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

• Published in: Progress in oceanography Vol. 130; pp. 157 - 171
• Main Authors: Kopp, Dorothée, Lefebvre, Sébastien, Cachera, Marie, Villanueva, Maria Ching, Ernande, Bruno
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.01.2015; Elsevier
• Subjects: Carbon; case studies; coasts; Consumers; continental shelf; diet; EEC; English Channel; European Union; Foods; Joining; Marine; Networks; nitrogen; particulate organic matter; predatory fish; Sciences of the Universe; stable isotopes; statistical models; trophic levels; Variance; ANE, Europe, English Channel; English Channel
• DOI: 10.1016/j.pocean.2014.11.001
• ISSN: 0079-6611

•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. 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 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.