DOI: 10.5061/dryad.0vt4b8h0r
title: Data for Land-sea linkages depend on macroalgal species, predator invasion history in a New Zealand archipelago

description: 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.
description: 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 <em>et al.</em> 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
 δ<sup>15</sup>N was analysed using an elemental analyzer
 (Costech ECS 4010) coupled with a mass spectrometer (Thermo-Finnigan
 DELTAplus Advantage) with atmospheric N<sub>2</sub> 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
 (<em>Ecklonia radiata, Xiphophora chondrophylla</em>,
 <em>Carpophyllum maschalocarpum, C. flexuosum, C.
 plumosum,</em> and<em> Vidalia colensoi</em>). 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
 δ<sup>15</sup>N using an elemental analyzer (Costech ECS 4010)
 coupled with a mass spectrometer (Thermo-Finnigan DELTAplus Advantage)
 with atmospheric N<sub>2</sub> 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 δ<sup>15</sup>N in macroalgae are depth and wave
 exposure. Wave exposure is mean weighted fetch calculated using the
 <em>fetchR</em> package [31]. Fetch-based indices are reliable
 in quantifying wave exposure across large areas in shallow systems [32].
 Seabird influence is measured as soil δ<sup>15</sup>N, which
 is highly correlated with burrow density and not independent (Fukami
 <em>et al.</em> 2006). Burrow density is calculated as burrow
 openings per m<sup>2</sup>, as a proxy for seabird activity
 (Fukami <em>et al.</em> 2006), within a 5m radius (78.54
 m<sup>2</sup>) around each sample site.