Sandy beach bioindicators: How each benthic taxon tells its own story

The use of biological indicators is one of the main strategies for monitoring anthropogenic impacts on sandy beaches. However, most studies performed so far have focused only on a few groups, mostly supralittoral and intertidal taxa, and did not consider how the environmental variability influences the performance of bioindicators, nor if these organisms can act as proxies of biodiversity or assemblage-level responses. The goal of this study was to evaluate the performance of several popular and under-evaluated sandy beach bioindicators from distinct groups to act as indicators of urbanization impacts, but controlling for and disentangling the effects of natural variability. We sampled a large set of sandy beach sites (n = 90) with distinct morphodynamic conditions and urbanization degrees to encompass a wide variability of natural and anthropogenic conditions. Almost all taxa affected by urbanization were also affected by morphodynamics. Natural variability explained a higher variance than urbanization in most cases, including for species such as the ghost crab O. quadrata and the polychaete Scolelepis spp. Although rarely included in environmental impact assessments, species inhabiting the sandy beach lower zones, such as polychaetes, showed good prospective as bioindicators. The potential of species to act as biodiversity surrogates was limited, and only the polychaete Scoloplos treadwelli was associated with a higher local diversity, an important measure of ecosystem ecological integrity. Overall, our results show that the use of bioindicators may provide a good cost-benefit approach in beach monitoring; however, management should consider beach morphodynamic type as individual units when choosing the adequate bioindicator. When the scale of management comprises multiple beaches with distinct morphodynamic characteristics, different bioindicators should be used to untangle the effects of beach type and anthropogenic activities. To expand the array of suitable bioindicators, our results indicate that sublittoral taxa should be better evaluated. The potential of benthic bioindicators to act as biodiversity surrogates was limited, and alternatives, such as biotic indices, may be more appropriate to uncover effects at the assemblage-level.