Vertical distribution and diel migration of zooplankton and micronekton in Polcevera submarine canyon of the Ligurian mesopelagic zone (NW Mediterranean Sea)
Introduction
Marine organisms are linked to each other by complex trophic webs. Zooplankton is a key contributor to the ocean biological pump, and is expected to play a crucial role in channeling phytoplankton production through grazing and transfering of organic matter through diel vertical migration (DVM, Longhurst, 1976, Longhurst, 1985). In the mesopelagic realm, no-migratory animals remain in deep water layers, and use the energy transported from the epipelagic zone via physical or biological conveyer system (Gorsky et al., 1991, Gorsky et al., 2002, Stemmann et al., 2008, Peña et al., 2014). In daytime, several zooplankton and micronekton groups forming the Deep Scattering Layers (DSL) are located at a depth of 400–600 m (Sameoto, 1982, Olivar et al., 2012, Cartes et al., 2013), but during nighttime they split into a component that rises towards the surface and another that remains stationary. Euphausiids, pelagic shrimps and midwater fishes, in addition to siphonophores, are the main component of the DSL (Olivar et al., 2013, Battaglia et al., 2016). They are distributed worldwide (Reid et al., 1991, Nishikawa et al., 2001) and for this reason may support key biological processes in meso- and bathypelagic trophic webs (Cartes et al., 2008).
Within the micronekton, mesopelagic fish dominate the mesopelagic zone of all oceanic regions (Kloser et al., 2009, Young et al., 2015), moreover they are important components in several oceanic ecosystem models since they feed on zooplankton (Christensen et al., 2009). The typical pattern of the DVM of zooplankton consists of an ascent during dusk and descent during dawn with shallow residence at night and deep residence in daytime (Marshall, 1960, Andersen and Sardou, 1992, Timonin, 1997, Taki, 2008). Predator avoidance, food availability and metabolic capacity modify patterns of DVM in many zooplankton taxa (Buchholz et al., 1995). Bottom topography and geomorphology, and oceanographic features are important constraints for zooplankton distribution and aggregation. Often, some species aggregate near the water-bottom interface (so-called “near bottom-zooplankton”) (Cartes et al., 2010, Guglielmo et al., 2011) and close to thermohaline fronts (Lopez Garcia et al., 1994). The Mediterranean Sea has been considered oligotrophic since Jespersen (1923). However, several studies have shown occasional high primary production in limited areas (Margalef, 1985, Estrada, 1996). The Ligurian Sea is one of the productive upwelling systems of the western Mediterranean Sea and hosts several marine mammal species (Forcada et al., 1996, Panigada et al., 1999). Euphausiids and copepods dominate the mesozooplankton communities in this region and are an important trophic link between primary producers and higher trophic levels (Andersen et al., 2001a, b, 2004). The main pelagic crustacean in this region is primarily the northern krill Meganyctiphanes norvegica (Wiebe and d’Abramo, 1972, Andersen et al., 1998) that, together with squids and small fishes, are important elements in the diets of top predators including fin whales (Panigada et al., 1999).
Micronekton is a key faunal group, forming multiple trophic linkages between meso/macro zooplankton and top predators in all oceans. Previous studies on the micronekton and macroplankton ecology of the western Mediterranean Sea have largely concerned specific taxonomic groups such as euphausiids (Casanova, 1970, Casanova, 1974), pelagic crustacean decapods (Casanova, 1977), pteropods (Rampal, 1967, Rampal, 1975), or the hydromedusa Solmissus albescens (Benovic’,1973). At the same time and for the following years standing stock and specific composition, seasonal abundance, vertical distribution/migration, and feeding habits of particular animal taxa of mesopelagic zooplankton and micronekton communities have been the focus of a number of studies in the western Mediterranean (see Andersen et al., 1998, Andersen et al., 2001a, Andersen et al., 2001b, Andersen et al., 2004, Kovalev et al., 2003 and references therein). More recently, several studies on mesopelagic realm functioning have been carryed out, focusing on both short-term and seasonal time frames (see Cartes et al., 2013, Bozzano et al., 2014, Clavel-Henry, 2015, Olivar et al., 2017 and references therein). However, in comparison to neritic zooplankton (see Licandro and Ibanez, 2000, Fernandez de Puelles et al., 2007, Molinero et al., 2008 and references therein), the biology and ecology of deep micronekton community, including trophic pathway, distribution patterns, and even biomass, have not been intensively studied. With regard to our study area of the Ligurian Sea, a strong contribution to the knowledge of the diversity, vertical distribution and daily migration of the main key species forming the trophic structure of the mesopelagic environment was given by Valerie Andersen and her research group (Andersen and Sardou, 1992, Andersen et al., 1992, Sardou and Andersen, 1993, Andersen and Sardou, 1994, Sardou et al., 1996, Andersen et al., 1998, Andersen et al., 2001a, Andersen et al., 2001b, Andersen et al., 2004, Andersen and Prieur, 2000).
The objectives of this study were to determine changes in the vertical diel distribution and migration patterns (DVM) of the major mesozooplankton groups (excluding copepods), macroplankton and micronekton in the upper 1300 m of the water column, in a selected station of the northern Ligurian Sea open waters. Abundance, biomass and community structure were also investigated, and the relative importance of some actively vertically migrating dominant species of crustaceans, gelatinous taxa and mesopelagic fishes was assessed. In addition, structure of water column to estimate the influence of temperature, salinity, oxygen and fluorescence on the vertical distribution of different species was examined.
Section snippets
Study area
The region of interest of the Biolig Cruise is the Ligurian Sea. In particular the St. 8 sampled with BIONESS (Fig. 1) is located just downstream from the confluence of the two main currents both flowing northward along each side of northern Corsica: the Tyrrhenian Current, which intrudes into the basin through the Corsica Channel, and the West Corsica Current (WCC). Both veins feed a well-defined cyclonic circulation moving westward along the upper part of the continental slope of the
Environmetal variables
The temperature and salinity profiles of the sampled water column are shown in Fig. 2a. Values recorded by the BIONESS revealed a marked vertical thermohaline structure, highlighting the different water masses along the profile. The warmer and less salty water masses from the surface until the pycnocline (about 5–20 m depth), featured typical values of the spring Tyrrhenian MAW (T ≈ 16.09 – S ≈ 37.63, Vignudelli et al., 2000). Beneath it, an intermediate layer extending to approximately 250 m
Vertical patterns and day-night migrations
WMD of different macroplankton and micronekton species were reported in Fig. 10, Fig. 11. Highest values of Solmissus albescens abundance found in this study are in agreement with Goy et al. (1989) that reported in winter and spring its maximum abundance. Our data confirm the extensive vertical migration of the entire compact population as been reported previously by different authors (Andersen et al., 1992, Sardou and Andersen, 1993, 1996). Our data agree with Benovic’s (1973) results that
Conclusion
Most of the strong migrant species found with great abundances in our study were euphausiids, that are reported as the most important animals involved in the high carbon demand in many areas of the Mediterranean Sea (Minutoli and Guglielmo, 2009, Minutoli and Guglielmo, 2012, Minutoli et al., 2014), and of other Oceans (Werner and Buchholz, 2013, Kwong, 2016). The seasonal differences in abiotic and/or biotic parameters influence the zooplankton carbon demand from the organic non-living rain
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful to the Captain, crew and technicians of the R/V Minerva Uno for excellent support on board and to all our colleagues who participated in the survey and helped with the collection of samples. Special thanks are due to Lina Rodriguez Valdes for the efficient collaboration in daily and nightly collection of BIONESS biological samples and for assistance in sorting of the micronekton and macroplankton species.This study was undertaken in the framework of the Italian Flagship
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