Ground-truthing Coral as a Proxy: An Examination of the δ15N of Four Coral Species across the Bermuda Pedestal

Abstract

In recent years, scleractinian corals have been used as a proxy for nitrogen conditions and as a tool for reconstructing the past. However, these coral proxies have not undergone extensive ground-truthing to ensure their robustness as proxies. To further complicate the picture, it is unknown how corals fulfill their nitrogen needs in the wild despite observations of both autotrophic and heterotrophic feeding in laboratory settings. Wang et al. found evidence that coral-symbiont nitrogen recycling may break down with prevalent food supply which results in ammonium excretion and internal variability of nitrogen within corals. This places corals on tentative ground as proxies. For this groundtruthing exercise, four coral species (Diploria labyirinthiformis, Montastrea cavernosa, Porites astreoides, and Madracis decactis) were collected from five sites across the Bermuda pedestal ranging from the edge of the reef to close proximity to the island. Additionally, macroalgae, Serpulidae feather duster worms, PON, and plankton net tows were collected to be utilized as representative N pools and species. The δ15N of all species was analyzed to determine which nitrogen pools the corals were using to fulfill their energetic needs. It was found that corals reflected the increasing nitrogen gradient with proximity to the island. The corals correlated closely to the gradients of the PON and net tows of the system which suggests that the corals are feeding from both the PON and plankton populations. No evidence was found to support a decoupling of coral-symbiont nitrogen recycling.