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Trophic relationships in demersal communities of Western Mediterraneo occidentale: case studies from coastal and deep-sea ecosystems

AuthorsFanelli, Emanuela CSIC ORCID
AdvisorCartes, Joan Enric CSIC ORCID; Badalamenti, Fabio
KeywordsTrophic relationships
Western Mediterranean
Tyrrhenian Sea
Balearic basin
Feeding ecology
Stable isotopes analysis
Marine protected areas
Issue Date8-Oct-2008
PublisherUniversità degli studi della Tuscia di Viterbo
AbstractThe present thesis deals with the analysis of the trophic web structure and spatio-temporal variation in trophic levels of prey and predator in two different ecosystems: a coastal environment up to 80 m in the Northern Sicily (Italy) and a deep-sea ecosystem, the middle slope (450-750 m deep) of the Balearic basin (Cabrera Archipelago-Spain). As trophic relationships are fundamental to understand biological interaction, a detailed study of the food and feeding ecology of demersal fish species, in the coast and deep-sea, is attempted. Three fish species from costal ecosystems and three from deep-sea were selected as representative of different trophic guilds (benthophagous, suprabenthic feeder and planktophagous). Moreover to define the trophic structure of coastal and deep-sea communities a complete examination of prey (i.e. zooplankton, suprabenthos and benthos, where possible) and predator based on stable isotope analysis was carried out. Particularly suprabenthic resources were generally ignored or scarcely considered in trophic web studies, thus the role of suprabenthos as a link between benthic and pelagic environments and its importance as forage for juveniles of coastal fish species and for deep-sea fish is explored.
The data used for the part of the thesis regarding coastal ecosystem were collected within the framework of a project where sampling was performed in four gulfs of Northern Sicily. Among the four areas, the gulfs of Castellammare and Patti are two fishery exclusion zones, where trawl fishery was banned in 1990 by a Regional Act, while the other two areas (the gulfs of Termini Imerese and S. Agata) were intensively trawled.
Three chapters of the thesis (2, 3 and 5) deal with the dynamic of suprabenthos in both coastal and deep-sea ecosystems. In chapter 2 the trophodynamics of suprabenthic communities in the Gulf of Castellammare at a seasonal scale (from November 2004 to June 2005) was analyzed. In addition changes in species composition and assemblage structure were studied; by means of stable isotopes analysis of N and C seasonal variations in trophic levels and source of carbon of some dominant suprabenthic species were analyzed. Temporal changes were correlated to environmental variables such as the Chla concentration, recorded at different time intervals.
In chapter 3 the suprabenthic assemblages of four Gulfs of Northern Sicily were defined, moreover an attempt to distinguish different trophic levels within suprabenthos and to determine spatial changes in trophic levels and source of carbon was given.
In chapter 4 the spatial variations in feeding habits and trophic level of Pagellus erythrinus, Arnoglossus laterna and Merluccius merluccius, three coastal demersal fish inhabiting muddy bottoms of Northern Sicily, were studied. Polychaetes and decapods were the preferred prey of P. erythrinus in the four gulfs. Differences in diet of samples from protected and unprotected areas were evident, with a more specialist diet in the protected gulfs. A. laterna fed mainly on decapods and gobiid fish. Ontogenetic changes were evident with juveniles mainly preying on suprabenthic species, e.g. mysids and small decapods. Suprabenthic species, natantian decapods and fish were dominant in the diet of juveniles of European hake, M. merluccius. Juveniles smaller than 160 mm of total length, mainly fed on crustaceans while a shift to a strongly pelagic diet occurred in larger specimens. Stable isotopes analysis confirmed gut contents results with significance differences in d15N signatures between protected and unprotected areas only for the benthic feeder P. eryhtrinus. Stable isotopes evidenced a complex food web in coastal demersal community with at least two trophic levels among suprabenthic species and with fish as top-predators.
In chapter 5 the trophodynamics of suprabenthos and zooplankton was analyzed off Cabrera Archipelago. Four stations situated at 150 m (shelf-slope break), and at bathyal depths on 350, 650 and 750 m were sampled at bi-monthly intervals during six cruises performed between August 2003 and June 2004.
In chapter 6 temporal variations in the diet of three deep-sea demersal fish on the continental slope off Cabrera were analyzed. Samples of Hoplostethus mediterraneus, Hymenocephalus italicus and Nezumia aequalis were collected within the framework of the Project IDEA, at bi-monthly intervals, between 550 and 750 m of depth. Classical stomachs contents and stable isotopes analyses were contemporarily applied in order to detect temporal variations of diet. An experiment using formalin preserved specimens and frozen specimens as control were also performed in order to verify if preserved material can be used for stable isotopy. H. mediterraneus mainly preyed on isopod and on lesser extent on amphipods and mysids. H. italicus fed on mysids and amphipods, captured in the Benthic Boundary Layer, while N. aequalis seemed to exhibit a more benthic behaviour with benthopelagic and benthic prey in its diet. Strong temporal variations were observed in the diet of H. mediterraneus, with February as a period of change in feeding habits. Temporal changes were less evident in the two macrourids and depth seemed to be a more structuring factor of diet than month. The experiment on frozen and fixed-preserved specimens showed that formalin did not affect 15N signatures, while time of preserving strongly influenced 13C values, with different pattern among the three species. Temporal variations of δ15N values were evident in the three species: the lowest values were observed in September and the highest in April-June. Among the three species N. aequalis had the highest trophic level (4,47), occupying a high position in the deep-sea trophic web. In general deep sea trophic webs were complex, with at least three trophic levels among benthopelagic resources and with fish as top-predators: marine snow seemed to be the only primary source of material
Description327 pages.-- PhD Tesis carried out at the Universitá degli Studi di Viterbo "La Tuscia", Dipartimento di Ecologia e Sviluppo Sostenibile, at the Institut de Ciències del Mar (ICM-CSIC) and at the Laboratorio di Ecologia Marina, IAMC-CNR
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