Challenging trophic position assessments in complex ecosystems: calculation method, choice of baseline, trophic enrichment factors, season and feeding guild do matter. A case study from Marquesas Islands coral reefs

Assessments of ecosystem functioning are a fundamental ecological challenge and an essential foundation for ecosystem-based management. An understanding of species trophic positions (TP) is essential to characterize food web architecture. However, despite the intuitive nature of the concept, empirically estimating TP is a challenging task due to the complexity of trophic interaction networks. Various alternative methods are proposed to assess TPs, including different approaches to account for the different sources of organic matter at the base of the food web (the “baseline”). However, it is often not clear which methodological approach and which baseline choices are the most reliable. Using an ecosystem-wide assessment of a tropical reef (Marquesas Islands, French Polynesia, with available data for 70 coral reef invertebrate and fish species), we tested whether different commonly used TP estimation methods yield similar results and, if not, whether it is possible to identify the most r..., The data set for the case study for the method comparison was obtained in Nuku Hiva, the largest of the Marquesas Islands (8°54’S, 140°02’W), French Polynesia. Major local environmental characteristics and the sampling methods were already described in detail in previous studies (Galzin et al., 2016; Fey et al., 2020, 2021). Briefly, the studied area, named the “Baie du controleur”, has relatively strong hydrodynamic conditions, and hosts a marine seafloor dominated by rocky habitats, characterized mainly by steep scree slopes of volcanic rock mixed with patches of soft-bottom habitats, algae groves, coral habitats and caves. The benthic community is composed mainly of algal turf, macroalgae, scattered coral colonies and sponges. Other distinctive features of the studied site include the absence of Acropora spp. corals, which are common across other Polynesian coral reefs, and a mean live coral cover of only ~ 5%. Sampling was realized at two seasons, in August 2016 (austral winter) and..., , # Data from: Challenging trophic position assessments in complex ecosystems: calculation method, choice of baseline, trophic enrichment factors, season and feeding guild do matter. A case study from Marquesas Islands coral reefs

This dataset relates to the nitrogen isotopic signatures of various marine species in the Marquesas Islands (French Polynesia), for a total of about 750 fish and invertebrates, as well as almost 110 samples of macroalgae and phytoplankton. The samples were collected in August 2016 (austral winter) and March 2017 (austral summer); sampling details are given in the articles by Fey et al. 2020 ([https://doi.org/10.1007/s00227-020-03703-z](https://doi.org/10.1007/s00227-020-03703-z)) and Fey et al. 2021 ([https://doi.org/10.1038/s41598-021-00348-w](https://doi.org/10.1038/s41598-021-00348-w)). The data are presented in tables in Word format but can be easily copied and pasted into Excel or equivalent spreadsheet format.

The first table concerns “bulk” nitrogen iso...