Shelf-slope fronts separating low-salinity coastal waters from high-salinity open-sea waters are common along continental shelves (Wang et al. 1988, Houghton 1997). Physical and biological coupling in these frontal zones shows strong spatio-temporal variability as a result of hydrographic complexity and the activity of the organisms (Mackas et al. 1985, Sournia 1994). In general, shelf-slope fronts are highly productive due to the accumulation and active growth of microalgae and zooplankters (Sabatés et al. 1989, Fernández et al. 1993, Mann and Lazier 2006). These phenomena determine the distributions and abundance of many groups of zooplankton (e.g. Kahru et al. 1984, Nishikawa et al. 1995, Sabatés and Olivar 1996).

Gelatinous zooplankton are abundant in pelagic communities, playing an important role in food-web dynamics due to their great trophic impacts and rapid population growth, which sometimes results in seasonal blooms (Graham et al. 2001, Pagès et al. 2001). Appropriately classified as plankton, gelatinous organisms have limited horizontal mobility, so their abundance and distribution patterns depend on hydrodynamic features such as gyres, clines and fronts. However, explicit evidence for this bio-physical coupling is scarce (e.g. Pagès and Gili 1992, Graham et al. 2001, Pavez et al. 2010).

Off the Catalan coast (NW Mediterranean), the shelf-slope density front is a permanent structure defined by strong salinity gradients, separating low-salinity shelf waters from the more saline waters offshore (Font et al. 1988, Alvarez et al. 1996). It is present in the upper 300 to 400 m of the water column and usually intersects the surface over the 1000 m isobath. Associated with the front is the Northern Current flowing southwestward following the continental slope at 20 to 30 cm s^–l (Font et al. 1995). In spring, northern Catalan coastal waters experience strong spatial and temporal variability due to the large inputs of continental runoff, mainly from the Rhône River in the northern Gulf of Lions (Masó and Tintoré 1991, Sabatés et al. 2007). These relatively low-salinity waters, advected by the Northern Current along the shelf break, increase the mesoscale activity at the shelf-slope front, generating oscillations and eddies (Alvarez et al. 1996, Flexas et al. 2002). The continental shelf in the study area (Fig. 1) is relatively narrow, with a submarine canyon whose head is close to the coast.

Previous studies in the area have analysed the role of the front and the associated current in primary and secondary production (Estrada 1991, Estrada et al. 1999, Alcaraz et al. 2007), and in zooplankton and ichthyoplankton distribution (Sabatés et al. 1989, Sabatés and Olivar 1996, Sabatés et al. 2004). Recently, the importance of the front in the distributions of medusan species forming blooms, such as Pelagia noctiluca (Forsskål, 1775), has been reported (Sabatés et al. 2010). However, little is known about the influence of the shelf-slope front on the most abundant planktonic cnidarians, especially at short timescales.

The planktonic cnidarian community along the Catalan coast in spring is dominated by a few species. Siphonophorae constitute the bulk of the community, and the calycophoran Muggiaea atlantica Cunningham, 1892 is by far the most abundant and representative species. Among Hydromedusae, Aglaura hemistoma Péron and Lesueur, 1810, is the most abundant and widespread. Both species are neritic and epipelagic, accounting in the area for up to 95% of the planktonic cnidarian community in spring (Gili et al. 1987a, b, 1988). Although these two species are present in the Mediterranean all year around (Bouillon et al. 2004), the highest abundances of M. atlantica in the northwestern basin have been recorded from April to June (Gili et al. 1987b, 1988, Licandro et al. 2012), and peaks of Aglaura hemistoma occur between June and September (Gili et al. 1987b, 1988, Licandro and Ibañez 2000). Both species are particularly abundant in the first 50 m of the water column but can occur down to 200 m (Gili et al. 1987a, b, Batistić et al. 2004). The value of knowing the abundance and distribution patterns of planktonic cnidarians derives from their predation on most other zooplankton (e.g. Biggs 1977, Purcell 1997, Colin et al. 2005), affecting the structure and dynamics of the whole planktonic community.

Our goal was to investigate how the variability of hydrodynamic structures determines the mesoscale distributions of planktonic cnidarians. Our approach was to study the coupling between short-term variability in the location of the shelf-slope front and the distributions of M. atlantica and A. hemistoma. To achieve this aim, we analysed the changes in abundance and spatial distribution of both species during three cross-frontal surveys carried out at approximately 10-day intervals.

The study area is located off the northern Catalan coast, NW Mediterranean (Fig. 1). Three oceanographic cruises were carried out from mid-May to late June 1992, at approximately 10-day intervals (13-21 May, 2-9 June and 18-25 June). On each survey, 43-44 stations were sampled for environmental and biological parameters. Stations were located approximately 8.5 km apart, and distributed along seven transects perpendicular to the shoreline, from near the coast to beyond the shelf-slope front. An additional transect in the northernmost part of the area (grey dots in Fig. 1) was conducted for environmental measurements only. Vertical profiles of basic hydrographic variables (temperature, salinity and fluorescence) were obtained at each station using a Mark-III Neil Brown CTD probe equipped with a Sea Tech fluorometer. Maps of the horizontal distribution of each environmental parameter (at 10 m depth) were generated by gvSIG (OADE-2010) and ArcGIS 10.2 software, applying the spline interpolations with a cell size of 200 m (see Fig. 2). The Catalano-Balearic Sea bathymetric chart (2005) was used to represent the bathymetry at 100 m intervals.

Zooplankton samples were collected using a bongo net of 60 cm mouth diameter and 300 µm mesh. Oblique hauls were performed, integrating the water column from a maximum depth of 200 m (or 5 m above the bottom in stations shallower than 200 m) to the surface. Samples were preserved immediately after collection in a 5% solution of formaldehyde in seawater buffered with borax. The volume of filtered water was estimated by means of a flowmeter placed in the centre of the net mouth.

In the laboratory, after the cruises took place, the siphonophore M. atlantica (polygastric stage) and the hydromedusa A. hemistoma were identified and counted under a stereomicroscope. Counts were standardized to number of individuals per 1000 m³. Recently, to complete the study with the rest of the cnidarian community, analysis of those same samples were carried out, however we found that the morphological conditions of the individuals had impoverished so much that the taxonomical identifications were not possible. The exceptional oceanographic conditions in which the cruises were performed and the ecological importance of these two species encouraged us to proceed with the study presented here.

The temporal scale of the sampling allowed us to identify the short-term spatial variability of the shelf-slope density front and the responses of M. atlantica and A. hemistoma species to associated environmental changes. The shifting position of the front is characteristic of the spring period off the northern Catalan coast, and results from the advection of low-salinity waters by the Northern Current (Alvarez et al. 1996, Masó et al. 1998, Sabatés et al. 2007). The low-salinity waters originating in the Gulf of Lions, mainly due to the River Rhône outflow, help strengthen the gradient of the shelf-slope density front. The temporal scale at which the frontal system oscillates has been reported to exert a decisive influence on processes affecting the concentration and dispersal of zooplankton and fish larvae (Sabatés and Olivar 1996, Masó et al. 1998, Sabatés et al. 2004). The gelatinous zooplankton followed a similar trend, varying in their spatial distributions at short time scales. The two cnidarian species studied are epipelagic, mainly occurring in the surface layer between 0 and 50 m (Gili et al. 1987a, b), and holoplanktonic, making them particularly susceptible to surface-water dynamics (Mackie et al. 1987, Blackett et al. 2014).

The abundance values recorded for both species are in accordance with previous reports in the area during the same season (Gili 1986, Gili et al. 1987b, Sabatés et al. 2010). The higher mean abundance of M. atlantica, compared with A. hemistoma, is usual in the NW Mediterranean (Gili et al. 1988, Licandro and Ibañez 2000, Sabatés et al. 2010), and it has been observed since M. atlantica replaced the formerly dominant, congeneric species Muggiaea kochii (Will, 1844) (Kršinić and Njire 2001, Batistić et al. 2007, Licandro et al. 2012). However, the hydromedusa can display peaks of greater abundance at some periods of the year (Gili et al. 1987b, Licandro and Ibañez 2000). Both species varied considerably in their mean abundance between surveys. For M. atlantica the temporal abundance sequence agrees with the seasonal trend previously observed in the NW Mediterranean. The highest values are generally recorded from April to June, significantly decreasing at the end of June and in July (Gili et al. 1987b, 1988, Licandro and Ibañez 2000). The highest abundance of A. hemistoma was observed in May, and it decreased markedly in June. That pattern contrasts with previous observations of the highest seasonal densities for it in June and July, after much lower values in May (Gili et al. 1987b, 1988, Licandro and Ibañez 2000). The inflow of low-salinity waters detected in early June inshore of the front could have negatively affected the abundance of A. hemistoma (Fig. 3: B2, Table 2). However, as this species has been observed inhabiting areas of similar and lower salinity (e.g.: Gili et al. 1988, Nagata et al. 2014), factors other than salinity per se probably affected its abundance.

Muggiaea atlantica and A. hemistoma displayed high spatio-temporal variability in the brief study period, apparently driven by the rapid onshore-offshore displacements of the shelf-slope front. Both species occurred predominantly inshore of the front, with significantly higher abundances there than on the offshore side (Figs 3 and 4). Previous studies in the region have also reported the highest concentration of M. atlantica and A. hemistoma on the inshore side of the front (Sabatés et al. 2010). The front acts as a natural barrier, limiting the distribution seaward of both species; this phenomenon has also been documented for larvae of coastal fish in the study area, with the front preventing their displacement to the open sea (Sabatés and Olivar 1996, Sabatés et al. 2004). In the case of the jellyfish, in addition to the barrier effect of the density front, we must consider that since they are mainly water with the same ionic concentration as the surrounding seawater, they tend to remain in waters of similar salinity (Graham et al. 2001). The accumulation over the shelf, very strong for M. atlantica when the front was close to the coast, could lead to high predation pressure on their planktonic prey; this species is known to be an effective predator, particularly on copepods (Purcell 1982), and when siphonophores are very abundant they can significantly affect planktonic populations (Purcell 1981, Purcell and Kremer 1983). Other dominant jellyfish species in the area, the siphonophores Lensia subtilis (Chun, 1886) and Chelophyes appendiculata (Eschscholtz, 1829) and the hydromedusae Rhopalonema velatum Gegenbaur, 1857 and Solmundella bitentaculata (Quoy and Gaimard, 1833), have also been reported to display similar patterns, with maximum abundances on the coastal side of the density front (Sabatés et al. 2010). However, oceanic species such as the siphonophore Lensia conoidea (Keferstein and Ehlers, 1860) and the scyphomedusa Pelagia notiluca were more abundant in the frontal area and offshore (Sabatés et al. 2010). Thus, the front seems to exert a barrier effect for both neritic and oceanic species, limiting their offshore and inshore displacement, respectively.

Studies conducted in other geographical areas have also shown the role of fronts shaping the distributions of gelatinous zooplankton. Analogies are found, for instance, in the salinity-driven mesoscale front in the Southern California Bight, where most gelatinous zooplankton organisms were located on the inshore side of the front (Luo et al. 2014). Pavez et al. (2010) also found the highest abundance of gelatinous zooplankton (hydromedusae, siphonophores and ctenophores) at the neritic inshore stations of a density front off central Chile. However, M. atlantica, evenly distributed over the shelf and slope, did not prove to be influenced by the position of the front. By contrast, the seasonal thermohaline front in the southern Benguela Region delimited the distribution of most species to the offshore side of the front (Pagès and Gili 1992). High abundances of A. hemistoma and M. atlantica were detected over the edge of the continental shelf, offshore of the front, in relation to the intrusion of Agulhas water, whose input increases the gelatinous zooplankton density and diversity. In general, the different hydrodynamic variability associated with each frontal system is a key factor explaining the spatial heterogeneity of plankton distribution (Le Fèvre 1986).

The measured environmental parameters appeared to have important effects on the distributions and abundance of the two species. Bottom depth and salinity were the variables most closely related to the distribution patterns (Tables 3 and 4). When the front was located away from the coast, M. atlantica and A. hemistoma abundances gradually decreased with bottom depth. This trend has already been documented by other studies on the area (Gili et al. 1987a, b, 1988) and is in agreement with the neritic character of both species. Abundances also declined at the higher salinity values (see Figs 3, 4 and 5) characterizing waters on the seaward side of the front. In particular, when the front occurred over the shelf in early June, salinity was the variable most strongly related to the spatial distributions of both species (Tables 3 and 4). Salinity has previously been reported as a determinant factor for the distributions and abundance of gelatinous zooplankton at various locations in the NW Mediterranean (Gili et al. 1988, Licandro et al. 2012) and the North Atlantic (Blackett et al. 2014, Greer et al. 2015).

High fluorescence values were clearly associated with the presence of low-salinity waters. These waters come from the Rhône River runoff, advected by the Northern Current along the Catalan coast, and are highly productive at surface; the offshore location of these low-salinity waters is variable due to the horizontal oscillation of the shelf-slope front (Sabatés et al. 2007). Abundance of both species in relation to fluorescence showed different trends (almost opposite) in mid-May to those of the two June cruises (Figs 4 and 5). In mid-May high fluorescence values were located offshore, coinciding with low abundance values for M. atlantica and A. hemistoma. In June high fluorescence values were detected on the inshore side of the front at stations where the two species were particularly abundant (Figs 2 and 3). This suggests that productive waters per se had no direct effect on the abundance of either species, depth being in fact the responsible variable.

No clear trend in the distribution of the species was detected regarding temperature. Although both species showed lower abundance in warmer waters, the narrow temperature variability within each survey and the short seasonal period we covered prevent any temperature pattern or preference from being detected. Overall, our results show the key role of the position of the front, rather than values of the measured environmental parameters per se, as an explanation for the abundance and distributions of M. atlantica and A. hemistoma. In addition, the topography of the area, with the presence of a submarine canyon, seems to have affected the observed distribution patterns. The presence of the Palamós submarine canyon has been reported to modify the circulation in the area, inducing a shelfward deflection on the upstream side of the canyon and an offshore flow on the downstream side (Alvarez et al. 1996, Jordi et al. 2005). In relation to these shelf-slope exchanges, high abundances of both species were observed on the canyon edges, particularly when the shelf-slope front intersected the canyon mouth (mid-May and late June surveys).

In summary, the shelf-slope front was the main factor controlling the abundance and distribution of the two most abundant and representative species of planktonic cnidarians in the NW Mediterranean, the siphonophore M. atlantica and the hydromedusa, A. hemistoma. A high degree of coupling was observed between the short timescale variability of the front’s location and the spatio-temporal distributions of the species. The front seemed to act as a barrier preventing their offshore displacement, as was reflected by the fact that the bottom depth and salinity among the analysed variables best explained the distributions and abundances. The strong hydrographic variability associated with shelf-slope fronts largely determines the seasonal and interannual variability of gelatinous zooplankton and their predation impacts on the planktonic community in this region.