Abstract
Stable isotopes have provided important insight into the trophic structure and interaction in many ecosystems, but to date have scarcely been applied to the complex food webs of coral reefs. We sampled white muscle tissues from the fish species composing 80% of the biomass in the 4–512 g body mass range at Cape Eleuthera (the Bahamas) in order to examine isotopic niches characterised by δ13C and δ15N data and explore whether fish body size is a driver of trophic position based on δ15N. We found the planktivore isotopic niche was distinct from those of the other trophic guilds suggesting the unique isotopic baseline of pelagic production sources. Other trophic guilds showed some level of overlap among them especially in the δ13C value which is attributable to source omnivory. Surprising features of the isotopic niches included the benthivore Halichoeres pictus, herbivores Acanthurus coeruleus and Coryphopterus personatus and omnivore Thalassoma bifasciatum being close to the planktivore guild, while the piscivore Aulostomus maculatus came within the omnivore and herbivore ellipses. These characterisations contradicted the simple trophic categories normally assigned to these species. δ15N tended to increase with body mass in most species, and at community level, the linear δ15N–log2 body mass relationship pointing to a mean predator–prey mass ratio of 1047:1 and a relatively long food chain compared with studies in other aquatic systems. This first demonstration of a positive δ15N–body mass relationship in a coral reef fish community suggested that the Cape Eleuthera coral reef food web was likely supported by one main pathway and bigger reef fishes tended to feed at higher trophic position. Such finding is similar to other marine ecosystems (e.g. North Sea).
Highlights
In coral reef food webs, fishes are typically categorised into strict trophic guilds (e.g. Hiatt and Strasburg 1960; Jennings et al 1995; Polunin 1996; McClanahan et al 1999; Hughes et al 2003; MacNeil et al 2015; D’Agata et al 2016; Graham et al 2017; Stamoulis et al 2017; Hadi et al 2018; Moustaka et al 2018)
Isotopic niches at species and trophic guild levels
Mean species δ13C ranged from − 17.4 ± 0.2‰ (Halichoeres pictus) to − 9.5 ± 0.0‰ (Lutjanus griseus), while mean δ15N ranged from 3.9 ± 0.8‰ (S. aurofrenatum) to 9.9 ± 1.5‰ (Sphyraena barracuda). δ15N values were significantly but weakly correlated with δ13C (p < 0.05, ra2djusted = 0.29 ) at the species level
Summary
In coral reef food webs, fishes are typically categorised into strict trophic guilds (e.g. Hiatt and Strasburg 1960; Jennings et al 1995; Polunin 1996; McClanahan et al 1999; Hughes et al 2003; MacNeil et al 2015; D’Agata et al 2016; Graham et al 2017; Stamoulis et al 2017; Hadi et al 2018; Moustaka et al 2018). Hiatt and Strasburg 1960; Jennings et al 1995; Polunin 1996; McClanahan et al 1999; Hughes et al 2003; MacNeil et al 2015; D’Agata et al 2016; Graham et al 2017; Stamoulis et al 2017; Hadi et al 2018; Moustaka et al 2018). Combining δ13C and δ15N delineates ‘isotopic niches’ (Leibold 1995; Newsome et al 2007) which inform feeding strategies and trophic
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