Food web structure in relation to environmental drivers across a continental shelf ecosystem

Abstract

Quantification of the physical and biological factors that influence the spatial structuring of food webs is central to inform effective resource management. We used baseline-corrected stable isotope ratios (δ 13 C and δ 15 N) of 63 invertebrate and fish to investigate food web structure across a continental shelf gradient-the Celtic Sea Shelf in the Northeast Atlantic Ocean. Hierarchical clustering on δ 13 C and δ 15 N showed that the shelf food web is characterized by four trophic levels with trophic groups spread across pelagic and benthic trophic pathways. Four biomass-weighted isotopic diversity metrics provided indicators on the status of the system, showing a relatively complex food web with high trophic redundancy at intermediate trophic levels suggesting resilience to disturbances. Two sets of statistical models, at the community scale and for each trophic group, identified five distinct trophic assemblages associated with different chlorophyll a concentrations, water depth, and bottom temperature. A cold, vertically mixed-water assemblage over the outer shelf comprised the largest habitat and most diverse assemblage, highlighting the importance of cold productive conditions in the Celtic Sea. Trophic group model results were used to generate spatial area predictions to compare functioning of groups using isotopic overlap (similarity and nestedness) metrics. Isotopic niche area was larger (spanning two trophic levels) in shallow habitats, but not in habitats underlying high primary production or nutrient-rich water masses, suggesting stronger benthic-pelagic trophic coupling in inner shelf habitats. Results suggest that depth and intensity of pelagic production are major drivers of trophic structure and functioning of Celtic Sea communities.

Functional structure and diversity of marine benthos reflect food availability and quality, as evidenced by isotopic diversity

“…The classical ecological theory (Macarthur and Levins 1967) predicts that a high overlap of the feeding niche between species will lead to an increase in competition, which would result in either ecological differentiation or local extinction. It was shown that in communities with high feeding niche overlap dominant species tend to maximize the resource use through higher feeding plasticity (Lesser et al 2020; Walters et al 2021). Similarly, higher FEve at deeper sites both at the open coast and in the river vicinity (Fig.…”

EcoDiet: A hierarchical Bayesian model to combine stomach, biotracer, and literature data into diet matrix estimation

“…Because of the environmental diversity characterizing the Celtic Sea (Hernvann et al, 2020; Walters et al, 2021), the generalist feeding behavior and ontogenetic diet shifts of some species (Day et al, 2019; Issac et al, 2017; Rault et al, 2017), the functional redundancy of several groups (Mérillet et al, unpublished manuscript), and benthic–pelagic coupling (Hernvann et al, 2020), data exhibited high intragroup variability and among‐group overlap in isotope signatures. Therefore, posterior distributions of dietary proportions were often diffuse, pointing out the limits of bulk SIA for studying such complex ecosystems.…”

Isotopic Niche and Trophic Position of the Invasive Portunid Portunus segnis Forskål, (1775) in Elounda Bay (Crete Island, Eastern Mediterranean)

“…The choices of appropriate baseline species and Trophic Enrichment Factors (TEF hereafter) were key steps in ensuring robust trophic position estimations [52]. Suspension feeding bivalves have long been recognized as reliable baseline references in studies of freshwater [53,54] and, more recently, marine food webs [55,56], since they have a relatively long biological cycle, a low metabolic rate, and are able to integrate highly variable isotope values among primary producers [53,57]. Here we used the cumulated δ 13 C and δ 15 N values of the two suspensivore bivalves Modiolus barbatus and Pinctada radiata as baseline, assuming a trophic level λ = 2 after a PERMANOVA indicated negligible inter-specific differences (see Results).…”