Flooding affects vertical displacement of intertidal macrofauna: A proxy for the potential impacts of environmental changes on sandy beaches

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In the context of global environmental changes, sea level rise is expected to cause long-term floods in upper zones of sandy beaches, while increased rainfall may intensify freshwater floods with extreme salinity reductions. However, the effects of these changes on benthic macrofauna behavior, especially the species vertical displacement, remain largely overlooked in sandy beach ecology. Here, we used natural and manipulative experiments to evaluate whether the predicted effects of saltwater and freshwater floods would affect the distribution of beach macrofauna within the sediment column (i.e. vertical distribution). We first tested the effects of natural tidal flooding by analyzing the upper beach macrofauna at different depths within the sediment column during low and high tide periods. Then, we simulated saltwater and freshwater floods in manipulative experiments to examine their effects on the vertical distribution of the semiterrestrial crustacean Excirolana armata within the sediment. We observed that the macrofauna was concentrated closer to the sediment surface layer during low tide and in deeper areas during high tide. We also found that both saltwater and freshwater floods induced a movement of E. armata towards deeper layers of the sediment; yet this effect was dependent on salinity given that fewer individuals reached deeper sediment layers during freshwater floods. Overall, our results show that floods induce movements of sandy beach macrofauna within the sediment column, thereby changing the vertical distribution of benthic species. It is possible that these changes may affect ecological processes and services provided by sandy beach biodiversity such as bioturbation, biomass production and the flux of energy. In this regard, we suggest that future studies focus on the understanding of behavioral responses of macrofauna as a proxy for the impacts of global environmental changes, attempting to upscale their ecological consequences from the short to long-term effects.