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

term, be minimal. Instead, seeding efforts may be better directed at sites where there are dis- persal barriers to oyster settlement (perhaps introduced by coastal structures), but where envi- ronmental conditions are otherwise suitable for their survivorship, growth and reproduction [49]. The functional groups of organisms that responded most strongly to the manipulations of habitat structure and seeding with oysters were those whose body size most closely matched the dimensions of the interstices formed by the interventions. That is, our experimental manipulations which created microhabitats at the scale of cms, were most effective for enhanc- ing the species density and/or abundances of benthic algae and invertebrates and crypto- benthic fishes but not pelagic fishes on seawalls [38, 50–52]. After 12 months, the species density of sessile algae and invertebrates and the percentage cover of S. glomerata was greater on the tiles for which one or both types of interventions were manipulated relative to the flat unseeded tiles. In contrast, the species density and abundances (counts or MaxN) of mobile taxa and cryptobenthic fishes on the tiles, were only influenced, positively, by seeding with oys- ters. These results suggest that even through the habitat structure provided by the seeded S. glo- merata varied through time due to morality and growth of the oysters, there may be lasting effects of the initial seeding on mobile taxa and cryptobenthic fishes. The similar species density of sessile taxa recorded between tiles irrespective of whether

they contained habitat structure, seeded oysters or both suggests there was some redundancy in the effects of the treatments, or, alternatively, because oyster cover did not differ between the three treatments, that oyster cover was the driver of species density. Oysters may influence species density by: enhancing the surface area for attachment and grazing; by providing micro- habitats between their shells that are protected from extreme low-tide air temperatures, desic- cation stress and/or predators; through their biological functions of filter feeding, which can reduce the abundance of competitively dominant spores, or create currents that facilitate set- tlement; and through biodeposition which provides organic material [53–55]. The species den- sity of mobile invertebrates and cryptobenthic fishes, may have responded more strongly to the initial seeding treatment than sessile taxa as they can move onto tiles within hours of their deployment to graze and utilise the microhabitat. In contrast, the colonisation of sessile taxa is dependent on larval settlement and may take longer to establish. Yet despite the rapid response of mobile species density to seeding with oysters, their abundances first displayed effects of habitat structure at 6 months and seeding at 12 months, Differences in the species density of sessile taxa among flat tiles and those with habitat structure may reflect the settlement choices of the organisms for cooler, shaded, moist and potentially less wave exposed surfaces [56],

greater post-settlement mortality on unprotected surfaces, or reduced competition for space where more hard substrate is available for settlement. Our experimental manipulations of seeded oysters were predicted to influence the species density and MaxN of pelagic fishes by enhancing prey availability and altering their foraging behaviour. However, contrary to our expectations, there were no temporally persistent effects of seeding on the species density or abundances of pelagic fishes. Previous research suggests that the relatively short-lived effect of seeding on the MaxN of pelagic fish, which disappeared after 6 months, was related to changing patterns of foraging on the tiles [7]. The most abun- dant pelagic fishes at 1 and 6 months were the Yellowfin bream (Acanthopagrus australis) and the Fanbelly leather jacket (Monacanthus chinensis). Both of these species are common around artificial habitats [57, 58] and are known predators of the Sydney rock oyster, S. glomerata [59–61], including those growing on the unprotected surfaces of seawalls [7, 62]. The absence of an effect of seeding on pelagic fishes after 6 months may reflect the loss of the oyster prey resource due to growth of oysters out of size classes most vulnerable to predation [7], and/or 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 15 / 21