Is the trophic diversity of marine benthic consumers decoupled from taxonomic and functional trait diversity? Isotopic niches of Arctic communities

It is predicted that a diverse array of functional traits in species‐rich assemblages can lead to strong resource partitioning among coexisting species and moderate a wider spectrum of resource use. We compared two benthic communities in an Arctic fjord: a species‐rich community (in an outer basin) and an impoverished community (in a glacially impacted bay) and explored (1) if high species richness was translated into high functional trait richness and (2) if high taxonomic and functional diversity promoted high trophic diversity in terms of resource use (indicated by isotopic niche measures). We documented higher functional trait richness in the outer basin (computed based on traits describing feeding mode, mobility, food source, body size and life habit), but the area occupied by consumers in the δ15N vs. δ13C iso‐space (a proxy for total trophic resource use) did not differ between the two sites. A wide array of functional traits used to acquire food may extend the benthic community trophic niche spatially (where and how animals forage) without impacting the isotopic niche breadth (in this system, mostly reflecting “what animals feed on”) due to the relatively homogenous distribution of isotopic characteristics of detritus pool across vertical scales in marine sediments. Moreover, this trend could indicate that a species‐poor community tends to exploit all the available food items, possibly due to the low food availability for primary consumers in a glacially impacted environment. Communities in glacial bays could therefore be particularly sensitive to future changes in glacial inputs and associated organic matter fluxes.