21 Aug 2023
 | 21 Aug 2023
Status: this preprint is currently under review for the journal BG.

Zooplankton as the primary diet for cold-water scleractinian corals (CWCs): implications for the CWC marine N cycle proxy and trophic ecology

Josie Mottram, Anne Gothmann, Maria Prokopenko, Austin Cordova, Veronica Rollinson, Katie Dobkowski, and Julie Granger

Abstract. The nitrogen (N) isotope composition (δ15N) of cold-water corals is a promising proxy for reconstructing past ocean N cycling, as a strong correlation was found between the δ15N of the organic nitrogen preserved in coral skeletons and the δ15N of sinking particulate organic matter exported from the surface ocean. However, a large offset of 8–9 ‰ between the δ15N recorded by the coral and that of export remains unexplained. The 8–9 ‰ offset may signal a potential sensitivity of the proxy to food web structure, an unusual large trophic isotope effect or a biosynthetic δ15N offset between the coral’s soft tissue and skeletal tissues, 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 scleractinan cold water coral Balanophyllia elegans. 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 of 291 ± 15 days in the well-fed incubations indicates that the coral skeleton is not apt to record seasonal difference in diet δ15N. Specimens of B. elegans from the shallow subtidal zone near San Juan Channel (WA, USA) revealed a modest difference between soft and skeletal tissue δ15N of 1.2 ± 0.6 ‰. The δ15N 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 the latter 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 µm, implicating a two-level trophic transfer between phytoplankton material and coral tissue. These results portend a sensitivity of cold-water coral δ15N to regional food web structure that must be heeded in paleoceanographic studies of ocean N cycling.

Josie Mottram et al.

Status: open (until 18 Oct 2023)

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  • RC1: 'Comment on bg-2023-127', Philip Riekenberg, 20 Sep 2023 reply

Josie Mottram et al.

Data sets

Project: Collaborative Research: Refining the use of scleractinian cold-water coral skeleton-bound d15N as a proxy for marine N cycling Anne Gothmann, Julie Granger, Maria Prokopenko

Josie Mottram et al.


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Short summary
Knowledge of ancient ocean N cycling can help illuminate past climate change. This study ground-truths a promising proxy for marine N cycling, the N isotope composition of cold-water coral (CWC) skeletons, through field and lab studies. CWCs likely record surface N via a two-step trophic transfer, consuming mainly zooplankton. We estimate N turnover in coral tissue, quantify isotope effects between tissue, diet and skeleton, and suggest that this proxy may be sensitive to food web structure.