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

and tile (random, microhabitat nested in tile nested in seeding and site, 5 levels). Analyses of cover, abundance and MaxN data also included the factor time (random repeated measure, 3 levels). As no pelagic fishes were recorded on the tiles, we did not test the effects of microhabi- tat identity on the species density or MaxN of this functional group. A third set of GLMMS tested for differences in the range of temperature (minimum, maxi- mum and standard deviation), humidity and light levels among microhabitats of both the complex and flat plates (2 fixed levels = crevice or ridge vs. flat) and between the two levels of seeding with native oysters (2 fixed levels = unseeded vs. seeded). These analyses also had the factors site, area and time (random repeated measures, 6 random levels for temperature, 3 lev- els for humidity and 2 levels for light). All statistical analyses were undertaken in R 3.5.0 [46]. The generalised linear mixed effects models were conducted using the packages lme4 and figures were produced using ggplot 2 [47]. Likelihood ratio tests comparing models with and without the random effects were used to obtain p-values for the random effects. In the models, species density, percentage cover and counts (i.e. mobile invertebrate abundance and fish MaxN) were modelled using a Poisson dis- tribution with a log link function. The average and standard deviation of temperature, average humidity and light levels were modelled using a Gaussian distribution. All models were

checked for over-dispersion and spatial and temporal autocorrelation with plots and the resid- uals were visually inspected for heteroscedasticity. Where appropriate, post hoc comparisons were undertaken using eemeans to identify sources of treatment effects. Results A total of 80 taxa (excluding S. glomerata, the seeded species) were identified on tiles during the study (S1 Table). These taxa were classified into four main groups: sessile algae and inverte- brates (9 taxa), mobile invertebrates (39 taxa), cryptobenthic fishes (7 taxa) and pelagic fishes (13 taxa) (S1 Table). At 12 months, the number of different species recorded across all tiles of each treatment was: flat unseeded—24 taxa; flat seeded—43 taxa; complex unseeded—34 taxa; and complex seeded—40 taxa. The most abundant taxa identified during the in-situ sampling were for each of the four main groups: the barnacle Elminius modestus (sessile algae and invertebrates; cover, mean = 19.650%, SE = +/- 1.947%), the conniwink Bembicium nanum (mobile invertebrates; count per tile, mean = 2.308, SE = +/-1.342); the oyster blenny, Omobranchus anolius (crypto- benthic fishes; MaxN, mean = 0.329, SE = +/- 0.053) and the yellowfin bream Acanthopagrus australis (pelagic fishes; MaxN, mean = 0.428, SE = +/- 0.019). Destructive sampling at month 12 revealed the barnacle Elminius modestus (cover, mean = 16.996%, SE = +/- 2.358%) was the most abundant sessile species and the limpet Patelloida mimula (count per tile, mean = 5.925, SE = +/- 1.089) was the most abundant mobile invertebrate on the tiles. For each analysis,

except for the number of live seeded S. glomerata and the abundance of mobile taxa, there was no significant difference in the effects of treatments between sites (S2–S4 Tables), allowing sites to be pooled together for the post-hoc tests. Effect of habitat structure on the number of live seeded S. glomerata, and of habitat structure and seeding on the percentage cover of oysters The effect of habitat structure on the number of live seeded oysters differed between the two study sites (Fig 2A and 2B, S2 Table). At site 1, where survivorship of the oysters was greater, there were no detectable differences in the number of live S. glomerata between the complex and the flat tiles, at any of the sampling times (Fig 2A, S2 Table). At site 2, where greater mor- tality was observed, the number of live S. glomerata was across all sampling times significantly 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 8 / 21