Spatial heterogeneity of Pelagia noctiluca ephyrae linked to water masses in the Western Mediterranean

Abstract

Pelagia noctiluca is the most common jellyfish in the Western Mediterranean Sea, living in oceanic waters with a holoplanktonic lifecycle. Frequent outbreaks have been well documented in coastal areas, yet little is known about their offshore distribution. In this study we address the relationship between oceanographic structures and the distribution of P. noctiluca ephyrae along the central continental slope of the Western Mediterranean, covering a wide latitudinal gradient, during July-August 2016. The region is characterized by a rich and complex mesoscale surface circulation driven by the inflow of Atlantic Water into the Western Mediterranean through the Strait of Gibraltar. The results revealed a high variability in the ephyrae spatial paterns related with different water masses and the resulting mesoscale hydrographic features. Their horizontal distribution showed a clear latitudinal gradient with high abundances in the south, associated with recent Atlantic Water, and low abundances or absence in the north, in coincidence with the old Atlantic Water transported by the Northern Current. Ephyrae showed diel vertical migrations of short-extent in the first 50 m, with a wide distribution above the thermocline and the Deep Chlorophyll Maximum during daytime, being more concentrated towards the surface at night. The results suggest the population connectivity of P. noctiluca between the Atlantic and the Mediterranean. In that case, the abundance variability of the species in the Mediterranean could be modulated by its entrance associated with the inflow of Atlantic Water through the Strait of Gibraltar.

Fig 1. Study area in the Western Mediterranean.

Hydrographic (black and red crosses) and plankton (red crosses) stations during the survey. The schematic lines indicate the main currents, Northern Current (NC) and Balearic Current (BC), and recent AW inflow through the Eivissa and Mallorca Channels. Coastline is from Natural Earth [41] and Instituto Hidrográfico de la Marina [42], and isobaths (every 400 m depth) are from European Marine Observation and Data Network (EMODnet, [43]).

Fig 2. TS diagram of the water layer from 0 to 100 m depth.

Recent Atlantic Water (blue dots), south of 39.5°N; old Atlantic Water (green dots), north of 39.5°N.

Fig 3

Horizontal distribution of a) salinity and b) temperature at 10 m depth from CTD data. Dots show the sampling stations. Images were created using ODV [44].

Fig 4

Vertical distribution of a) salinity, b) temperature and c) fluorescence in the upper 100 m depth from CTD data, along the continental slope. Horizontal axis indicates latitude range. Vertical dotted lines represent 1 m binned CTD profiles data.

Fig 5. Mean velocity field for August 2016 at 10 m depth from CMEMS reanalysis [45].

The current velocity is depicted through the colour scale and arrows show the current direction. Coastline is from Natural Earth [41] and Instituto Hidrográfico de la Marina [42].

Fig 6

Vertical distributions of ephyrae in night time (dark grey bars) and daytime (light grey bars) and temperature (red), salinity (blue), and fluorescence (green). Data of six stations representative of the hydrographic conditions at the a) north (41.0°N, 2.0°E), b) centre (39.5°N, 0.5°E) and c) south (38.0°N, 0.2°E) of the sampling area. Ephyrae relative abundance by station. Values in parenthesis indicate ephyrae abundance 100 m^-3.

Fig 7

Horizontal distribution of P. noctiluca ephyrae overlaid on the mean a) salinity and b) temperature at 10 m depth for the sampling period from CMEMS reanalysis [45]. Coastline is from Instituto Hidrográfico de la Marina [52] and isobaths (every 400 m) are from EMODnet [43].

Fig 8. Depth-integrated ephyrae abundances in relation to mean surface (5–10 m depth) salinity and temperature at plankton stations.