Food-web structure and trophodynamics of mesopelagic–suprabenthic bathyal macrofauna of the Algerian Basin based on stable isotopes of carbon and nitrogen

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The trophodynamics of mesopelagic (macrozooplankton/micronekton) and benthic boundary layer (suprabenthos=hyperbenthos) faunas from the Algerian Basin were characterized on a seasonal scale through stable carbon and nitrogen isotopic analyses of a total of 34 species and two broad taxa (Copepoda and Cumacea). This is the first study simultaneously focused on trophodynamics of deep-sea zooplankton and suprabenthos. Samples were collected southeast of Mallorca (Algerian Basin, Western Mediterranean), on the continental slope close to Cabrera Archipelago, at 650–780 m depths, ca. bi-monthly between August 2003 and June 2004. Mean δ 13C values of suprabenthos ranged from −21.1‰ ( Munnopsurus atlanticus) to −16.7‰ ( Cyclaspis longicaudata). Values of δ 15N ranged from 2.8‰ ( Lepechinella manco) to 9.9‰ (larvae of Gnathia sp.). The stable isotope ratios of suprabenthic fauna displayed a continuum of values, confirming a wide spectrum of feeding guilds (from filter feeders/surface deposit feeders to predators). According to the available information on diets for suprabenthic species, the highest annual mean δ 15N values were found for the hematophagous isopod Gnathia sp. parasite on fish (represented by Praniza larvae) and carnivorous amphipods (e.g. Rhachotropis spp., Nicippe tumida) consuming copepods, and the lowest δ 15N values were found for two cumaceans ( Cyclaspis longicaudata and Platysympus typicus) feeding on detritus. Assuming a 15N-enrichment factor of 2.5‰ and deposit feeders as baseline, we found three trophic levels in suprabenthic food webs. δ 13C ranges were particularly wide among deposit feeders (ranging from −21.8% to −17.3‰) and omnivores (from −20.5% to −18.8‰), suggesting exploitation of particulate organic matter (POM) of different characteristics. Our isotopic analyses revealed lower ranges of δ 13C and δ 15N for macrozooplankton/micronekton, compared with suprabenthos. δ 13C values of zooplankton taxa ranged from −21.1‰ (the hyperiid Phrosina semilunata) to −19.9‰ (the decapod Pasiphaea multidentata), while δ 15N values ranged from 3.9‰ ( P. semilunata) to 7.5‰ ( P. multidentata). Among zooplankton, more enriched δ 15N values were found among carnivores (e.g. the fish Cyclothone spp. and Pasiphaea multidentata) preying on copepods, hyperiids, euphausiids and small fish. The lowest δ 15N values were found for hyperiids that feed on the mucus nets of salps (e.g. Vibilia armata). After contrasting isotope analysis with dietary data, we conclude there were two trophic levels among zooplankton/micronekton. Strong correlation between the mean annual δ 15N and δ 13C values was found for zooplankton ( R 2=0.7), but not for suprabenthos, which suggests a single source of carbon for plankton. We found a general seasonal trend for δ 13C enrichment from late autumn (November) to late winter–spring (February–April) for both suprabenthos and zooplankton. The δ 13C enrichment in February–April was correlated in zooplankton with higher surface chlorophyll a concentration 1 month before sampling. As evidenced by δ 13C–δ 15N correlations, the response of zooplankton to the peak of surface primary production was almost immediate (an increase of δ 13C–δ 15N correlations in February), and stronger than for suprabenthos. The response among suprabenthos was weak, with slight increase in δ 13C–δ 15N relationships in April–June.