Utilization of nitrogen derived from seabird guano by terrestrial and marine plants at St. Paul, Pribilof islands, Bering sea, Alaska

Dense, conspicuous colonies of seabirds and pinnipeds breed on ocean islands throughout the world. Such colonies have been shown to have local impacts on prey populations, but whether or not they affect nutrient cycling has been debated. We determined the natural abundance levels of the stable isotopes (C and N) of primary producers, seabirds and other consumers at and near St. Paul Island, Pribilof Islands, Bering Sea, in summer 1993. Marine primary producers (phytoplankton, as particulate organic matter, and kelp) collected near seabird colonies were ca. 6.5 parts per thousand, enriched in both N-15 and C-13 relative to those collected further from shore. Terrestrial plants collected near the seabird colonies were enriched in N-15 (delta(15)N ca. 22 parts per thousand) compared with conspecifics collected away from the colonies (delta(15)N ca. 11 parts per thousand). The trend towards higher delta(15)N values in marine and terrestrial plants near bird colonies is consistent with their uptake of ornithogenic N. This N-15-enrichment of plants using ornithogenic N can be attributed to a combination of two processes: trophic enrichment, and volatilization of ammonia produced during degradation of terrestrially deposited guano. Seabird breeding colonies at St. Paul Island appear to be significant sources of recycled nitrogen for terrestrial plants in the vicinity of colonies and for phytoplankton in the nearshore zone.