This study was conducted in February, April and June 2009 at three stations in the southern Adriatic. Occurrence of the dinoflagellate Noctiluca scintillans and tunicates Salpa fusiformis and Salpa maxima in high abundances for the oligotrophic open sea, indicates the importance of physical forcing (vertical mixing) and inflow of nutrient-enriched Atlantic water, due to the Bimodal Oscillating System (BiOS) mechanism, into the Adriatic Sea thus creating an environment favourable for their rapid increase. This is the first time a bloom of N. scintillans has been recorded in the open southern Adriatic (OSA). High abundance of Noctiluca and salp populations in the OSA was characterized by low abundance of phytoplankton and other zooplankton, with obvious trophic implications (reduction of food availability to crustacean primary consumers). Moreover, during the S. maxima bloom in June 2009, calanoid copepods and appendicularians were almost completely absent (<1 ind. m−3).

We analyzed stable isotopes of carbon and nitrogen to investigate the trophic structureof the subtidal food web around the Mingan Islands, northern Gulf of St. Lawrence, easternCanada. All benthic consumers were enriched in 13C (mean δ13C of–17.1‰) compared to particulate organic matter (POM: –23.3‰). Nitrogen stable isotoperatios ranged from 6‰ to 14‰ and the organisms studied fell into three distinct trophicgroups, primary producers, primary consumers (herbivores), and predators. The sea starCrossaster papposus and the sculpin Myoxocephalusscorpius, known to be top level predators, had slightly higher δ15Nthan other predators. Although the average isotope signature of the sea starSolaster endeca placed it among regular predators, the δ15Nincreased with sea star size and large individuals could be considered as top predators.The relatively small number of organisms located at intermediate trophic levels suggests alow level of omnivory in the Mingan Islands’ system, which contrasts with previouslydescribed benthic systems that exhibit a continuum between herbivores and predators. Lowomnivory, in addition to low diversity, suggests that this ecosystem may be relativelyunstable if exposed to natural and/or anthropogenic disturbances such as exploitation andclimate change.

A single trophic interaction between two major biotic components within similar ecosystems can reveal high variability. In fact, in situ measurements of the trophic interactions, as provided by the field application of stable isotopes, are likely to reveal strong differences in patterns. An illustration of such a geographical plasticity is provided when regarding the trophic interaction between two major components of intertidal habitats namely, the oyster Crassostrea gigas and the microphytobenthos. The contribution of benthic diatoms to oysters' diets ranged from 4.4±3.5% in the Mont St Michel bay to 69.6±1.1% in the Marennes-Oléron bay, as estimated from δ13C and δ15N analyses. Concerning food web investigations, these results point out serious limits for extrapolating qualitative and quantitative observations related to the major trophic links between identical food sources and consumers inhabiting similar ecosystems.