Food web structure of the epibenthic and infaunal invertebrates on the Catalan slope (NW Mediterranean): Evidence from δ13C and δ15N analysis

The food-web structure of the epibenthic and infaunal invertebrates on the continental slope of the Catalan Sea (Balearic basin, NW Mediterranean) was investigated using carbon and nitrogen stable isotopes on a total of 34 species, and HPLC pigment analyses for three key species. Samples were collected close to Barcelona (NE Iberian Peninsula), between 650 and 800 m depth and between February 2007 and February 2008. Mean δ13C values ranged from −21.0‰ (small Calocaris macandreae and Amphipholis squamata) to −14.5‰ (Sipunculus norvegicus). Values of δ15N ranged from 4.0‰ (A. squamata) to 12.1‰ (Molpadia musculus). The stable isotope ratios of benthic fauna displayed a continuum of values (e.g. δ15N range of 8‰), confirming a wide spectrum of feeding strategies (from active suspension feeders to predators) and complex food webs. According to the available information on diets of benthic fauna, the lowest values were found for surface deposit feeders (small C. macandrae and the two ophiuroids A. squamata and Amphiura chiajei) and active suspension feeders (Abra longicallus and Scalpellum scalpellum) feeding on different sizes of particulate organic matter (POM), among which small particles may exhibit lower δ15N. High annual mean δ15N values were found among sub-surface deposit feeders, exploiting refractory or frequently recycled organic matter that is enriched in δ15N. Carnivorous polychaetes (Nephtys spp., Oenonidae and Polynoidae) and large decapods (Geryon longipes and Paromola cuvieri) also displayed high δ15N values. δ13C ranges were particularly wide among surface deposit feeders (ranging from −21.0‰ to −16.4‰), suggesting exploitation of POM of both terrigenous and oceanic origins. Correlation between δ13C and δ15N was generally weak, indicating multiple carbon sources, likely due to the consumption of different kinds of sinking particles (e.g. marine snow, phytodetritus, etc.), sedimented and frequently recycled POM, together with macrophyte remains. The stronger δ13C–δ15N correlations found in February and April suggest that during the period of water column homogeneization (winter–spring), the benthic community was sustained by phytodetritus inputs originating from the peak of surface primary production in February. Conversely, weaker δ13C–δ15N correlations were observed during the period of water column stratification (beginning in June–July), suggesting that the benthic community in this period was sustained, with a delay of ca. 2/3 months, by multiple carbon sources including continental inputs from river discharge (with the maxima in April–May). Thus both advective and vertical fluxes seem to be food sources for benthos on the Catalonian slope. Pigments in the guts of key species were generally degraded, and only the active suspension feeder A. longicallus ingested fresh chlorophyll during periods of high primary production at the surface (February and April 2007).

Research highlights► Food web structure of the epibenthic and infaunal invertebrates is complex. ► It is articulated in three trophic levels from ASF and SDF to carnivores. ► SSDF showed high δ15N values as they consume refractory or recycled organic matter. ► Main food sources are phytodetritus (vertical) and continental inputs (advective). ► Temporal variations in isotopic signatures are due to variation in the food source.