Marine Pollution Bulletin xxx (2013) xxx–xxx Contents lists available at SciVerse ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul Modeling jellyfish Pelagia noctiluca transport and stranding in the Ligurian Sea ⇑ L. Berline a,b,c, , B. Zakardjian a, A. Molcard a, Y. Ourmières a, K. Guihou a a Université du Sud Toulon-Var, Aix-Marseille Université, CNRS/INSU, IRD, Mediterranean Institute of Oceanography (MIO), UM 110, 83957 La Garde Cedex, France b UPMC Univ Paris 06, UMR 7093, LOV, Observatoire Océanologique de Villefranche, 06234 Villefranche-sur-Mer, France c CNRS, UMR 7093, LOV, Observatoire Océanologique de Villefranche, 06234 Villefranche-sur-Mer, France article info abstract Keywords: Jellyfish blooms are generally attributed to a biological response to the environment, neglecting the role Medusae of transport patterns in redistributing existing populations. Here, we use high-resolution (1.25 km) ocean Lagrangian model modeling to examine the role of transport in the onshore arrival and abundance of the pelagic stinging Connectivity jellyfish Pelagia noctiluca on the Ligurian Sea coast. Jellyfish are modeled as Lagrangian particles with a Beaching 0–300-m diel vertical migration typical of P. noctiluca. Over the course of a year, onshore arrivals are Mediterranean not restricted to the summer. Arrivals are concentrated at capes, but abundance can reach maxima in Northern current Wind bays and in the lee of capes. Two factors impact jellyfish arrivals at the coast: the position of the Northern Current and the wind. A comparison of summer 2006 and available onshore jellyfish observations sug- gests a correct capture of the main stranding events by the model. These results have implications for understanding long-term fluctuations. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction throughout its life cycle, i.e., there is no benthic polyp phase. The presence of Pelagia in nearshore waters is episodic. Goy et al., Among zooplankton groups, jellyfish are one of the best known (1989), analyzing a 112-year-long time series of Pelagia presence by the general public because of their large size and recurrent in the Mediterranean basin, detected periods of presence and ab- stranding on beaches. Although they are generally harmless, in- sence with an apparent periodicity of 12 years (Goy et al., 1989; creased interest from the public and scientists has been motivated Kogovsek et al., 2010). The drivers of these fluctuations are not recently by at least two factors. First, recent surges in jellyfish had obvious. Correlative analyses detected dry, warm and anticyclonic important socio-economic effects. One instance is the 1980s up- late-spring conditions as favoring Pelagia presence and blooming surge of Pelagia noctiluca (Cnidaria: Scyphozoa, Forskål, 1775) in during the following summer (Goy et al., 1989). One plausible the Mediterranean Sea (CIESM, 2001; Mariottini et al., 2008), mechanism linking these weather conditions and Pelagia pullula- which significantly impacted fishing and tourism activity. Other tion is that early stages (ephyra larvae) are highly sensitive to food jellyfish blooms also disturbed fish farming and power-plant activ- and water-temperature conditions during late spring (Morand ity (Malej and Vukovic, 1984). Second, several authors hypothe- et al., 1992). Recent abundance data collected on the Ligurian Sea sized that jellyfish and, more broadly, gelatinous zooplankton, beaches from 1980 to 2008 confirmed periods of presence and ab- may have benefitted from the trophic reorganization driven by sence and identified the last period (1998–present) as peculiar, both ocean warming and removal of top predators by overfishing with long-lasting and nearly continuous abundance (Bernard (Purcell, 2005; Lynam et al., 2006; Attrill et al., 2007). et al., 2011). In the Mediterranean Sea and the Atlantic Ocean (Mariottini To interpret the onshore observations of a pelagic species such et al., 2008; Doyle et al., 2008), P. noctiluca (hereinafter Pelagia), as Pelagia, one must consider the onshore transport of organisms also known as the ‘‘mauve stinger’’, is well known for its sting, by currents. Indeed, Pelagia is passively transported by horizontal which is very unpleasant due to the numerous venomous nemat- currents, so its distribution in time and space emerges from the ocysts covering its body. Although it is found near shore, its habitat interplay between population dynamics and dispersal by currents. is in pelagic waters offshore, and this species is planktonic In the offshore Ligurian Sea, the predominant circulation pat- tern is a permanent coastal-density current named the Northern Current (hereafter, NC). The NC flows westward along the shore ⇑ Corresponding author at: Université du Sud Toulon-Var, Aix-Marseille Univer- (Millot, 1999), with a core located between 5 and 40 km from sité, CNRS/INSU, IRD, Mediterranean Institute of Oceanography (MIO), LOV, CNRS- the coast (e.g., Boucher et al., 1987; Niewiadomska et al., 2008; UPMC UMR7093 06234 Villefranche-sur-Mer, France. Tel.: +33 493 76 38 11; fax: Ourmières et al., 2011). Going seaward from the coast, three hydro- +33 493 76 38 34. graphical zones are defined: the so-called coastal or peripheral E-mail address: [email protected] (L. Berline). 0025-326X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.marpolbul.2013.02.016 Please cite this article in press as: Berline, L., et al. Modeling jellyfish Pelagia noctiluca transport and stranding in the Ligurian Sea. Mar. Pollut. Bull. (2013), http://dx.doi.org/10.1016/j.marpolbul.2013.02.016 2 L. Berline et al. / Marine Pollution Bulletin xxx (2013) xxx–xxx zone, the frontal zone where the NC core is located, and the central km resolution (Fisher et al., 2005) in 2006 and REMO with 18-km zone (Prieur, 1981). The frontal region hosts a high biological pro- resolution (Jacob et al., 2001) for years 1999–2001 (see Ourmières ductivity thanks to vertical transfers of nutrients (Boucher et al., et al. (2011) for details). The model has a 1/64° (1.25 km) hori- 1987; Niewiadomska et al., 2008). According to Morand et al., zontal resolution and 130 vertical levels, with higher resolution (1992), adult Pelagia abundance is maximal ‘‘on each side of the in the first 100 m to better represent the upper boundary layer. [surface density] front’’, while ephyra is more abundant in the Comparisons of simulations outputs with current meter observa- peripheral zone. Recent observations (Ferraris et al., 2012) report tions and altimetry products show that the NC is properly simu- a larger abundance in the NC itself. The general circulation is lated in term of position and intensity (Ourmières et al., 2011; alongshore, thus it is not favorable to cross-shore transport of jel- Guihou et al., submitted for publication). Daily averages of the lyfish; however, the NC shows meandering, especially in winter velocity fields were used. (Millot, 1999), which can transfer offshore waters to the coastal zone. Additionally, episodic wind bursts can induce intense 2.1.2. Lagrangian particle tracking cross-shore surface currents. The jellyfish is considered a particle transported only by ocean As many zooplankters, Pelagia achieves diel vertical migration, current, with no direct influence of the wind. The advection of par- staying at the surface at night and in deep water during the day. ticles is computed offline using the Ariane code (Blanke and Ray- The extent of the migration is not precisely known. During the naud, 1997), modified to implement diel vertical migration. The day, Pelagia was observed below 300 m in the Mediterranean Sea Ariane code is well suited to the Arakawa C-grid and is fast and (Franqueville, 1971), and only at 185 m in New England and reasonably accurate. Unlike most Lagrangian advection schemes, 90 m in Florida areas (Larson et al., 1991). Once in shallow waters, which compute the space step travelled by a particle during a de- Pelagia survival is strongly reduced, as physical stresses damage its fined time step, Ariane computes the time needed by a particle to gelatinous body (bottom contact, wave mixing), diel migration is leave the model grid cell containing the particle. Thus, no time step significantly disturbed and fish predation is important (Axiak and is needed. No time interpolation of the velocity field is performed. Civili, 1991). Therefore, it is reasonable to assume that no popula- We did not account for subgrid scale dispersion, as mesoscale stir- tion can become resident of shallow areas and that Pelagia individ- ring is resolved in this high-resolution configuration. At the lateral uals found onshore are recent offshore expatriates. boundaries (coasts and domain boundary), the model had free slip Thus, the cross-shore transport of jellyfish potentially results conditions. Thus, particles reaching the last ocean grid cell did not from processes that are strongly variable in time and space. These stop. The position of each particle along the trajectory is saved on processes are difficult to assess from observations alone as no file every hour. in situ or remote observation provides a continuous picture of On the vertical, the jellyfish is considered to migrate actively the circulation. High resolution ocean circulation models overcome with a diel cycle. The maximum depth of migration is 305 m, this problem, allowing for the simulation of the evolution in time which is an intermediate value among the estimates from observa- and space of currents, temperature and salinity, by solving the tions (Franqueville, 1971). The diel vertical migration pattern fol- equation of fluid motion with adequate boundary and initial condi- lows the solar cycle as follows: the jellyfish stays at 305 m depth tions. Lagrangian particle tracking using these model velocity out- during the day from 9 am to 9 pm and at 5 m depth from 11 pm puts reproduce the patterns of transport observed with real buoys to 6 am of the next day.