Abstract
Cold-water corals (CWC) are frequently reported from deep sites with locally accelerated currents that enhance seabed food particle supply. Moreover, zooplankton likely account for ecologically important prey items, but their contribution to CWC diet remains unquantified. We investigated the benthic food web structure of the recently discovered Santa Maria di Leuca (SML) CWC province (300 to 1100 m depth) located in the oligotrophic northern Ionian Sea. We analyzed stable isotopes (δ13C and δ15N) of the main consumers (including ubiquitous CWC species) exhibiting different feeding strategies, zooplankton, suspended particulate organic matter (POM) and sedimented organic matter (SOM). Zooplankton and POM were collected 3 m above the coral colonies in order to assess their relative contributions to CWC diet. The δ15N of the scleractinians Desmophyllum dianthus, Madrepora oculata and Lophelia pertusa (8 to 9‰) and the gorgonian Paramuricea cf. macrospina (9 to 10‰) were consistent with a diet mainly composed of zooplankton (6 to 7‰). The antipatharian Leiopathes glaberrima was more 15N-depleted (7 to 8‰) than other cnidarians, suggesting a lower contribution of zooplankton to its diet. Our δ13C data clearly indicate that the benthic food web of SML is exclusively fuelled by carbon of phytoplanktonic origin. Nevertheless, consumers feeding at the water–sediment interface were more 13C-enriched than consumers feeding above the bottom (i.e. living corals and their epifauna). This pattern suggests that carbon is assimilated via 2 trophic pathways: relatively fresh phytoplanktonic production for 13C-depleted consumers and more decayed organic matter for 13C-enriched consumers. When the δ13C values of consumers were corrected for the influence of lipids (which are significantly 13C-depleted relative to other tissue components), our conclusions remained unchanged, except in the case of L. glaberrima which could assimilate a mixture of zooplankton and resuspended decayed organic matter.
Highlights
Like their shallow-water tropical counterparts, coldwater corals (CWC; called deep-water corals) are organisms of major ecological importance in deep ocean environments (Roberts et al 2006)
particulate organic matter (POM) collected 3 m above the bottom on Santa Maria di Leuca (SML) coral mounds exhibited C:N values (~7 to 9) similar to data reported for surface POM in the southern Adriatic and northern Ionian Seas (Socal et al 1999, Boldrin et al 2002) and match C:N ratios of living phytoplankton. δ13C values of POM (~–22 ‰) reflect the typical signature of marine phytoplanktonic production of this Mediterranean area (Martinotti et al 1997, Sarà 2006)
The present study provides a first outlook of the isotopic composition of deep POM in the vicinity of SML Cold-water corals (CWC), but there is clearly a crucial need to better characterise the quality and availability of this potential food source
Summary
Like their shallow-water tropical counterparts, coldwater corals (CWC; called deep-water corals) are organisms of major ecological importance in deep ocean environments (Roberts et al 2006). The distribution of CWC has been directly related to the presence of hard topographic highs and enhanced currents that are strong enough to prevent burial by deposition of fine sediment and provide rapid transport of fresh and labile food particles to the coral polyps (Thiem et al 2006, Kiriakoulakis et al 2007) These benthic ecosystems are likely indirectly fuelled by surface primary production via sinking fluxes of particles (phytodetritus, fecal pellets) (Duineveld et al 2004). Δ13C data obtained for CWC worldwide (typically between –22 and –18 ‰), and in particular for the 2 cosmopolitan scleractinians Lophelia pertusa and Madrepora oculata, evidenced that these species depend (at least indirectly) on phytoplanktonic production for their food (Duineveld et al 2004, Sherwood et al 2008, Becker et al 2009, Le Guilloux et al 2009). We evaluated the trophic role of zooplankton in the diet of several co-occurring cnidarian species, including the ubiquitous scleractinian species Lophelia pertusa and Madrepora oculata
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