LAUSR

Abstract Arctic colonial seabirds are recognized as effective fertilizers of terrestrial ecosystems by delivering marine-origin nutrients to the vicinities of their nesting sites. A proportion of this ornithogenic matter is then thought to return to the sea and, concentrated within a smaller area, locally provides additional nutrients for the nearshore marine communities. The aim of this study was to assess the presence and impact of local ornithogenic enrichment on two important elements of the Arctic coastal food web: (1) the planktonic pathway originating in the surface water, and (2) the benthic pathway based on benthic primary production. We sampled two areas in Isfjorden (Spitsbergen): one located below a coastal mixed breeding colony of guillemots and kittiwakes, and a control area not influenced by the colony. Slightly higher nitrogen stable isotope ratios (δ15N) were found in particulate organic matter suspended in the surface water (POM), sedimentary organic matter (SOM) from outside the zone of dense kelp forest, and the predatory/scavenging whelks Buccinum sp. collected below the seabird colony (the components recognized as following the planktonic path). In contrast, no ornithogenic isotopic enrichment was detected in the herbivorous gastropod Margarites helicinus or in SOM from the kelp zone (benthic path). The data are compatible with those obtained from the same location a year before, showing δ15N enrichment in predatory/scavenging hermit crabs Pagurus pubescens below the seabird, and no such changes in kelps Saccharina latissima or their presumed consumers, sea urchins Strongylocentrotus droebachiensis (Zmudczynska-Skarbek et al., 2015a ). The results suggest that, in the conditions of periodic, short-term pulses of ornithogenic nutrient inputs to the local marine environment, which typify the short High Arctic summer, planktonic organisms are the initial organisms to incorporate these nutrients before transfer to the benthic food web via pelagic–benthic coupling. However, the supply of ornithogenic resources alone is insufficient to support benthic producers and the trophic pathways based on them. Overall, the ornithogenic subsidies are not a major nutrient source for marine organisms from below the seabird colony.