Interacting effects of habitat structure and seeding with oysters on the intertidal biodiversity of seawalls

data loggers (Thermodata Pty. Ltd. Warrnambool, Australia). We placed one iButton haphaz- ardly on each flat tile (unseeded and seeded), and two iButtons (one in the crevice and one on the crest of the ridge) haphazardly on each complex tile (unseeded and seeded), either inside the oyster clumps (for seeded tiles) or outside the oyster clumps (for unseeded tiles). The iBut- tons were waterproofed with Plastidip rubber coating (Plasti Dip International, Blaine, Minne- sota, USA). The iButtons were attached to the tile with a cable tie, which was threaded through a plastic straw, glued to the tile with non-toxic epoxy (RMF—Vivacity Marine Ltd Pty). This enabled iButtons to be interchanged, so as to not exceed memory capacity, throughout the experiment, without damaging the tiles or the colonising organisms. The iButtons were pro- grammed to record temperatures at 20 min intervals, across a one-month period, with 0.5˚C accuracy. These measurements were used to calculate the minimum, maximum, range and standard deviation of temperature recorded across each month (16). One, 6 and 12 months, after the tiles were deployed, we took measurements of humidity with six DS1923 Hygrochron iButtons (Thermodata Pty. Ltd) and readings of light with the external fibre optic sensor from a Diving PAM fluorometer (Walz Pty. Ltd.). As the Hygro- chron iButtons could not get wet and the Diving PAM could only be used to make spot mea- surements, readings of humidity and light were limited to low tide, with tiles sampled in

random order with respect to treatment over two consecutive days. For each of humidity and light, we took one haphazardly positioned measurement from each flat tile and two measure- ments from each complex tile (one in the crevice and one on the crest of the ridge). The Hygrochron iButtons took measurements at 2 min intervals for 16 min, with the 8 measure- ments per tile averaged to give a single value that was used in analyses. Analyses For each of the four functional groups (i.e. sessile taxa excluding S. glomerata, mobile inverte- brates, cryptobenthic fishes, pelagic fishes), we calculated two univariate metrics: (1) species density (i.e. the number of different species per tile, crevice or ridge), and (2) either cover (for sessile organisms), counts (for mobile invertebrates) or MaxN (for fish). Species density and abundance were calculated separately, at the level of tile and, for complex tiles, also separately for crevices and ridges to assess how the two different microhabitats influenced the biodiver- sity. The species density was calculated at the final time point, 12 months, via destructive sam- pling for sessile and mobile organisms or in situ sampling for cryptobenthic or pelagic fishes, while the abundance metrics were calculated separately at each time point using in situ sam- pling and at month 12 using destructive sampling (for sessile and mobile taxa). We used generalised linear mixed models (GLMMs) to test the effects of the initial treat- ment conditions, adding habitat structure (fixed, 2 levels: flat, complex), seeding with oysters

(fixed, 2 levels: unseeded, seeded), site (random, 2 levels) and tile surface area (offset, m2), on the species density of each of the four functional groups at 12 months. Analogous GLMMs including these same factors as well as month (random repeated measures, 3 levels) were used to test for the effects of adding habitat structure and seeding with oysters on the percentage cover of S. glomerata (live and dead, transplants and colonising oysters) as well as the abun- dance or MaxN, of the sessile and mobile taxa and cryptobenthic fishes and pelagic fishes. The effect of adding habitat structure on the number of live seeded S. glomerata (transplants only) was assessed using a GLMM that excluded the factor seeding. To determine the effect of microhabitat identity on each of the same metrics for the sessile and mobile taxa and cryptobenthic fish, a second set of GLMMs were run. For species density data collected at 12 months, these had the factors: microhabitat (fixed 2 levels: crevice, ridge), seeding with oysters (fixed, 2 levels: unseeded, seeded), site (random, 2 levels), area (offset m2) PLOS ONE Effects of microhabitats and seeding with oysters on intertidal biodiversity PLOS ONE | https://doi.org/10.1371/journal.pone.0230807 July 16, 2020 7 / 21