Search

Simple Summary: In the Adriatic Sea, there is little knowledge concerning the role of medium-sized pelagic fish species such as Scomber spp. and Trachurus spp. in the local food web. To better depict their role, stomach content and stable isotope analyses were performed on specimens caught during routine acoustic surveys carried out along the western Adriatic coast. The results show that the two Trachurus species (T. trachurus and T. mediterraneus) share a similar diet but present spatial segregation along a latitudinal (i.e., thermal) gradient, while the two Scomber species (S. scombrus and S. colias) differ for prey preferences and present spatial segregation too, along a bathymetric gradient. The positions (trophic niches) of these species in the food web only partially overlap. This fact allows good resource partitioning and the coexistence of these species in the Adriatic Sea, limiting the risk of possible future collapses of some of these fish populations, with inevitable cascade effects on the entire marine food web. The Mediterranean is one of the most overfished seas of the world where mesopredators are severely threatened. The trophic strategies of four pelagic species that inhabit the Adriatic Sea (Scomber spp. and Trachurus spp.) were investigated through an integrated approach of stomach contents and stable isotopes analyses. Our study demonstrated that Scomber colias feeds mainly on strictly pelagic prey, with fish larvae as a secondary prey in the Southern Adriatic Sea, while S. scombrus feeds on prey belonging to higher trophic levels. Smaller specimens of Trachurus mediterraneus have a diet mainly based on pelagic prey, while larger fishes rely on prey such as benthic decapods, showing an ontogenetic shift in the diet of the species. Trachurus trachurus shows a preference for offshore and deeper areas and a diet such as that of its congeneric, but no clear ontogenetic shift was observed. This spatial segregation allows the co-existence of these two species of Trachurus. Scomber colias mainly inhabits southern areas and S. scombrus shows a preference for the northern sectors. This latitudinal gradient avoids the overlap of their trophic niches. Bayesian mixing models confirmed that the trophic niches of these species only partially overlap in the middle of the trophic web. [ABSTRACT FROM AUTHOR]

The pelagic ecosystem in the Ross Sea has one central component that is very important for energy exchanges between upper and lower trophic levels: the Middle Trophic Level. Krill species are the most important and abundant organisms within this level. Several acoustic surveys were conducted in the western Ross Sea over the past 25 years, revealing that Euphausia superba is by far the most abundant species of krill in the Ross Sea during austral summer, and that its core distribution is concentrated in the northern part, bordering the Southern Ocean. Euphausia crsytallorophias, the second most abundant krill species, is more concentrated in the central Ross Sea, generally near the coast. Data on krill biomass were collected in December and January from 1994 to 2016 and analyzed together with key environmental parameters by means of two-way ANOVA in order to explain species behavior and identify possible environmental drivers. Temperature and dissolved oxygen influenced the biomass of both species of krill, while other environmental parameters only affected one species. In conclusion, the biomass of both species has varied over the years, possibly due to a complex synergy of environmental drivers. [ABSTRACT FROM AUTHOR]

Zooplankton are critical to the functioning of ocean food webs because of their utter abundance and vital ecosystem roles. Zooplankton communities are highly diverse and thus perform a variety of ecosystem functions; thus changes in the community or food web structure may provide evidence of ecosystem alteration. Assemblage structure and trophodynamics of mesozooplankton communities were examined across the Adriatic basin, the northernmost and most productive basin of the Mediterranean Sea. Samples were collected in June–July 2019 within the framework of the MEDIAS (MEDiterranean International Acoustic Survey) project, along coastal–offshore transects and from the surface to ca. 200 m depth, covering the whole western Adriatic side; consistently environmental variables were also recorded. Results showed a clear separation between samples from the northern-central Adriatic and the southern ones, with a further segregation, although less clear, of inshore vs. offshore stations, the latter being mostly dominated in the central and southern stations by gelatinous plankton. Such patterns were mainly driven, based on the outputs of the distance-based linear model, by fluorescence (as a proxy for primary production) for northern-central stations, i.e. closer to the Po River input, and by dissolved oxygen, together explaining 44 % of the total variance. Overall, at the basin level, the analysis of stable isotopes of nitrogen and carbon allowed for identifying a complex food web characterized by three trophic levels from filter feeders–herbivores to carnivores, passing through a general pattern of omnivory with varying preference towards herbivory or carnivory. Stable isotope signatures spatially varied between inshore vs. offshore communities and across sub-areas, with the northern Adriatic exhibiting greater δ15 N and more variable δ13 C than the other two sub-areas, likely attributable to the occurrence in the area of organic matter of both terrestrial and marine origin. Our results contribute to the knowledge of mesozooplankton community and trophic structure, at the basin scale across a coastal–offshore gradient, also providing a baseline for the future assessment of pelagic food webs within the European Council (EC) Marine Strategy Framework Directive. [ABSTRACT FROM AUTHOR]

Zooplankton are critical to the functioning of ocean food webs because of their utter abundance and vital ecosystem roles. Zooplankton communities are highly diverse and thus perform a variety of ecosystem functions, thus changes in their community or food web structure may provide evidence of ecosystem alteration. Assemblage structure and trophodynamics of mesozooplantkon communities were examined across the Adriatic basin, the northernmost and most productive basin of the Mediterranean Sea. Samples were collected in June-July 2019 along coast-offshore transects covering the whole western Adriatic side, consistently environmental variables were also recorded. Results showed a clear separation between samples from the northern-central Adriatic and the southern ones, with a further segregation, although less clear, of inshore vs. off-shore stations, the latter mostly dominated in the central and southern stations by gelatinous plankton. Such patterns were mainly driven by chlorophyll-a concentration (as a proxy of primary production) for northern-central stations, i.e. closer to the Po river input, and by temperature and salinity, for southern ones, with the DistLM model explaining 46 % of total variance. The analysis of stable isotopes of nitrogen and carbon allowed to identify a complex food web characterized by 3 trophic levels from herbivores to carnivores, passing through the mixed feeding behavior of omnivores, shifting from phytoplankton/detritus ingestion to microzooplankton. Trophic structure also spatially varied according to sub-area, with the northern-central sub-areas differing from each other and from the southern stations. Our results highlighted the importance of environmental variables as drivers of zooplanktonic communities and the complex structure of their food webs. Disentangling and considering such complexity is crucial to generate realistic predictions on the functioning of aquatic ecosystems, especially in high productive and, at the same time, overexploited area such as the Adriatic Sea. [ABSTRACT FROM AUTHOR]