F. Lüskow, H. U. Riisgård *

Transcription

1 Vie et milieu - Life and environment, 2016, 66 (3-4): Population predation impact of jellyfish (aurelia aurita) controls the maximum umbrella size and somatic degrowth in temperate Danish waters (Kertinge Nor and Mariager Fjord) F. Lüskow, H. U. Riisgård * Marine Biological Research Centre, University of Southern Denmark, Hindsholmvej 11, 5300 Kerteminde, Denmark * Corresponding author: hur@biology.sdu.dk SCYPHOZOA PREDATION IMPACT FOOD WEB INTERANNUAL VARIABILITY SOMATIC GROWTH ABSTRACT. The population density and individual size of the common jellyfish Aurelia aurita were studied during 2014 and 2015 in two Danish fjord systems, Kertinge Nor and Mariager Fjord in order to obtain a better understanding of the driving forces for somatic degrowth (shrinkage) of medusa during late summer and autumn. In both fjord systems the numerous medusae were characterized by their small body size and by a distinct phase of degrowth. The population predation impact of A. aurita, with estimated zooplankton half-lives of only about 1 to 3 d, indicated that shortage of prey controls the maximum umbrella size of only 60 (Kertinge Nor) to 100 mm (Mariager Fjord) and the subsequent degrowth. When jellyfish were brought into the laboratory in early spring and the late degrowth period and continuously fed with zooplankton (Artemia salina), equivalent to 5.8 µg C l 1, this resulted in initial growth rates of 11.3 and 24.4 % d 1 in two series of experiments with 56.9 and 5.5 mm umbrella diameter, respectively, and considerably longer survival than of jellyfish in their natural environment. The degrowth rates in Kertinge Nor ( 1.2 % d 1 ) and Mariager Fjord ( 1.5 and 0.7 % d 1 in 2014 and 2015, respectively) were slightly lower than observed in laboratory starvation experiments ( 3.2 and 4.2 % d 1 ), indicating that the starvation of jellyfish in nature was less pronounced, i.e. some prey organisms may have been available although the short estimated half-lives of zooplankton suggested a pronounced predation impact exerted by the jellyfish. INTRODUCTION The importance of jellyfish in marine food webs has often been overlooked, but increasing attention is now being paid to the predation impact of jellyfish on zooplankton and early life history stages of fish (Elliott & Leggett 1995, Hansson et al. 2005, Shoji et al. 2005, Titelman & Hansson 2006, Ohata et al. 2011, Riisgård et al. 2012), and to the competition of jellyfish with zooplanktivorous fish for the same prey, causing damages on commercially important fish populations (Purcell & Arai 2001, Purcell et al. 2007, Boero et al. 2008, Flynn et al. 2012, Haraldsson et al. 2012), and further, intra-guild predation may also regulate jellyfish populations (e.g. Båmstedt et al. 1997, Hansson 1997, Pitt et al. 2014). The common jellyfish Aurelia aurita (Linnaeus, 1758) has been described as a cosmopolitan generalist (Dawson & Martin 2001) which occurs in a wide variety of coastal ecosystems (Möller 1980, Lucas 1996) and can be very abundant and exert a considerable predatory impact (Bailey & Batty 1984, Båmstedt 1990, Schneider & Behrends 1994, 1998, Behrends & Schneider 1995, Uye et al. 2003, Møller & Riisgård 2007a, b, Matsakis & Conover 2011). A. aurita has a life cycle which includes a pelagic medusa and a benthic polyp stage. Medusae reproduce sexually, and females release planula larvae that settle and metamorphose into polyps which produce ephyrae that develop into medusae (Møller 1980, Hernroth & Gröndahl 1985, Lucas 2001). In temperate waters an annual life cycle of A. aurita is typical (Hamner & Jenssen 1974, Lucas 2001), and thus in temperate Danish waters, ephyrae are released in spring resulting in a distinct cohort of medusae that reproduce sexually during summer, followed by loss of body mass ( degrowth ) and disappearance of medusae in late autumn (Ussing 1927, Olesen et al. 1994, Goldstein & Riisgård 2016). The population density, individual size and sexual reproduction of Aurelia aurita were investigated during 2013 and 2014 in the shallow Danish cove Kertinge Nor (and the adjacent Great Belt) by Goldstein & Riisgård (2016) in order to explain the mechanisms controlling the seasonal dynamics of jellyfish populations in temperate waters. After sexual maturation in early summer, the medusae were characterized by a distinct phase of degrowth, which was followed by the absence of A. aurita medusae during winter, and it was found that the production of planula larvae accounted for less than about 10 % of total size-specific energy losses (Goldstein & Riisgård 2016). Shortage of zooplankton prey during autumn was suggested to be the main factor controlling degrowth, and results indicated that seasonal variability in food supply, rather than energy allocation to reproduction determines the life span of A. aurita medusae in temperate Danish waters (Goldstein & Riisgård 2016).

2 234 F. LÜSKOW, H. U. RIISGÅRD In the present work, we have further studied the seasonal growth and degrowth patterns of jellyfish in two Danish waters, Kertinge Nor and Mariager Fjord, in order to obtain a better understanding of the reasons for degrowth of medusa and why they disappear during winter. The hypothesis whether food limitation starting in late summer is the main reason for the onset of degrowth in natural jellyfish populations was tested by a combination of field observations and laboratory feeding and growth experiments. MATERIALS AND METHODS Study sites Mariager Fjord: This fjord is located on the Danish northeast coast and connected to the Kattegat (Fig. 1). The fjord is 42 km long and mostly narrower than 2 km, the inner part is between 10 to 30 m deep and the water in the inner deep basins is almost stagnant below 15 m. The outer part of Mariager Fjord consists of shallow tidal plains, around 1 to 2 m depth, apart from a narrow channel (4 to 6 m depth) held open by the tide (Fallesen et al. 2000). Compared to the open sea and other Danish fjords, the nutrient concentrations (N and P) in the surface layer in Mariager Fjord are among the highest, and the nutrients give rise to a considerable primary production dominated by a few diatom species (Fallesen et al. 2000). The salinity in Mariager Fjord is typically between 12 to 17 at the surface and between 18 to 24 and 20 to 25 in the inner and outer part, respectively. The tidal range is about 20 to 30 cm. The topography of Mariager Fjord, with a deep inner part and a very shallow outer part, implies that the water exchange with the open sea (i.e. Kattegat) is very limited (Ellegaard et al. 2006), and it takes approximately 17 months for half of the bottom water in the inner part to be replaced. The deep water is only exchanged when high-saline water from the Kattegat enters the fjord (Fallesen et al. 2000). Only small freshwater streams flow into the fjord. Water masses in the upper layer are usually well windmixed while a permanent halocline at around 10 to 15 m depth separates it from the deep water (Fallesen et al. 2000, Petersen et al. 2002, Zuendorf et al. 2006). Below the halocline the water is nearly always anoxic and only a diverse bacterial community exists (Fenchel et al. 1995). During summer, when the stratification is most stable and the biological oxygen consumption is at Fig. 1. Collecting sites for jellyfish in two Danish waters. A: Kertinge Nor ( N, E); B: Mariager Fjord ( N, E).

3 JELLYFISH DEGROWTH IN DANISH WATERS 235 a maximum, the anoxic zone gradually moves upwards, and in 1997 the water column became anoxic to the surface resulting in total defaunation (Fallesen et al. 2000, Hansen et al. 2002, Petersen et al. 2002). During the jellyfish sampling in 2014 and 2015, temperature (9.8 to 16.8 C and 6.8 to 19.7 C), salinity (15.4 to 15.9 and 14.3 to 16.4), and chlorophyll a concentrations (6.6 to 9.1 µg l 1 and 8.3 to 11.3 µg l 1 ) were measured by means of a CTD sea bird (GMI probe 45, Geological Marine Instrumentation, Denmark). Kertinge Nor: The fjord system consisting of Kerteminde Fjord and Kertinge Nor (Fig. 1) covers an area of 8.5 km 2 and has a mean water depth of approximately 2 m and a maximum depth of 8 m. The fjord has a sill at its mouth to the open sea (Great Belt). The discharge over the sill is forced by a diurnal tide with average amplitude of approximately 20 cm. The salinity in the fjord system varies typically between 15 and 21 over the year, and the water exchange is governed by density-driven circulation (Riisgård et al. 1996, Nielsen et al. 1997, Riisgård 1998, Goldstein & Riisgård 2016) which results from salinity variations in Great Belt due to changing flow situations (Jürgensen 1995, Møller 1996). Extremely high abundance of small jellyfish, Aurelia aurita, in Kertinge Nor causes shortage of prey and thus restriction of their own growth, and therefore the maximum diameter of the medusa umbrella is usually only a few centimeters (Olesen et al. 1994, Olesen 1995, Riisgård et al. 1995, 1996, Frandsen & Riisgård 1997, Nielsen et al. 1997, Riisgård et al. 2008, 2010). The high population density of A. aurita in combination with small umbrella diameters remained remarkably unchanged during the period 1991 to 2009, although all sewage effluent to the fjord system was cut off in 1990 (Riisgård et al. 2010). But in years with particularly high densities of jellyfish, this has been correlated with relatively small diameters (Riisgård et al. 2010), a pattern that has been confirmed by recent jellyfish studies conducted in Kertinge Nor in 2013 and 2014 by Goldstein & Riisgård (2016). During the sampling in 2015 temperature (5.3 to 22.4 C), salinity (13.9 to 19.8), and chlorophyll a concentrations (1.9 to 12.6 µg l 1 ) were routinely measured at 1 m depth using an YSI 650 (Yellowstone Scientific Instruments, Big Sky, Montana, USA). Collection of jellyfish: In Mariager Fjord (Fig. 1) Aurelia aurita medusae were collected every second week with a 500 µm meshed plankton net (mouth area = 0.20 m 2, KC Denmark A/S, volume filtered per haul was 10 to 100 m 3 ) at the deepest point during short boat trips in 2014 and One to 3 horizontal hauls were performed at a depth of 1 m at a speed of 1.5 to 2.5 knots (haul length between 50 and 500 m) for population density estimates. After each haul, the number of medusae was counted either on-board the small research boat R/B Maria or within 20 h after sampling, and the population density was estimated. The inter-rhopalia umbrella diameter of 50 A. aurita individuals was determined to the nearest mm on millimetre paper. In Kertinge Nor (Fig. 1) jellyfish were collected every week by means of 3 horizontal sub-surface hauls using a 500 µm meshed plankton net (mouth area = 0.25 m 2, KC Denmark A/S, volume filtered per haul was 2.5 to 5.0 m 3 ). Mean population densities were estimated from 3 hauls, and the diameter of 50 individuals was determined similar to the nearest mm as in Mariager Fjord. Laboratory feeding and growth experiments: Jellyfish were fed in controlled feeding experiments with 3-day old Artemia salina (Linnaeus, 1758) nauplii obtained from cysts (OOO Biotrade, Barnaul, Russia). Therefore, every day a new cohort of A. salina was started in illuminated air-mixed 1 liter flasks containing artificial seawater (25 C, salinity of 15) (Tetra Marine Sea Salt, Melle, Germany). Before adding A. salina to the feeding tank, unhatched cysts were allowed to float up and were removed by skimming. By means of a dosing pump (Ole Dich, Type 102 SA. CH. 4., Hvidovre, Denmark) A. salina were transferred to the jellyfish holding aquarium, and the same water volume was simultaneously taken out by a second pump. The number of A. salina that was taken out by the activity of the second pump was tested to be insignificant. By adjusting the number of Artemia added to the jellyfish tank to match the steadily increasing clearance rate of the growing jellyfish a steady-state prey concentration of 4.9 ± 2.0 Artemia l 1 was ensured during the 2 series of feeding experiments (see Supplementary Material). Every week all jellyfish were carefully taken out and placed with the aboral side on a millimeter paper to measure their umbrella diameter. Water movement to prevent the jellyfish to bunch together was generated by a gentle, air-bubbling driven water-circulation system (Fig. S1, Supplementary Material). Once a week, the dosing pump was switched off to measure the clearance rate of the jellyfish by following the exponential reduction of Artemia (Fig. S2, Supplementary Material), and subsequently by adjusting the concentration of Artemia in the stock-flask the dosing pump ensured a steadystate concentration of 5 Artemia l 1 in the jellyfish tank. The concentration of prey organisms in the jellyfish growth tank was measured every day by taking out samples for counting under a stereo microscope to adjust the number of prey items that had to be added. Series #1 was started on 22 December 2014 and conducted in a 713 l tank with slowly circulating seawater (11.8 ± 2.0 C, salinity of 15) with 10 jellyfish (d = 56.9 ± 10.3 mm) collected in Kertinge Nor. Before the experiment started plenty small parasitic amphipods (Hyperia galba Montagu, 1813) were carefully removed with forceps from the stomach of the jellyfish. The feeding experiment ran for 39 d before the food supply was stopped, followed by a 39 d starvation period, before the dosing pump was switched on again for further 119 d. Series #2 was started on 25 March 2015 using a 370-L tank with circulating seawater (17.1 ± 3.1 C, salinity of 15) added 60 small jellyfish (d = 5.5 ± 2.4 mm) collected in Kertinge Nor and fed during 77 d, followed by a starvation period of 23 d. Clearance rate and predation impact: The individual clearance rate (Cl ind, l d 1 ) of Aurelia aurita feeding on adult copepods in Kertinge Nor and Mariager Fjord was estimated from

4 236 F. LÜSKOW, H. U. RIISGÅRD the inter-rhopalia umbrella diameter (d, mm) by the equation (Møller & Riisgård 2007a; Fig. 5 therein): Cl ind = d 2.1 Eq. (1). The volume-specific population clearance rate (Cl pop, m 3 water filtered by the jellyfish population in one m 3 water per day = m 3 m -3 d 1 ) was estimated as the product of the individual clearance rate (Cl ind, l d 1 ) and the population density (D, ind. m 3 ) (Riisgård et al. 2010): Cl pop = Cl ind D/1,000 Eq. (2). The time (t 1 2, d) it takes for a population of jellyfish to reduce the concentration of prey organisms (i.e. copepods) by 50 % (i.e. the half-life time of prey) was estimated as (Riisgård et al. 2012): t 1 2 = ln2 Cl pop Eq. (3). Growth rate estimates: The inter-rhopalia umbrella diameter (d, mm) of Aurelia aurita was converted to individual body dry weight (DW, mg) using the following equation from Båmstedt et al. (1999; Fig. 1B therein) for ephyrae (d 10 mm) and from Olesen et al. (1994; Fig. 2 therein) for medusae: DW ephyra = d Eq. (4), DW medusa = d 2.82 Eq. (5). The weight-specific growth rate (µ, % d 1 ) of Aurelia aurita was determined using the equation (Olesen et al. 1994): µ = ln(dw t DW 0 ) t Eq. (6), where DW t and DW 0 express the mean individual body dry weight of jellyfish at time t and time 0, respectively. Statistical analyses: Analysis of variance (ANOVA) with repeated measures was carried out to compare interannual differences in umbrella diameter and density in the degrowth period between 2014 and 2015, as well as between Mariager Fjord and Kertinge Nor in Statistical analyses were performed in R (R Core Team 2015) version Hypothesis of significance was accepted for P < RESULTS The growth and subsequent degrowth patterns of Aurelia aurita in Kertinge Nor and Mariager Fjord appear from Figs 2, 3 and Tables I, II. In both locations, the maximum umbrella diameter was relatively small, 64.0 ± 16.1 mm in Kertinge Nor in 2015 (Fig. 2A, Table I), and 72.9 ± 33.5 and 42.3 ± 10.4 mm in Mariager Fjord in 2014 and 2015, respectively (Fig. 3A, Table II) and was reached soon after high initial growth rates of 4.0 % in Mariager Fjord (Fig. 3B) and 6.6 % in Kertinge Nor (Fig. 2B) in In 2015, jellyfish reached in Mariager Fjord significantly bigger umbrella diameters (ANOVA, Fig. 2. Aurelia aurita. Jellyfish collected in Kertinge Nor in A: Mean (± SD) umbrella diameter; B: Individual dry weight (cf. Eqs. 4&5) along with exponential regression lines and equations in growth ( ) and degrowth ( ) as a function of Julian date (d). Data from Table I. Fig. 3. Aurelia aurita. A: Mean (± SD) umbrella diameter of jellyfish collected in Mariager Fjord in 2014 during degrowth period (Δ), and in 2015 during growth ( ) and degrowth ( ) periods. B: Estimated individual dry weight (cf. Eqs. 4&5) along with exponential regression lines and equations for growth and degrowth. Data from Table II.

5 JELLYFISH DEGROWTH IN DANISH WATERS 237 F 1,4258 = 572.3, P = ) compared to Kertinge Nor, whereas the density relation was reverse (ANOVA, F 1,4258 = 98.2, P = ). In Kertinge Nor, the weightspecific degrowth rate (expressed by the negative exponent in the equation for the exponential regression line) was 1.2 % d 1 (Fig. 2B), which may be compared with 1.5 and 0.7 % d 1 in Mariager Fjord in 2014 and 2015, respectively (Fig. 3B). The jellyfish population density in Mariager Fjord was substantially lower in 2014 than in 2015 (Table II, ANOVA, F 1,2062 = 436.4, P = ) which is reflected in a significantly larger mean umbrella diameter in 2014 (ANOVA, F 1,2062 = 1,035.0, P = ) whereas the estimated relatively short half-lives of zooplankton are comparable, suggesting predation control of the zooplankton and that the maximum umbrella diameter is correlated with jellyfish density, i.e. the bigger the Table I. Aurelia aurita. Jellyfish collected in Kertinge Nor in D = population density, d = umbrella diameter, DW = individual dry weight (cf. Eqs. 4&5), Cl ind = individual clearance rate (cf. Eq. 1), Cl pop = population clearance rate (cf. Eq. 2), t 1/2 = estimated half-life of copepods (cf. Eq. 3, t 1/2 > 3 weeks are indicated by ). Mean ± SD are shown. Cruise #/ Date D (ind. m 3 ) d (mm) DW (mg) Cl ind (l d 1 ) Cl pop (m 3 m 3 d 1 ) t 1/2 (d) #1, 18 Mar 60.7 ± ± #2, 25 Mar ± ± #3, 8 Apr 35.3 ± ± #4, 15 Apr 77.9 ± ± #5, 22 Apr 15.7 ± ± #6, 29 Apr 16.0 ± ± #7, 6 May 30.1 ± ± #8, 12 May 78.1 ± ± #9, 20 May 83.7 ± ± #10, 2 Jun ± ± #11, 26 Jun 10.7 ± ± #12, 8 Jul 36.0 ± ± #13, 15 Jul 7.7 ± ± #14, 22 Jul 46.4 ± ± #15, 29 Jul 28.3 ± ± #16, 5 Aug 43.7 ± ± #17, 11 Aug 13.9 ± ± #18, 19 Aug 42.4 ± ± #19, 26 Aug 25.9 ± ± #20, 4 Sep 22.4 ± ± #21, 9 Sep 5.9 ± ± #22, 16 Sep 20.8 ± ± #23, 23 Sep 10.1 ± ± #24, 2 Oct 2.7 ± ± #25, 6 Oct 94.4 ± ± #26, 14 Oct 11.9 ± ± #27, 22 Oct 22.1 ± ± #28, 28 Oct 16.5 ± ± #29, 6 Nov 4.0 ± ± #30, 11 Nov 4.9 ± ± population density, the smaller the maximum diameter. Likewise, the estimated short half-lives of zooplankton in Kertinge Nor in 2015, typically less than one day from mid-may to late August (Table I), indicate predation control and food limitation being the decisive factor for maximum umbrella diameter. The data obtained from the two series of controlled laboratory feeding experiments with Aurelia aurita are shown in Figs 4, 5 and Tables III, IV. In Series #1, the mean initial umbrella diameter was 56.9 ± 10.3 mm which during the following 39 days increased to 91.2 ± 19.8 mm, indicating a 3.8 times increase in body dry weight along with a decrease in weight-specific growth rate from 11.3 to 0.1 % d 1 during this period (Table III). During the subsequent 39 days starvation period, the mean degrowth rate was 3.2 % d 1 (Fig. 4B), before the prey organism dosing pump was switched on again to keep jellyfish alive for further 119 days (last jellyfish died on 7 July 2015). The initial umbrella diameter of the jellyfish in Series #2 was only 5.5 ± 2.4 mm, but it rapidly increased to 77.5 ± 30.8 mm during the following 77 days, indicating a 1,227 times increase in body dry weight along with a decrease in weight-specific growth rate from initially 24.4 falling gradually to 1.7 % d 1 during this period (Table Table II. Aurelia aurita. Jellyfish collected in Mariager Fjord in 2014 and D = population density, d = umbrella diameter, DW = individual dry weight (cf. Eqs. 4&5), Cl ind = individual clearance rate (cf. Eq. 1), Cl pop = population clearance rate (cf. Eq. 2), t 1/2 = estimated half-life of copepods (cf. Eq. 3). Mean ± SD are shown. Cruise #/ Date D (ind. m 3 ) d (mm) DW (mg) Cl ind (l d 1 ) Cl pop (m 3 m 3 d 1 ) t 1/2 (d) 2014 #1, 9 Sep 2.1 ± ± #2, 25 Sep 4.6 ± ± #3, 8 Oct 0.7 ± ± #4, 22 Oct 3.6 ± ± #5, 6 Nov 2.8 ± ± #6, 18 Nov ± #7, 7 May 15.7 ± ± #8, 19 May 49.9 ± ± #9, 18 Jun 60.7 ± ± #10, 30 Jun 38.2 ± ± #11, 16 Jul 4.2 ± ± #12, 29 Jul 62.8 ± ± #13, 12 Aug 40.6 ± ± #14, 25 Aug 2.8 ± ± #15, 15 Sep 43.9 ± ± #16, 24 Sep 72.3 ± ± #17, 5 Oct 13.6 ± ± #18, 21 Oct 2.8 ± ± #19, 9 Nov 6.2 ± ± #20, 26 Nov 31.2 ± ±

6 238 F. LÜSKOW, H. U. RIISGÅRD Fig. 4. Aurelia aurita. Growth and degrowth of jellyfish in controlled laboratory feeding and starvation experiments (Series #1). A: Mean (± SD) umbrella diameter; B: Individual dry weight (cf. Eq. 5) in growth ( ) and degrowth ( ) periods along with exponential regression line and equation for degrowth. The dotted line indicates start of starvation period. Data from Table III. Table III. Aurelia aurita. Jellyfish in controlled laboratory feeding experiment (Series #1, Day 0 to 39) and subsequent starvation experiment (Day 40 to 78), d = mean (± SD) umbrella diameter, DW = individual dry weight (cf. Eq. 5), µ = weightspecific growth rate (cf. Eq. 6). Line indicates the start of the starvation phase. Day d (mm) DW (mg) µ (% d 1 ) ± ± ± ± ± ± ± ± ± ± ± ± Fig. 5. Aurelia aurita. Growth and degrowth of jellyfish in controlled laboratory feeding and starvation experiments (Series #2). A: Mean (± SD) umbrella diameter; B: Individual dry weight (cf. Eqs. 4&5) in growth ( ) and degrowth ( ) periods along with exponential regression line and equation for degrowth. Data from Table IV. Table IV. Aurelia aurita. Jellyfish in controlled laboratory feeding experiment (Series #2, Day 0 to 77) and subsequent starvation experiment (Day 78 to 100), d = mean (± SD) umbrella diameter, DW = individual dry weight (cf. Eqs. 4&5), µ = weight-specific growth rate (cf. Eq. 6). Line indicates the start of the starvation phase. Day d (mm) DW (mg) µ (% d 1 ) ± ± ± ± ± ± ± ± ± ± ± ± ± IV). During the following 23 days starvation period the mean degrowth rate was 4.2 % d 1 (Fig. 5B). The observed degrowth rates in Kertinge Nor ( 1.2 % d 1 ; Fig. 2B) and Mariager Fjord ( 1.5 and 0.7 % d 1 in

7 JELLYFISH DEGROWTH IN DANISH WATERS and 2015, respectively; Fig. 3B) are slightly lower than observed in the laboratory starvation experiments. DISCUSSION The present study indicates that the population predation impact on zooplankton exerted by numerous small Aurelia aurita controls the maximum umbrella size as well as the degree of subsequent somatic degrowth due to food limitation in both Kertinge Nor and Mariager Fjord. These findings are in good agreement with earlier statements by Möller (1980) and Schneider & Behrends (1994) who inferred an inverse relation between the medusa abundance and the mean umbrella diameter. Riisgård et al. (2010) showed based on an 19-year long record (period 1991 to 2009) of umbrella diameter and jellyfish density in August from Kertinge Nor an inverse yearly correlation of both parameters, and Goldstein & Riisgård (2016) showed that this correlation also applies to seasonal variability. After the release of ephyrae during early spring, Kertinge Nor is characterized by extremely high population densities of small Aurelia aurita medusae, and small maximum umbrella diameters of typically 4 to 5 cm are observed, before degrowth starts in early September along with a decline in zooplankton biomass (e.g. Olesen et al. 1994, Goldstein & Riisgård 2016). The predation impact on zooplankton of A. aurita in Kertinge Nor has over the years been documented in a number of studies: in 1991, 1992 (Olesen et al. 1994), 1995 (Nielsen et al. 1997), 2009 (Riisgård et al. 2010), and more recently in 2013 and 2014 (Goldstein & Riisgård 2016), and they show that the dense jellyfish population has the potential to cause a strong decrease in zooplankton biomass during summer and autumn. Estimated short zooplankton (copepod) halflives, typically less than a few days and frequently lower as also observed in the present study (Table I) suggest strong predatory control of the zooplankton community from April to November. Thus, food scarcity seems to be the main reason for seasonal degrowth of A. aurita medusae in Kertinge Nor. Lucas & Lawes (1998) pointed out that food limitation in combination with temperature effects limiting primary and secondary production in autumn may be the main explanation of medusa degrowth in Horsea Lake and Southampton waters, which is in agreement with observed degrowth of Aurelia in French Mediterranean lagoons (Marques et al. 2015) and degrowth of A. aurita in the Gulf of Gdansk (southern Baltic Sea) after October (Brulinska et al. 2016). Fallesen et al. (2000) mentioned that A. aurita might play a pronounced role in the predation on zooplankton in Mariager Fjord, as clearly demonstrated in the present study which indicates that this fjord shares important similarities with Kertinge Nor, i.e. high population densities of small jellyfish, maximum umbrella diameter controlled by the density of jellyfish (Fig. 3, Table II), short half-lives (Table II), and pronounced degrowth during late summer and autumn with interannual variation (Fig. 3), and further, the mean zooplankton biomass in Mariager Fjord (which was 4.6 µg C l 1 in 2006, Fig. S3, Supplementary Material) and may be compared to approximately 5 µg C l 1 in Kertinge Nor (Olesen et al. 1994). For comparison, it can be mentioned that Zervoudaki et al. (2009) analysed the zooplankton concentrations for a number of locations in Danish waters and in the western Baltic Sea, and the summer zooplankton concentrations for the coastal regions were 75 to 200 µg C l 1 whereas the winter concentrations were between 10 to 20 µg C l 1. Compared to the dense populations of small medusae in Kertinge Nor and Mariager Fjord, Aurelia spp. in other temperate regions show much larger maximum umbrella diameters, reflecting higher growth due to their generally low abundance as typical for larger A. aurita medusae in e.g. Kiel Bight, Germany (Möller 1980, Schneider 1989), western Wadden Sea (van der Veer & Oorthuysen 1985), North Sea (Hay et al. 1990, Barz & Hirche 2007), Inland Sea of Japan and Tokyo Bay (Ishii & Tanaka 2001, Uye & Shimauchi 2005), eastern Pacific (Albert & Walsh 2014), Black Sea (Mutlu 2001), northwestern Mediterranean Sea (Marques et al. 2015), and Great Belt, Denmark (Goldstein & Riisgård 2016). However, the seasonality in growth and degrowth patterns, and the less than one year life span, as indicated here for jellyfish in Kertinge Nor and Mariager Fjord, may apply for jellyfish in other temperate waters due to shortage of zooplankton prey during autumn, and therefore it seems likely that availability of prey organisms determines the life span of A. aurita medusae in these waters. When jellyfish in the present study were brought into the laboratory during the degrowth period and fed with realistically (naturally) low zooplankton concentrations, this resulted in positive somatic growth (Figs 4, 5; Tables III, IV) and long-term survival for considerably longer than jellyfish survived in their natural environment. However, the used diet was exclusively based on Artemia salina nauplii and probably did not led to maximum growth (not aimed for) because it has been shown by Graham & Kroutil (2001) that mixed diets covering a variety of essential nutrients typically result in greatest growth response. In other waters, such as the Roscoe Bay (west coast of Canada) A. labiata is not restricted to an annual life span and occurs far within the next season (Albert & Walsh 2014). Likewise, Hamner & Jensen (1974) stated that A. aurita is not an obligatory annual species, in agreement with observations from Japanese coastal waters (Omori et al. 1995, Makabe et al. 2012), and the tropical Jellyfish Lake in Palau (Hamner et al. 1982). The present work has further added to our understanding of the reason for degrowth of medusa and why Aurelia aurita medusae disappear during winter in temperate waters, although this phenomenon is still poorly described and documented for less dense populations of large umbrella-sized jellyfish in the open sea.

8 240 F. LÜSKOW, H. U. RIISGÅRD Ack n o w l e d g e m e n t s. Thanks to skippers S Bråten and JC Thorhauge aboard the R/B Maria, and to H Munk Sørensen and ML W Maarup at the Danish Nature Agency, Ministry of Environment and Food, for suppling environmental data, and to B Lüskow for making the map and the technical drawing. Thanks are due to P Andrup for skillfully conducting the first series of laboratory feeding and growth experiments, and to J Goldstein for practical assistance, constructive comments on the manuscript and help with statistics. Thanks for technical assistance go to CS Gros, S Delaunay, A Charitonidou, JB Wiersma, B Tang, D Prömper, ST Ortiz, J Baltzer, NC W Møller, L Kumala, MT Pedersen, A Maxwell, J Larsson, KD Piroz, and A Corregidor and to the anonymous reviewer for constructive comments. This study was financially supported by a grant (HUR) from the Danish Agency for Science, Technology and Innovation (Det Frie Forskningsråd, DFF ). 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10 242 F. LÜSKOW, H. U. RIISGÅRD Supplementary Material POPULATION PREDATION IMPACT OF JELLYFISH (AURELIA AURITA) CONTROLS THE MAXIMUM UMBRELLA SIZE AND SOMATIC DEGROWTH IN TEMPERATE DANISH WATERS (KERTINGE NOR AND MARIAGER FJORD) F. LÜSKOW, H. U. RIISGÅRD* Marine Biological Research Centre, University of Southern Denmark, Hindsholmvej 11, 5300 Kerteminde, Denmark * Corresponding author: hur@biology.sdu.dk Prey concentration used in jellyfish feeding and growth experiments The energy (or carbon) budget of a jellyfish can be expressed as P = I R E = A R, where P = production, I = ingestion, R = respiration, E = excretion (faeces and urine), and A = assimilated food. The budget can also be written as P = (F C AE) R, where F = clearance rate, C = prey concentration, and AE = assimilation efficiency. The following equations were used to estimate dry weight (DW, mg) from umbrella diameter (d, mm) of Aurelia aurita medusae ( 10 mm): DW = d 2.82 (Olesen et al. 1994); 1 mg dry weight (medusae) = 2.24 J = 50 μg C (Schneider 1988); 1 μl O 2 = 0.46 μg C (Uye & Shimauchi 2005); 1 Artemia (3-day-old) = 1.16 μg C (Szyper 1989). The filtration rate (F, l d 1 ) as a function of umbrella diameter (d, mm) using 3 d old Artemia salina as prey organism is given by the equation (Møller & Riisgård 2007): F Ar,3 = 0.032d 2.3. The respiration (R, µl d 1 ) as a function of body weight (DW, mg) is given by the equation (Frandsen & Riisgård 1997): R = 10.89DW The number of Artemia (3 d old) to be added to cover the maintenance of for example a 56.9 mm umbrella diameter Aurelia aurita (initial size in Series #1) so that it does not starve and lose weight (P = 0) was calculated using the following data: d = 56.9 mm and therefore DW = ( mm 2.82 =) mg R = ( mg 0.86 = µl O 2 d 1 = 828.5µl O 2 d µg C =) µg C d 1 F Ar,3 = ( mm 2.3 =) l d 1 P = 0 or P = (F C AE) R = 0, or F C AE = R. If AE = 90 % then: n 0.9 = 381.1, or n = ( ) = 1.0 Artemia l 1. In the present feeding and growth experiments, a prey concentration 5 times above the maintenance (i.e. 5 Artemia l 1, equivalent 5.8 µg C l 1 ) was prejudged to result in fast growth and therefore used in both Series #1 and Series #2. The number of Artemia needed to be added to maintain a steady-state prey concentration of 5 Artemia l 1 was calculated as equivalent to the predation rate = F Ar,3 n 5. Thus in the case of the initial period of Series #1 with 10 jellyfish of diameter 56.9 mm, the addition rate of prey organisms was adjusted to l d 1 10 ind. 5 (24 60) = 12 Artemia min -1. The mean number of Artemia in the culture was counted from Lugol preserved subsamples (1 ml, n = 3) under a stereo microscope and used to calculate the concentration of Artemia in the stock-flask from which a dosing pump with fixed rate continuously added the decided number of Artemia to the jellyfish feeding tank (Fig. S1). In Fig. S2 an example of clearance experiment in the jellyfish tank after switching off the dosing pump is depicted which were conducted regularly to check the jellyfish filtration activity. Preliminary cruise and zooplankton in Mariager Fjord On 25 September 2014 a cruise spanning 5 stations was conducted in the inner part of Mariager Fjord. Positions, maximal water depths, mean (± SD) density (D, ind. m 3 ), Fig. S1. Schematic drawing of setup for feeding and growth experiments with Aurelia aurita. A smooth circulation was established by in- and outlets on the left and right side of the tank. Via a dosing pump 3 day-old Artemia were continuously added into the jellyfish tank while a second peristaltic pump ensured a constant volume of water in the tank.

11 JELLYFISH DEGROWTH IN DANISH WATERS 243 Fig. S2. Aurelia aurita. Semi-ln plot of Artemia concentration (C, ind. ml 1 ) as a function of time in a clearance experiment with 10 jellyfish. Linear regression line along with its equation is shown. Slope of regression line volume of tank = population filtration rate. Fig. S3. Zooplankton biomass in Mariager Fjord during The mean (± SD) biomass was 4.6 ± 5.8 µg C l 1. Plot based on data from the Danish Nature Agency, Ministry of Environment and Food. and mean (± SD) umbrella diameter (d, mm) of jellyfish are shown in Table SI. It appears from the survey data that the jellyfish were evenly distributed throughout the inner Mariager Fjord, and therefore it was decided to follow the jellyfish population by monitoring only one location (i.e. Station #1, Dybet).Data on biomass and composition of zooplankton in Mariager Fjord were obtained from the Danish Nature Agency, Ministry of Environment and Food. The most recent data on zooplankton are from 2006, but assuming no pronounced changes in recent years these data may apply to the present study. In 2006 zooplankton was collected by integrated vertical tows from 10 m depth to the surface using a WP2 plankton net (mesh size 60 µm, mouth area 0.28 m 2 ) equipped with a flowmeter. All samples were preserved in acidic 1 % Lugol s solution (6 % iodine-potassium, 4 % iodine solution) for further analysis in the laboratory. Zooplankton composition and abundance was determined under a dissecting microscope according to the official guidelines from the national monitoring programme of water and nature (NOVANA, Novana 2011). The composition and biomass of zooplankton over the year is shown in Fig. S3. REFERENCES Frandsen K, Riisgård HU Size dependent respiration and growth of jellyfish (Aurelia aurita). Sarsia 82: Møller LF, Riisgård HU Feeding, bioenergetics and growth in the common jellyfish Aurelia aurita and two hydromedusae, Sarsia tubulosa and Aequorea vitrina. Mar Ecol Prog Ser 346: Novana Tekniske Anvisninger NOVANA (technical instructions for environmental monitoring, in Danish): saerligt-interesserede/fagdatacentre/fdcmarintny/ta / Olesen NJ, Frandsen K, Riisgård HU Population dynamics, growth and energetics of jellyfish Aurelia aurita in a shallow fjord. Mar Ecol Prog Ser 105: Schneider G Chemische Zusammensetzung und Biomasseparameter der Ohrenqualle Aurelia aurita. Helgol Wiss Meeresunters 42: Szyper JP Nutritional depletion of the aquaculture feed organisms Euterpina acutifrons, Artemia sp. and Brachionus plicatilis during starvation. J World Aquacult Soc 20(3): Uye SI, Shimauchi H Population biomass, feeding, respiration and growth rates, and carbon budget of the scyphomedusa Aurelia aurita in the Inland Sea of Japan. J Plankton Res 27(3): Table SI. Aurelia aurita. Jellyfish collected in Mariager Fjord on 25 September 2014: D = population density, d = umbrella diameter. Mean ± SD are shown and based on each 150 specimens. Station #/Name Coordinates Water depth (m) D (ind. m 3 ) d (mm) #1, Dybet N, E ± ± 33.5 #2, Sandhagen N, E ± ± 35.2 #3, Røkkedal N, E ± ± 31.3 #4, Gl. Spølledning N, E ± ± 31.7 #5, Hobro Havn N, E ± ± 32.1