Isotopic approaches to estimating the contribution of heterotrophic sources to Hawaiian corals

Abstract Corals obtain nutrition from the photosynthetic products of their algal endosymbionts and the ingestion of organic material and zooplankton from the water column. Here, we use stable carbon (δ 13 C) and nitrogen (δ 15 N) isotopes to assess the proportionate contribution of photoautotrophic and heterotrophic sources to seven Hawaiian coral species collected from six locations around the island of O‘ahu, Hawaiʻi. We analyzed the δ 13 C and δ 15 N of coral tissues and their algal endosymbionts, as well as that of dissolved inorganic matter, particulate organic matter, and zooplankton from each site. Estimates of heterotrophic contribution varied among coral species and sites. Bayesian mixing models revealed that heterotrophic sources (particulate organic material and zooplankton) contributed the most to Pocillopora acuta and Montipora patula coral tissues at 49.3% and 48.0%, respectively, and the least to Porites lobata at 28.7%, on average. Estimates of heterotrophic contribution based on the difference between δ 13 C of the host and algal endosymbiont (δ 13 C h–e ) and isotopic niche overlap often differed, while estimates based on δ 15 N h–e were slightly more aligned with the estimates produced using Bayesian mixing models. These findings suggest that the utility of each approach may vary with coral health status, regions, and coral species. Overall, we find that the mean heterotrophic contribution to Hawaiian coral tissues ranges from 20% to 50%, suggesting a variety of trophic strategies. However, these findings did not always match past direct measurements of heterotrophic feeding, indicating that heterotrophically acquired nutrition does not necessarily get incorporated into tissues but can be respired or exuded in mucus.