The Effect of Temporal Changes and Environmental Trophic Condition on the Isotopic Composition (ω13C and ω15N) of Atherina boyeri (Risso, 1810) and Gobius niger (L., 1758) in a Mediterranean Coastal Lagoon (Lake of Sabaudia): Implications for Food Web Structure

Abstract. ω¹³C and ω¹⁵N of organic matter sources and consumers were employed to analyse trophic differentiation between a benthic consumer, Gobius niger (L., 1758) (Pisces, Osteichthyes), and a pelagic consumer, Atherina boyeri (Risso, 1810) (Pisces, Osteichthyes) in a Mediterranean coastal lagoon (Lake of Sabaudia) in winter and summer 1999. Trophic differences between the two species throughout the two sampling periods were related to the environmental trophic condition (i. e. nutrient and phytopigment concentrations). Although these two fish have different habitats, they both exploited benthic organisms, above all in summer. When the nutrient and phytopigment concentrations were higher (summer), the most abundant benthic organisms were the main food sources for both species. In winter A. boyeri apparently shifted its feeding preferences to include zooplankton, confirming its opportunistic feeding strategy. Par-ticulate organic matter and algae seemed to play the main trophic role in the food web structure. Benthic invertebrates used both sources, while zooplankton seemed to rely exclusively on the particulate fraction. The overlapping and very depleted signatures of sedimentary and particulate organic matter highlights a strong link between sediments and water column, rarely observed in other aquatic ecosystems using stable isotopes. Such a finding further substantiates the importance of particulate organic matter as a food source in the study area.