Bottom-up Effects of Seabird Colonies on Arctic Benthic Communities Mediated by Kelp Forests

Arctic seabird colonies concentrate marine-derived nutrients on land through the deposition of guano, which fertilizes the vegetation and supports local consumers. However, the process in which nutrients derived from seabirds are not fully consumed on land and subsequently return to the sea, accumulating beneath bird cliffs, is not widely recognized, particularly in non-tropical and non-oligotrophic ecosystems. This study aimed to assess the impact of ornithogenic nutrient enrichment on the structure of the benthic community in the shallow, rocky subtidal zone beneath a seabird colony in the Svalbard archipelago. Significant differences in benthic community composition were observed between the seabird and Reference locations, with higher diversity and greater abundance of benthic filter-feeders beneath the bird cliffs. The nitrogen stable isotope ratio values (δ15N) of selected benthic invertebrates were higher in the studied taxa adjacent to colonies than in the reference areas, indicating the uptake of seabird-derived nitrogen. However, the presence of kelp forests modified this effect. Chitons (Tonicella sp.) and hermit crabs (Pagurus pubescens) appeared to use ornithogenic nutrients effectively beyond the kelp zone, whereas whelks (Buccinum sp.) appeared to utilise them closer to the shore among dense kelp. These findings improve our understanding of nutrient transfer processes and the complex interactions between terrestrial and marine ecosystems in the Arctic. The ornithogenic fertilization effect extends beyond the coastline, although it is probably spatially limited and more subtle in nearshore waters than on land as nutrients disperse with waves and currents.