Current understanding of the potential vulnerabilities of the soft-sediment food web of the Clarion-Clipperton Zone (CCZ) nodule fields to future mining impacts is based largely on benthic stock, taxonomic and modelling assessments, lacking in ground-truthing observations of trophic interactions. Using dual stable isotope analysis and related metrics, we investigated the trophic relationships of meiofauna and macrofauna as a whole and of their key taxa (Nematoda, Copepoda, Polychaeta, Tanaidacea, Isopoda, Amphipoda) across nodule-rich and nodule-free sediments at two sites within the Global Sea Mineral trophic niche overlap) among the two investigated size classes in nodule-free sediments where the two size classes likely share more of the present basal resources. The nodule-rich sediments represented a more structurally complex habitat, where the taxa belonging to both infaunal size classes may have been using alternative energy pathways, leading to enhanced trophic niche partitioning and a narrowing of their isotopic niches. The metrics highlighted the importance of meiofauna as a foundational trophic link for the reestablishment of the soft-sediment food web following mining. The larger trophic niche partitioning observed between Nematoda and Copepoda hinted at higher sensitivity of the smaller-sized metazoan assemblage to functional cascading, with a potentially crucial role of copepods in bridging between the sediment detrital pool and the next trophic levels. The macrofauna displayed more trophic redundancy between its taxa, known to increase functional resilience to disturbance. Polychaetes displayed smaller trophic niche widths in nodule-free compared to the nodule-rich sediments. This novel dataset and metrics analyses confirm results from a previous modelling study of the sediment food web at this contract area and suggest a need for future research to focus on the links between the meiobenthos and sediment prokaryotes in order to assess potential mining impacts. |
