Trophic ecology of deep-sea megafauna in the ultra-oligotrophic Southeastern Mediterranean Sea

Abstract

The trophic ecology of fourteen species of bathybenthic and bathypelagic fishes and six species of bathybenthic decapod crustaceans from the continental slope and rise of the Southeastern Mediterranean Sea (SEMS) was examined using stable isotope analysis. Mean δ13C values among fish species varied by more than 4.0‰, from -20.85‰ (Macroramphosus scolopax) to -16.57‰ (Conger conger and Centrophorus granulosus), and increased as a function of depth (200 - 1400 m). Mean δ13C values of the crustaceans showed smaller variation, between -16.38‰ (Polycheles typhlops) and -18.50‰ (Aristeus antennatus). This suggests a shift from pelagic to regenerated benthic carbon sources with depth. Benthic carbon regeneration is further supported by the low benthic-pelagic POM-δ13C values, averaging -24.7 ± 1.2‰, and the mixing model results, presenting a very low contribution of epipelagic POM to the bathyal fauna. Mean δ15N values of fish and crustacean species ranged 7.91 ± 0.36‰ to 11.36 ± 0.39‰ and 6.15 ± 0.31‰ to 7.69 ± 0.37‰, respectively, resulting in trophic position estimates, occupying the third and the fourth trophic levels. Thus, despite the proximity to the more productive areas of the shallow shelf, low number of trophic levels (TL ≈ 1.0) and narrow isotopic niche breadths (SEAc <1) were observed for bathybenthic crustaceans (TL = 3.62 ± 0.22) and bathypelagic fishes (TL = 4.33 ± 0.34) in the study area — probably due to the ultra-oligotrophic state of the SEMS resulting in limited carbon sources. Our results, which provide the first trophic description of deep-sea megafauna in the SEMS, offer insight into the carbon sources and food web structure of deep-sea ecosystems in oligotrophic marginal seas and can be further used in ecological modeling and support the sustainable management of marine resources in the deep Levantine Sea.