Stable Isotope composition of particulate organic matter and fauna in a deep Mediterranean cold-water coral bank

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 (delta13C and delta15N) 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 delta15N of the scleractinians Desmophyllum dianthus, Madrepora oculata and Lophelia pertusa and the gorgonian Paramuricea cf. macrospinawere consistent with a diet mainly composed of zooplankton. The antipatharian Leiopathes glaberrima was more 15N- depletedthan other cnidarians, suggesting a lower contribution of zooplankton to its diet. Our delta13C 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 delta13C 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.

Diatoms and laminae of Mediterranean sapropels

The origins of sapropels (sedimentary layers rich in organic carbon) are unclear, yet they may be a key to understanding the influence of climate on ocean eutrophication, the mechanisms of sustaining biological production in stratified waters and the genesis of petroleum source rocks (Rohling, 1994, doi:10.1016/0025-3227(94)90202-X; Castradori, 1993, doi:10.1029/93PA00756; Calvert et al., 1992, doi:10.1038/359223a0). Recent microfossil studies of foraminifera (Rohling, 1994, doi:10.1016/0025-3227(94)90202-X) and calcareous nannofossils (Castradori, 1993, doi:10.1029/93PA00756) have focused attention on a deep chlorophyll maximum as a locus for the high production inferred (Calvert et al., 1992, doi:10.1038/359223a0) for sapropel formation, but have not identified the agent responsible. Here we report the results of a high-resolution, electron-microscope-based study of a late Quaternary laminated sapropel in which the annual flux cycle has been preserved. We find that much of the production was by diatoms, both mat-forming and other colonial forms, adapted to exploit a deep nutrient supply trapped below surface waters in a stratified water column. Reconstructed organic-carbon and opal fluxes to the sediments are comparable to those at high-productivity sites in today's oceans, and calculations based on diatom Si/C ratios suggest that the high organic-carbon content of sapropels may be entirely accounted for by sedimenting diatoms. We propose that this style of production may have been common in ancient Palaeogene and Cretaceous seas, environments for which conventional appeals to upwelling-driven production to account for the occurrence of diatomites, and some organic-carbon-rich sediments, have never seemed wholly appropriate.