Abstract
Studies of trophic interactions give valuable insights into the functioning of ecosystems and can be used to identify qualitative differences among ecosystems. Here, we use natural stable isotope concentrations (δ13C and δ15N) to study the food-web structure in four contrasting locations on the northern European continental shelf: two sub-Arctic locations in the Lofoten-Vesteralen region (fjord vs. open shelf) and two temperate locations (northern vs. southern North Sea). Phytoplankton was identified as the major primary producer in all studied ecosystems, even in the sub-Arctic fjord, where macroalgae only played a minor role in the food web. We used mixing models to determine the relative reliance on prey of benthic affinity and found that reliance on benthic prey was higher in the North Sea than in the Lofoten-Vesteralen region. Atlantic cod (Gadus morhua) was identified as the single top-predator in the North Sea, utilizing food from both, benthic and pelagic trophic channels. More separate benthic and pelagic trophic channels characterize the Lofoten-Vesteralen region, where G. morhua acts as part of the pelagic food chain. Furthermore, our data indicate that the recent mesopredator outburst in the southern North Sea might have been enhanced by reduced predation pressure due to the collapse of the local cod stocks. We conclude that the resilience towards a high fishing pressure is higher in the Lofoten-Vesteralen region than in the North Sea.
Highlights
Food-web structure is a fundamental feature of marine ecosystems as it gives insight into how energy and contaminants are transferred from low trophic positions to upper trophic-level consumers (Hobson et al, 2002; Jardine et al, 2006; Schückel et al, 2015)
Sediment was significantly enriched in δ15N compared to particulate organic matter (POM) (p < 0.001 [Analysis of variance (ANOVA)]), while no difference in δ13C was observed between the two carbon sources
Our study has shown that the coupling of the benthic and pelagic compartments of food webs differs between the LofotenVesterålen region and the North Sea, and food webs might be affected differently by environmental changes (Krause et al, 2003)
Summary
Food-web structure is a fundamental feature of marine ecosystems as it gives insight into how energy and contaminants are transferred from low trophic positions to upper trophic-level consumers (Hobson et al, 2002; Jardine et al, 2006; Schückel et al, 2015). The northern European continental shelf, one such productive region, contains contrasting ecosystems, ranging from the temperate regions of the southern North Sea to the Arctic regions of northern Norway Despite these differences, the marine ecosystems of the northern European continental shelf are characterized by some key overarching features: (1) Pronounced seasonality in the ecosystems with a strong influence of the North Atlantic Current (Neumann and Kröncke, 2011; Silberberger et al, 2016), (2) Calanus as the key zooplankton taxon and an important link between primary production and higher trophic levels (Williams et al, 1994; Espinasse et al, 2016), and (3) large stocks of commercially important fish species are sustained by these ecosystems (Føyn et al, 2002; Ehrich et al, 2007). Comparing food web structures and their key trophic groups across climate zones might reveal potential consequences of natural and anthropogenic pressures on these ecosystems
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