Modelling present and future distribution of intertidal macroalgae to evaluate the efficacy of Western Australia's marine protected areas

Distribution patterns of intertidal macroalgae are often investigated to evaluate the efficacy of designated marine protected areas (MPAs), yet those assemblages are threatened by high sea surface temperatures, making them vulnerable to ocean warming. Here, we model the present distribution patterns of macroalgae on rocky intertidal shores of Western Australia and project those patterns onto different future climate scenarios to assess the effect of ocean warming. Present and future distribution patterns of those assemblages were then used to evaluate the efficacy of presently designated MPAs. We developed distribution models using hierarchical modelling of species communities based on species occurrence and biomass data of 36 species from 38 sites (18 degrees S to 34 degrees S). The efficacy of MPAs was evaluated by overlaying marine park boundaries with the map of species distributions. Macroalgae were projected to show decreases in species richness by 49%, reduction in total biomass by 20%, changes in species composition by 43%, increases in spatial homogenisation, poleward shift by 0.80 degrees, and loss in distribution area by 70% by 2100 based on RCP8.5 scenario. Nevertheless, half of the remaining distribution area of macroalgae was still inside existing MPAs. The current macroalgal bioregions all contained at least one MPA, but it is projected that a bioregion from 25 degrees S to 31 degrees S will be separated into two distinct bioregions (25 degrees S to 29 degrees S and 30 degrees S to 31 degrees S) by 2100 based on RCP8.5 scenario. Unfortunately, there is no MPA within the northernmost bioregion, and all studied species inside the MPA within the southernmost bioregion will go extinct under that scenario. Thus, the current system of MPAs will be inadequate in the future, based on our modelling.