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- Title
Quantifying the δ15N trophic offset in a cold-water scleractinian coral (CWC): implications for the CWC diet and coral δ15N as a marine N cycle proxy.
- Authors
Mottram, Josie L.; Gothmann, Anne M.; Prokopenko, Maria G.; Cordova, Austin; Rollinson, Veronica; Dobkowski, Katie; Granger, Julie
- Abstract
The nitrogen (N) isotope composition (δ15 N) of cold-water corals is a promising proxy for reconstructing past ocean N cycling, as a strong correlation was found between the δ15 N of the organic nitrogen preserved in coral skeletons and the δ15 N of particulate organic matter exported from the surface ocean. However, a large offset of 8 ‰–9 ‰ between the δ15 N recorded by the coral and that of exported particulate organic matter remains unexplained. The 8 ‰–9 ‰ offset may signal a higher trophic level of coral dietary sources, an unusually large trophic isotope effect or a biosynthetic δ15 N offset between the coral's soft tissue and skeletal organic matter, or some combinations of these factors. To understand the origin of the offset and further validate the proxy, we investigated the trophic ecology of the asymbiotic scleractinian cold-water coral Balanophyllia elegans, both in a laboratory setting and in its natural habitat. A long-term incubation experiment of B. elegans fed on an isotopically controlled diet yielded a canonical trophic isotope effect of 3.0 ± 0.1 ‰ between coral soft tissue and the Artemia prey. The trophic isotope effect was not detectably influenced by sustained food limitation. A long N turnover of coral soft tissue, expressed as an e -folding time, of 291 ± 15 d in the well-fed incubations indicates that coral skeleton δ15 N is not likely to track subannual (e.g., seasonal) variability in diet δ15 N. Specimens of B. elegans from the subtidal zone near San Juan Channel (WA, USA) revealed a modest difference of 1.2 ± 0.6 ‰ between soft tissue and skeletal δ15 N. The δ15 N of the coral soft tissue was 12.0 ± 0.6 ‰, which was ∼6 ‰ higher than that of suspended organic material that was comprised dominantly of phytoplankton – suggesting that phytoplankton is not the primary component of B. elegans' diet. An analysis of size-fractionated net tow material suggests that B. elegans fed predominantly on a size class of zooplankton ≥500 µ m, implicating a two-level trophic transfer between phytoplankton material and coral tissue. These results point to a feeding strategy that may result in an influence of the regional food web structure on the cold-water coral δ15 N. This factor should be taken into consideration when applying the proxy to paleo-oceanographic studies of ocean N cycling.
- Subjects
SAN Juan (P.R.); WESTERN Australia; DEEP-sea corals; SCLERACTINIA; CORALS; ALCYONACEA; DIET; FOOD chains
- Publication
Biogeosciences, 2024, Vol 21, Issue 5, p1071
- ISSN
1726-4170
- Publication type
Article
- DOI
10.5194/bg-21-1071-2024