Seabirds are keystone species, providing inter-habitat connectivity by transporting vast quantities of nutrients from their pelagic feeding grounds to terrestrial and marine ecosystems, dictating community structure and dynamics, and providing multiple ecosystem services. This flow of nutrients is being disrupted by human activities, such as invasive species, and seabirds are now one of the most threatened bird groups. Whether seabird nutrients are cycling through nearshore, temperate systems, in particular in the United Kingdom (UK), impacting biodiversity and productivity, and how these inputs may vary temporally, remains unknown. The impact of nutrient subsidies to the Farne Islands, Northumberland, areas of high seabird densities during breeding season, was compared to nearby low seabird density areas on the Northumberland coast. Seabird densities and nitrogen inputs of up to 2334 and 1054 times higher, respectively, on seabird islands resulted in both substantially enriched nitrogen stable isotope (δ15N) values and higher total nitrogen content in terrestrial and intertidal organisms, including soil, terrestrial plants, lichen, barnacles, and limpets, and just higher δ15N values in macroalgae and turf algae. Temporal variation was also detected, though, surprisingly, nitrogen loads were higher during early breeding season. Some support was found for increased biodiversity in intertidal areas of seabird islands, in higher densities of intertidal predators and marginally greater algal species richness, though diversity differences were small overall. A marginal difference in limpet size was found, whilst on plots artificially cleared to simulate a disturbance event, barnacles were unexpectedly found to recolonise faster on mainland sites, which was likely caused by the mainland’s increased exposure level. These findings provide evidence that seabirds are cycling pelagic nutrients into terrestrial and intertidal ecosystems on UK islands, though support for seabird nutrient input increasing biodiversity and productivity was mixed. Given the threats to seabirds globally, these findings promote seabird conservation, including the removal of invasive species, such as rat eradication programmes.