Trophic ecology of the supralittoral rocky shore (Roscoff, France): A dual stable isotope (δ13C, δ15N) and experimental approach

The present study investigates the trophic transfers on the upper littoral rocky shore (i.e. the supralittoral zone together with the upper midlittoral and adlittoral) of northern Brittany. The population mainly consists of four invertebrate species: the littorinids Littorina saxatilis and Melarhaphe neritoides, the isopod Ligia oceanica and the insect Petrobius maritimus. The utilisation of food sources available to these grazers was examined in a laboratory microcosm feeding experiment and a field study using stable isotopes (δ13C, δ15N). The results indicated that although Ligia oceanica preferentially occurs in the supralittoral zone, its trophic subsidies originate mostly from the adlittoral and lower intertidal zones. The stable isotope data also suggested that adlittoral terrestrial organic material may be the major food source of Petrobius maritimus. δ15N of Littorina saxatilis indicated a highly variable diet consisting of supralittoral lichens, midlittoral macroalgae and other food sources (e.g. microalgae). Both feeding experiments and stable isotope data show that only Melarhaphe neritoides has a clearly identifiable diet based on a mixture of lichens, mostly Verrucaria maura and Caloplaca marina, as estimated by an isotopic mixing model. Hence, the food web of this intertidal zone appears largely based on trophic subsidies from other habitats (i.e. upper and lower intertidal zones).