New contributions to the ecology of gelatinous zooplankton in the western mediterraneanabundance, distribution and diversity

Resumen

Gelatinous zooplankton has been studied in the Western Mediterranean for a long time, however, its spatial structure has been mainly addressed in coastal areas, during spring and summer periods. Hence, information regarding its large-scale distribution in the open sea, the role of hydrodynamic factors on the distribution of poorly known taxa and the characterization of gelatinous communities in the winter season remain unknown. In this context, the present Ph.D. thesis aims to improve the knowledge on the ecology of gelatinous zooplankton in the Western Mediterranean, focusing on the study of less known areas, species and seasonal periods. The first chapter addresses the spatial heterogeneity of Pelagia noctiluca ephyrae along a wide latitudinal gradient in the Western Mediterranean. The second chapter examines the oceanographic conditions and colony size shaping the mesoscale distribution of Pyrosoma atlanticum in the NW Mediterranean. Finally, the third chapter analyses the whole planktonic cnidarian community during winter in the NW Mediterranean. Ephyrae of P. noctiluca showed a heterogeneous spatial distribution in open waters of the Western Mediterranean linked to 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 the recent Atlantic Water, and low abundances or even absence in the north, in coincidence with the old Atlantic Water transported by the Northern Current. These results suggested the occurrence of P. noctiluca population connectivity between the Atlantic and the Mediterranean. Ephyrae were present in the upper 50 m of the water column, where they performed short-extent diel vertical migrations. During the day, ephyrae presented a wide distribution well above and within the thermocline, far from the Deep Chlorophyll Maximum, while during the night they were concentrated near the surface, in coincidence with their potential prey. The mesoscale and vertical distribution of P. atlanticum, a species scarcely studied so far in the Mediterranean, were shaped by oceanographic and biological features, as well as by the ontogenetic stage of the colonies. Large colonies (¿ 7 mm long) were found on the slope, all along the shelf-slope density front, which probably aggregates them and prevents their dispersion towards the open sea. Small (< 4 mm) and medium colonies (4¿6.9 mm) extended their distribution over the shelf because of front instabilities, being practically absent in the cold, low salinity coastal waters. The fine-scale analysis of their vertical distribution allowed to detect the onset of a migratory behaviour when colonies were 4¿6.9 mm long. At night, colonies of all sizes remained close to the surface, where chlorophyll-a levels were high, whereas during the day medium and large colonies migrated to deeper layers, with larger colonies reaching greater depths. This migratory behaviour might contribute to the transport of carbon to deeper layers of the water column. The species assemblages and the spatial characterization of the whole cnidarian community (Siphonophorae, Hydromedusae and Scyphomedusae) were studied in winter 2017 and 2018. High species richness was found in 2017 in relation to an advancement of the spring conditions and the subsequent phytoplankton bloom, favouring a mixture of winter and spring species. The ¿typical¿ winter oceanographic conditions in 2018 might have allowed the development of the winter species populations resulting in a lower number of species but a higher abundance of cnidarians. The most abundant species were Lensia subtilis, Muggiaea kochii, Chelophyes appendiculata, Abylopsis tetragona (eudoxid stage), Aglaura hemistoma and Velella velella larvae. In both winters, the species assemblages presented a coastal-offshore ordination resulting from the combined effect of environmental variables, particularly bathymetry, and oceanographic features, such as water masses and the shelf-slope density front. This ordination was only disrupted over the submarine canyons where coastal and offshore cnidarian communities coexisted. Overall, this Ph.D. thesis provides novel information on the ecology of gelatinous zooplankton in the Western Mediterranean, but at the same time points to other lines of research that still need to be investigated in order to obtain a global knowledge of these organisms in this temperate sea.