Land–sea linkages depend on macroalgal species, predator invasion history in a New Zealand archipelago

Abstract

Seabirds on islands create a circular seabird economy—they feed in the ocean, transport marine‐derived nutrients onshore to their breeding colonies, and then seabird‐derived nutrients runoff into the ocean, enriching nearshore ecosystems. Invasive predators reduce seabird‐driven nutrient subsidies; thus, predator eradication is critical for restoring seabird islands. Terrestrial recovery of seabird nutrients following predator eradication can take mere decades or longer, but few studies have linked nearshore marine recovery to terrestrial ecosystem attributes. We tested the influence of season, seabird variables, terrestrial abiotic variables, and marine variables on nearshore algal communities surrounding islands in a New Zealand island archipelago. The link from terrestrial to marine proved strong, with over 40% of the variation in macroalgae community composition being driven by a combination of seabird and terrestrial abiotic variables. Season and depth were the strongest contributors to nitrogen enrichment. Two macroalgae species had seabird‐derived nitrogen levels similar on eradicated and never invaded islands, but four more had lower seabird‐derived nitrogen on eradicated versus never invaded islands. Our results suggest seabird land–sea linkages are disrupted when predators invade, and are not restored three decades after invasive predator removal in the studied archipelago. Moreover, some marine algae species are better indicators of seabird influence than others. This study provides baseline information about which abiotic and ecological variables are important when studying the linkages from land to sea in island ecosystems.