Reorganization of a marine trophic network along an inshore–offshore gradient due to stronger pelagic–benthic coupling in coastal areas

Abstract

Recent theoretical considerations have highlighted the importance of the pelagic-benthic coupling in marine food webs. In continental shelf seas, it was hypothesized that the trophic network structure may change along an inshore-offshore gradient due to weakening of the pelagic-benthic coupling from coastal to offshore areas. We tested this assumption empirically using the eastern English Channel (EEC) as a case study. We sampled organisms from particulate organic matter to predatory fishes and used baseline-corrected carbon and nitrogen stable isotope ratios (δ13C and δ15N) to determine their trophic position. First, hierarchical clustering on δ13C and δ15N coupled to bootstrapping and estimates of the relative contribution of pelagic and benthic carbon sources to consumers' diet showed that, at mesoscale, the EEC food web forms a continuum of four trophic levels with trophic groups spread across a pelagic and a benthic trophic pathway. Second, based on the same methods, a discrete approach examined changes in the local food web structure across three depth strata in order to investigate the inshore-offshore gradient. It showed stronger pelagic-benthic coupling in shallow coastal areas mostly due to a reorganization of the upper consumers relative to the two trophic pathways, benthic carbon sources being available to pelagic consumers and, reciprocally, pelagic sources becoming accessible to benthic species. Third a continuous approach examined changes in the mean and variance of upper consumers' δ13C and δ15N with depth. It detected a significant decrease in δ13C variance and a significant increase in δ15N variance as depth increases. A theoretical two-source mixing model showed that an inshore-offshore decrease in the pelagic-benthic coupling was a sufficient Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site. condition to produce the δ13C variance pattern, thus supporting the conclusions of the discrete approach. These results suggest that environmental gradients such as the inshore-offshore one should be accounted for to better understand marine food webs dynamics. Highlights ► Marine food web is reorganized along a seaward gradient in the English Channel. ►Stronger coupling of the pelagic and benthic pathways from offshore to the coast. ►Shift of pelagic predators to the benthic pathway in shallow areas. ►Adaptive foraging of pelagic predators may explain food web reorganization.

Trophic ecology of large gadiforms in the food web of a continental shelf ecosystem

“…Trophic functioning in these areas is linked to terrigenous inputs that stimulate primary production and, as a result, benefit organisms of higher trophic levels and thus fisheries. In these ecosystems, the food web is structured mainly around interconnected pelagic and benthic pathways (Blanchard et al, 2011, Kopp et al, 2015. In the pelagic pathway, nutrients fuel the primary production consumed by zooplankton grazers that are themselves preyed on by invertebrate predators and pelagic fish.…”

“…Habitats can provide different prey depending on their environmental variables (depth, salinity, sediment type, etc.). Depth is an important factor as it regulates the benthic-pelagic connection, and it regulates the degree of interactions between the water column and the water near the bottom (Schindler and Scheuerell, 2002;Kopp et al, 2015;Giraldo et al, 2017).…”

Temporal variation in trophic relationships among three congeneric penguin species breeding in sympatry

“…The main difference between the isotopic niches of chinstrap vs Adélie and gentoo penguins was the large variation in individual δ 13 C in the former species. Variability in δ 13 C values in marine organisms can be associated with distance from shore and whether the organism feeds in the pelagic (Cherel & Hobson, ; Hobson et al., ; Kopp, Lefebvre, Cachera, Villanueva, & Ernande, ) or benthic food webs. Chinstrap penguins appeared to use a wider range of foraging habitats than the two other species, although this was not directly reflected in their individual δ 13 C values.…”

Are food web structures well represented in isotopic spaces?

“…Such generalization must, however, be made with caution, given the large variability in trophic structures encountered within biomes (Schoener 1989). For instance, a general comparison of the reliability of isotopic approaches in benthic and pelagic food webs is challenging, since benthic food webs tend to harbour less trophic levels (Schoener 1989) but larger levels of omnivory (Kopp et al 2015) than pelagic ones. These two types of variation in trophic structure are likely to have compensatory influences on the isotopic reliability within these two food web types (Fig.…”