Seabirds on islands create a circular seabird economy - whereby they feed in the ocean, transport marine-derived nutrients onshore to their breeding colonies, and then seabird-derived nutrients run off into the ocean, enriching nearshore ecosystems. Invasive predators reduce seabird colonies and nutrient subsidies; thus, predator eradication is critical for restoring seabird islands. Few studies have linked nearshore marine recovery to terrestrial ecosystem attributes and none have in temperate zones. Here, we tested the influence of seabird driven soil nutrients, terrestrial abiotic variables, and marine variables on nearshore algal communities using in-depth repeated sampling of four islands in a New Zealand island archipelago. We show seabird land-sea linkages are disrupted when predators invade, and are not restored three decades after invasive predator removal in the studied archipelago. Our study is the first account of how seabird influences on land impact nearshore marine environments on islands cleared of invasive predators in temperate ecosystems. Such information can help provide baseline information for which abiotic and ecological variables are most important when studying the linkages from land to sea in island ecosystems.

Terrestrial soil samples were taken approximately 25m apart to ensure spatial independence [27]. The biopedturbation of the soil by seabird burrowing activity has not been found to influence the vertical stratification of soil nutrients in soil depths up to 50cm (Fukami et al. 2006). Using a trowel, we took approximately 100g of soil at 2-10cm depth from three locations around each sample site to analyse for stable isotopes. In high density nesting areas, we took soil samples at the surface to avoid damaging burrows or disturbing birds. The three soil samples were homogenized in the field and pH was measured from a slurry of soil (10g) from 2-10cm depth with deionized water (50ml) using a calibrated Eketcity handheld pH meter (Anaheim, CA, USA). 

To analyse soil stable isotopes, samples were sieved with a 1mm sieve, oven dried (55°C, ~48 h), ball milled to a fine powder (400 rpms for 3 minutes; Retsch PM 100 Ball Mill, Germany), and weighed (15 mg, precision 0.01mg). Soil δ¹⁵N was analysed using an elemental analyzer (Costech ECS 4010) coupled with a mass spectrometer (Thermo-Finnigan DELTAplus Advantage) with atmospheric N₂ as the nitrogen standard.

To collect the nearshore marine samples, four to six sampling areas were established around each study island, grouped by invasion history (Fig. 1). Each sampling area consisted of a pair of SCUBA transects ~30m apart, resulting in 8-12 transects per study island/invasion history group. Transects began at the shoreline and ran through the intertidal zone, ending in the subtidal zone at a depth of 5 meters. Macroalgae species and their percent cover were documented within a 1-meter by 1-meter quadrat placed every meter in depth along the transects. Due to ocean conditions, it was not possible to sample every depth and transect in both seasons. The total number of quadrats ranged from 49–100 per island/group over both seasons (Supplemental Materials; Table S). Samples from the six most common macroalgae species found at the study islands were collected from quadrats for stable isotope analysis (Ecklonia radiata, Xiphophora chondrophylla, Carpophyllum maschalocarpum, C. flexuosum, C. plumosum, and Vidalia colensoi). Each macroalgae sample was cleaned of epiphytic algae, rinsed with deionized water, and dried at 55°C for 48-72 hours. The most recent growth from the tip of each sample was removed, ground into a fine powder, weighed (4-7mg, precision 0.01mg), packed into tin capsules, and analyzed for δ¹⁵N using an elemental analyzer (Costech ECS 4010) coupled with a mass spectrometer (Thermo-Finnigan DELTAplus Advantage) with atmospheric N₂ as the nitrogen standard.

Compaction is a proxy for soil porosity and water infiltration [29], which was measured ten times and an average taken at each terrestrial sample site (Fig. 1; Dickey-John soil compaction tester; Auburn, IL, USA). Runoff is a unitless measure of flow accumulation calculated as a raster of the total number of cells that drain into each individual cell. It is based on flow direction from a digital elevation model (DEM) [30] using the flow accumulation algorithm (Hydrology toolset, Spatial Analyst toolbox, ArcGIS).

Marine variables that could influence δ¹⁵N in macroalgae are depth and wave exposure. Wave exposure is mean weighted fetch calculated using the fetchR package [31]. Fetch-based indices are reliable in quantifying wave exposure across large areas in shallow systems [32]. Seabird influence is measured as soil δ¹⁵N, which is highly correlated with burrow density and not independent (Fukami et al. 2006). Burrow density is calculated as burrow openings per m², as a proxy for seabird activity (Fukami et al. 2006), within a 5m radius (78.54 m²) around each sample site.