0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D Aurelia sp.8 abundance log (n+1) ind. m -3 Ephyrae Medusae 2012 2013 2014 2015 Fig. 3. Aurelia sp.8 abundance in Bizerte Lagoon between 2012 and 2015 Tab. 1. Interannual variability in Aurelia sp.8 dynamics in Bizerte Lagoon between 2012 and 2015 2012 2013 2014 2015 Mean abundance (± SD) (ind. m -3 ) 1.0 ± 2.1 1.2 ± 2 0.6 ± 1.2 0.35 ± 0.6 Mean bell diameter (± SD) (cm) 8.0 ± 4.6 10.1 ± 4.8 11.1 ± 4.9 10.2 ± 3.2 Max bell diameter (cm) 20 23.4 22.3 16.1 Growth rate (mm . d -1 ) ND 1.3 1.7 ND Shrinkage rate (mm. d -1 ) ND -0.5 -0.5 ND Growth onset period ND February January February Reproduction onset period ND April April ND Conclusion Acknowledgments :This work was supported by the European project MED-JELLYRISK (ENPI–CBCMED/ref: I-A/1.3/ 098) and by the cooperation program between Tunisia and France (Jeunes Equipes AIRD; JEAI–ECOBIZ). Materials & Methods Fig. 2. Sub-surface visual counting • A bimonthly sampling took place from 2012 to 2015 in Bizerte Lagoon (37°12’46.0N”, 09°50’48.0”E ; SW Mediterranean Sea) in the North of Tunisia (Fig. 1). • Sea surface temperature (SST) and salinity (SSS) were recorded using a WTW ® multi- parameter probe. • Zooplankton was sampled by vertical towing using a WP2 net (200μm mesh size), then counted under microscope (Leica ® M125). • Aurelia sp.8 abundance was determined by WP2 net trawl for ephyrae and by a non- conventional method for adults by counting the medusae swimming near the surface (-1 m) along a transect line of 4.5 km in length and 2 m wide carried out from both sides (1 m transect wide off each side) of a boat (homogeneity of medusae vertical distribution was assumed due to the vertical distribution homogeneity of ST, SS, nutrients, phyto- microzooplankton (Sakka et al. 2008 ; Mensi et al. 2010) (Fig. 2.). • At each sampling event, 30-70 individuals were sampling for bell diameter measure. Fig. 4. Aurelia sp.8 bell diameter in Bizerte Lagoon between 2012 and 2015 Main Results Summary Field observations showed a winter ephyrae strobilation, followed by a medusae stage occurrence from spring to early summer (Fig. 3). Medusae abundance (ANOVA; ddl : 29; p < 0.05) and bell diameter (ANOVA; ddl : 1077; p < 0.05) showed significant annual variability (Tab. 1). Medusae somatic growth pattern was characterized by a growth stage from late winter to mid spring (1.3 – 1.7 mm. d -1 ) followed be a degrowth stage (-0.5 mm. d -1 ) continuing until the end of medusae occurrence (Fig. 4) concurrent with the breeding period. The earlier growth onset observed in 2014 (vs 2013) seemed to be related to a higher zooplankton abundance (1767 ind.m -3 in December 2014 vs 425 ind.m -3 in December 2013). Linear regressions showed that warmer SST and higher zooplankton density affect positively the somatic growth (p < 0.05), while low salinity and higher medusae abundance decrease the population bell diameters (p < 0.05). Disentangling drivers of Aurelia sp.8 growth in South Western Mediterranean Lagoon Sonia K.M. GUEROUN 1 , Delphine BONNET 2 , Juan Carlos MOLINERO 3 , Stefano PIRAINO 4 ,Md. Néjib DALY-YAHIA 1 1 Laboratory of Aquatic Systems Biodiversity and Functioning, Bizerte Sciences Faculty, Tunisia 2 Montpellier University, France 3 Helmholtz Centre for Ocean Research Kiel (GEOMAR), Germany 4 University of Salento, Italy E-mail : sgueroun@gmail.com soniakhadijamaite.gueroun@fsb.rnu.tn 0 5 10 15 20 25 30 35 40 0 2000 4000 6000 8000 10000 12000 JFMAMJJASONDJFMAMJJASONDJFMAMJJASONDJFMAMJJASOND Bell diameter (cm) Temperature (°C) & salinity (psu) Zooplankton abundance (ind. m -3 ) 2012 2013 2014 2015 salinity temperature zooplankton Fig. 5. Environmental factors and Aurelia sp.8 bell diameter interannual variability in Bizerte Lagoon between Introduction Massive proliferations of jellyfish have ecosystem-wide implications and constitute an increasing concern in many coastal areas at a global scale. Identifying the factors leading jellyfish growth and their life expectancy is essential to understand the proliferation causes. Here we assess the influence of the main environmental factors on the pelagic population dynamics and somatic growth of the Scyphozoa Aurelia sp.8 in Bizerte Lagoon (SW Mediterranean) (Fig. 1). The obtained information represents the first stage to unravel controlling factors that determine the life expectancy of this species in temperate waters and eventually predict (via modelling) its responses under climate change and anthropogenic impact scenarios. Fig. 1. Aurelia sp.8 (A) in Bizerte Lagoon (B) © Sonia KM Gueroun • Aurelia sp.8 displayed a winter strobilation and a spring-early summer adults occurrence • Aurelia sp.8 displayed an interannual variability in density, bell diameter and somatic growth onset period related to environmental factors; • Higher SST, SSS and zooplankton positively affect the somatic growth while high medusae abundance limited the population bell diameters (explained by an intraspecific competition) A B References Mensi F, Ksouri J, Hammami W, Romdhane MS (2010) Choix du site de culture de l’algue rouge Gracilaria verrucosa (hudson) papenfuss dans la lagune de Bizerte : caractéristiques physico-chimiques de l’eau. Bull l’Institut Nat Sci Technol Mer Salambô 37:133–144 Sakka Hlaili A, Grami B, Niquil N, Gosselin M, Hamel D, Troussellier M, Hadj Mabrouk H (2008) The planktonic food web of the Bizerte lagoon (south-western Mediterranean) during summer: Spatial distribution under different anthropogenic pressures. Estuar Coast Shelf Sci 78:61–77 1 m 1 m Deepth -1 m Observater