A first approach to decipher the megabenthic trophic network in the Angolan cold-water coral mounds through stable isotope analysis

The characterisation of trophic nets in the deep-sea is challenging due to the remoteness and the logistic constraints to reach and sample these habitats. Stable carbon and nitrogen isotope (δ¹³C and δ¹⁵N) analysis has proven to be a useful tool in identifying trophic relationships. The benthic trophic network of the fauna in the recently discovered Angolan cold-water coral (CWC) mounds is being investigated through carbon and nitrogen isotopic characterization. Samples were collected in 2016 during the M122 ("ANNA") expedition on board the R/V METEOR. Organisms belonging to different functional groups of Porifera, Cnidaria, Mollusca, Annelida, Bryozoa, Arthropoda, Echinodermata, and Chordata, as well as potential food sources (suspended particulate organic matter and organic matter of sediments) were analysed. The main aims of the study are: (1) to describe the megabenthic trophic guilds of the Angolan CWC mounds, (2) to estimate the total number of trophic levels, and (3) to compare this unexplored area with previous results from other CWC dominated communities. Our first results show that particulate organic matter presented a marine signature (δ¹³C from -20 to -25‰ V.P.D.B.). However, the analysed CWC species seem to prefer a diet which combined a mix of detrital and resuspended organic matter (δ¹³C from -15 to -20‰ V.P.D.B.), similar to what has been observed for Antarctic gorgonians and other CWC habitats. Porifera presented the highest δ¹⁵N ratios (20.19±1.81‰). We hypothesise this could be due to the assimilation of bacterioplankton as a food source which has been already documented for Porifera.