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

recorded on flat tiles as on ridges, irrespective of seeding (Fig 7A, S7 Table). The crevices and flat microhabitats displayed no difference in humidity at 1 month and displayed differences only when unseeded at 6 months, but by 12 months crevices were more humid than flat micro- habitats, independent of seeding (Fig 7A, S7 Table). Discussion Our study provides the first experimental test of how habitat structure (crevices/ridges) and seeding with habitat-forming species (oysters) interact to influence the species density and abundances of four functional groups of organisms (sessile and mobile taxa and cryptobenthic and pelagic fishes) on vertically orientated substrate, in a highly modified urban marine envi- ronment. The two types of experimental manipulations generally had positive effects on the species density and abundances. Nevertheless, whether the effects of habitat structure or seed- ing with oysters were independent or interactive varied among functional groups. The addition of habitat structure had a positive influence on the survival of seeded S. glo- merata at the site with the highest morality and, at both sites, on the percentage cover of seeded and colonising oysters. Consequently, by 12 months, we found a similarly high percentage cover of oysters on seeded and unseeded complex tiles, although in the absence of habitat structure, oyster cover was greater on seeded than unseeded tiles. The generally greater sur- vival and colonisation of oysters on the complex tiles relative to the flat tiles, appeared to be

due to the protective role of the crevices, which reduced oyster mortality. The crevices may have protected oysters against both predation by pelagic fishes, which were more abundant around the seeded than unseeded tiles during the first 6 months, and summer low tide temper- ature extremes, which can be lethal to juvenile oysters [7, 48]. Maximum temperatures recorded in the crevices were up to 6.1˚C cooler than on the ridges or flat plates in the first few months of our study, which corresponded to the Austral summer. Irrespective of the mecha- nism, these results highlight the importance of habitat structure in providing a refuge for juve- nile oysters against predatory and environmental stressors. Interestingly, there were no long-term effects of seeding with S. glomerata on the percentage cover of oysters on the complex tiles. Each of our study sites contained low density populations of wild oysters (relative to nearby rocky shores) [43] and oysters recruited in significant num- bers to crevices, swamping the effects of seeding. Hence, at sites with a supply of S. glomerata recruits, the benefits of seeding may for enhancing percentage cover of oysters, in the longer- Fig 7. Effect of microhabitat (flat vs crevice vs ridge) and seeding (unseeded vs seeded) on the mean (+/-SE) (a) humidity (%) and (b) light (mmol m-2 s-1), through time (months), (n = 5). Data are pooled across sites. https://doi.org/10.1371/journal.pone.0230807.g007 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 14 / 21