 121: 327-334 Feistel R, Nausch G, Matthaus W, Hagen E (2003) Temporal and spatial evolution of the Baltic deep water renewal in spring 2003. Oceanologia 45: 623-642 Graham WM, Kroutil RM (2001) Size-based Prey Selectivity and Dietary Shifts in the Jellyfish, Aurelia aurita. J Plankton Res 23: 67-74 Groendahl F (1988) A comparative ecological study on the scyphozoans Aurelia aurita, Cyanea capillata and C. lamarckii in the Gullmar Fjord, western Sweden, 1982 to 1986. Mar Biol 97: 541-550 Hamner WM, Gilmer RW, Hamner PP (1982) The physical, chemical and biological characteristics of a stratified, saline, sulfide lake in Palau. Limnol Oceanogr 27: 896-909 Hernroth L, Groendahl F (1983) On the biology of Aurelia aurita (L.). 1. Release and growth of Aurelia aurita (L.) ephyrae in the Gullmar Fjord, western Sweden, 1982-83. Ophelia 22: 189-199 Janas U, Witek Z. (1993) The occurence of medusae in the Baltic and their importance in the ecosystem, with special emphasis on Aurelia aurita. Oceanologia 34: 69-84 Lischka M (1999) Abundanz, Verteilung und Nahrungsökologie von Scyphomedusen in der zentralen Ostsee. MS Thesis, Kiel Margonski P, Horbowa K (1995) Vertical distribution of cod eggs and medusae in the Bornholm basin. Scientific Papers Presented at the Polish-Swedish Symposium on Baltic Cod., Mir, Gdynia (Poland), 1995. Medd Haviskelab Lysekil 327: 7-17 Matsakis S, Conover RJ (1991) Abundance and feeding of medusae and their potential impact as predators on other zooplankton in Bedford Basin (Nova Scotia, Canada) during spring. Can J Fish and Aquat Sci 48: 1419-1430 Moeller H (1980a) Population Dynamics of Aurelia aurita Medusae in Kiel Bight, Germany (FRG). Mar Biol 60: 123-128 Moeller H (1980b) Scyphomedusae as Predators and Food Competitors of Larval Fish. Mar Res 28: 90-100 Moeller H (1984) Data on the biology of jellyfish and youngfish in Kiel Bight. Verlag H. Moeller, Kiel (FRG) Nissling A, Kryvi H, Vallin L (1994) Variation in egg buoyancy of Baltic cod Gadus morhua and its implications for egg survival in prevailing conditions in the Baltic Sea. Mar Ecol Prog Ser 110: 67-74 Olesen N (1995) Clearance potential of jellyfish Aurelia aurita, and predation impact on zooplankton in a shallow cove, Mar Ecol Prog Ser 124: 63-72 Olesen NJ, Frandsen K, Riisgard HU (1994) Population dynamics, growth and energetics of jellyfish Aurelia aurita in a shallow fjord. Mar Ecol Prog Ser 105: 9-18 Postel L (1996) Zooplankton. In: Rheinheimer, G. (ed.) Meereskunde der Ostsee. Springer, Berlin, pp 150-160 Purcell JE (1985) Predation on fish eggs and larvae by pelagic cnidarians and ctenophores. Bull Mar Sci 37: 739-755 Purcell JE, Cresswell FP, Cargo DG, Kennedy VS (1991) Differential ingestion and digestion of bivalve larvae by the scyphozoan Chrysaora quinquecirrha and the ctenophore Mnemiopsis leidyi . Biol Bull 180: 103- 111 Purcell JE (1992) Effects of predation by the scyphomedusan Chrysaora quinquecirrha on zooplankton

8 Barz and Hirche: Predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic) CM 2004/J: 12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ populations in Chesapeake Bay, USA. Mar Ecol Prog Ser 87: 65-76 Purcell JE (1997) Pelagic cnidarians and ctenophores as predators: Selective predation, feeding rates, and effects on prey populations. Ann Inst Oceanogr 73: 125-137 Purcell JE (2003) Predation on zooplankton ba large jellyfish, Aurelia labiata, Cyanea capillata and Aequorea aequorea, in Prince William Sound, Alaska. Mar Ecol Prog Ser 246: 137-152 Schneider G (1989) Estimation of food demands of Aurelia aurita medusae populations in the Kiel Bight/western Baltic. Ophelia 31: 17-27 Schneider G, Behrends G (1994) Population dynamics and the trophic role of Aurelia aurita medusae in the Kiel Bight and western Baltic. ICES J Mar Sci 51: 359-367 Schneider G, Behrends G (1998) Top - down control in a neritic plankton system by Aurelia aurita medusae - a summary. Ophelia 48: 71-82 Schulz J. Hirche H.-J. (2004) Zooplankton distribution in the Bornholm Sea – A seasonal overview. ICES, ASC, CM 2004/P: 19 Suchman CL, Sullivan BK (1998) Vulnerability of the copepod Acartia tonsa to predation by the scyphomedusa Chrysaora quinquecirrha: Effect of prey size and behaviour. Mar Biol 132: 237-245 Sullivan BK, Garcia JR, Klein-MacPhee G (1994) Prey selection by the scyphomedusan predator Aurelia aurita. Mar Biol 121: 335-341 van der Veer HW (1985) Impact of coelenterate predation on larval plaice Pleuronectes platessa and flounder Platichthys flesus stock in the western Wadden Sea. Mar Ecol Prog Ser 25: 229-238 Viitasalo M, Flinkman J, Viherluoto M (2001) Zooplanktivory in the Baltic Sea: A comparison of prey selectivity by Clupea harengus and Mysis mixta, with reference to prey escape reactions. Mar Ecol Prog Ser 216: 191-200 Wieland K (1989) Small-scale vertical distribution and mortality of cod and sprat eggs in the Bornholm Basin (Baltic Sea) during two patch studies in 1988. ICES Council Meeting 1989. 15pp

9 Barz and Hirche: Predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic) CM 2004/J: 12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Figures and Tables

Sweden 11a 89 11 11b

2 3 4 5 6 7 10 12 12a 12b

40m X 1 X19 X18 X17 16 13 15 14 60m 80m X20 21 22 23 24 25 26

Bornholm 32 31 30 29 28 27

33 34 35 36 37

42b 41 40 39 38 42 43b 43a 43 44 45

0 1 2 4 6 8 ind. 100m-3

Fig. 1: Study area with spatial distribution of Aurelia aurita in August 2002. X = no data on cruise, focus stations underlined

10 Barz and Hirche: Predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic) CM 2004/J: 12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯

4,0 22

A. aurita 20 3,5 C. capillata Temp. [°C] 5m 18 3,0 16 ]

2,5 14 °C [ -3 12 2,0 100m .

10 erature d p n I 1,5 8 Tem 6 1,0 4 0,5 2

0,0 0 Jun Jul Aug Sep Oct Nov Dec Jan

Fig. 2: Mean abundance of A. aurita and C. capillata in the Bornholm Basin and water temperature in 5m in 2002-2003

Ind. 100m-3

Fig. 3: Vertical distribution of Aurelia aurita and hydrographic situation in September 2002 in the Bornholm Basin.

11 Barz and Hirche: Predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic) CM 2004/J: 12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Tab. 1: Prey composition of gut contents from Aurelia aurita, July to October 2002. 0m = caught by dip net from surface, all others = caught by BIOMOC (%) (%) (%)

(%) ) SD spp (%) % ± ( spp. (%) Month Depth (m) No. of medusae Size (cm) means Total prey Bosmina coregoni maritima Evadne nordmanni Podon Pseudocalanus acuspes (%) Acartia Temora longicornis Centropages hamatus (%) Eurytemora hirundoides Bivalve larvae (%) Gastropod larvae Jul 0 10 8.7 ± 2.1 3356 87.53 1.35 4.02 0.09 0.37 1.93 3.87 00.0 0.83 00.0 0-5 10 7.9 ± 0.9 641 48.21 1.72 36.35 1.09 0.00 11.54 1.09 0.00 0.00 0.00 0-10 20 6.6 ± 1.8 763 46.53 1.70 37.61 1.18 0.13 11.66 1.18 0.00 0.00 0.00 30-40 6 7.0 ± 3.3 326 75.77 1.53 16.56 0.61 0.92 2.15 2.45 0.00 0.00 0.00 70-80 9 9.9 ± 2.5 915 64.04 1.20 19.34 3.17 1.75 6.01 3.61 0.00 0.87 0.00 Aug 0 10 14.4 ± 0.7 6692 87.60 0.45 1.90 0.00 0.34 0.19 0.46 0.00 9.03 0.00 Sep 0 10 15.2 ± 1.6 2290 75.02 9.83 0.26 0.13 0.30 1.53 1.26 00.0 11.12 0.39 0-5 10 10.9 ± 1.6 815 78.77 5.15 6.87 1.60 0.25 4.17 1.96 0.00 1.23 0.00 10-15 10 11.9 ± 1.1 178 26.40 21.91 13.48 2.25 1.12 23.03 9.55 0.00 2.25 0.00 40-45 10 13.6 ± 1.6 303 28.05 10.89 6.27 0.99 3.30 38.61 6.93 0.00 4.62 0.33 80-85 6 14.7 ± 4.7 138 55.80 7.97 5.07 5.80 4.35 15.94 5.07 0.00 0.00 0.00 Oct 0 10 14.0 ± 3.0 3364 75.42 1.84 6.18 0.42 9.09 1.24 0.80 2.76 1.66 0.50

12 Barz and Hirche: Predatory impact of Aurelia aurita on the zooplankton community in the Bornholm Basin (central Baltic) CM 2004/J: 12 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ Tab. 2: Predatory impact of A. aurita on copepods a. and cladocerans b. from July to Aug. 2002, calculated with abundance of medusae and prey in the upper 20m for sta. 23, additional focus stations with high medusae abundance and averaged for all focus stations, parenthesised the range for al focus stations. For size and number of analysed medusae see Tab. 1. Digestion time was 3h, 24 h feeding assumed. a. Date Station No. Cop. Medusae No. cop. eaten No. cop. eaten Copepods % eaten d-1 medusa -1 m-3 med.-1 d-1 m-3 d-1 m-3 (range) July 23 20.4 0.02 163.2 3.26 4267 0.08 41 20.4 0.04 163.2 6.53 2358 0.28 average 20.4 0.01 ± 0.01 163.2 1.42 ± 2.20 5028 ± 5695 0.03 (0.00-0.28) Aug. 23 6.9 0.07 55.2 3.86 9139 0.04 41 6.9 0.18 55.2 9.94 4192 0.24 average 6.9 0.09 ± 0.06 55.2 4.73 ± 3.15 4601 ± 2855 0.10 (0.03-0.27) Sep. 23 7.4 0.10 59.2 5.92 5971 0.10 03 7.4 0.11 59.2 6.51 3101 0.21 41 7.4 0.09 59.2 5.33 3450 0.15 average 7.4 0.07 ± 0.03 59.2 3.88 ± 1.60 5328 ± 7198 0.07 (0.01-0.21) Oct. 23 48.2 0.03 385.6 11.57 19328 0.06 12 48.2 0.12 385.6 46.27 4013 1.15 average 48.2 0.05 ± 0.03 385.6 17.35 ± 13.36 6144 ± 5517 0.41 (0.06-1.15) b. Date Station No. clad. Medusae No. clad. eaten No. clad. eaten Cladocerans % eaten d-1 medusa -1 m-3 med.-1 d-1 m-3 d-1 m-3 (range) July 23 302.6 0.02 2420.8 48.42 43280 0.11 41 302.6 0.04 2420.8 96.83 32154 0.30 average 302.6 0.01 ± 0.01 2420.8 21.01 ± 32.65 43812 ± 19278 0.05 (0.00-0.30) Aug. 23 601.9 0.07 4815.2 337.06 174118 0.19 21 601.9 0.15 4815.2 722.28 9178 7.87 41 601.9 0.18 4815.2 866.74 95501 0.91 average 601.9 0.09 ± 0.06 4815.2 412.04 ± 272.62 75749 ± 67946 0.54 (0.13-7.87) Sep. 23 198.8 0.10 1590.4 159.04 6384 2.49 03 198.8 0.11 1590.4 174.94 12954 1.35 41 198.8 0.09 1590.4 143.14 3392 4.22 average 198.8 0.06 ± 0.03 1590.4 104.26 ± 42.90 17716 ± 10088 0.59 (0.20-4.22) Oct. 23 280.7 0.03 2245.6 67.37 37965 0.18 12 280.7 0.12 2245.6 269.47 6656 4.05 average 280.7 0.05 ± 0.03 2245.6 101.98 ± 78.54 12862 ± 11261 1.09 (0.18-4.05)