The threat of freshwater input on sandy beaches: A small-scale approach to assess macrofaunal changes related to...

Increased rainfall has become a key threat in recent decades for subtropical coastal regions. On sandy beaches that are associated with streams and rivers, the intensification of freshwater inputs is expected to reduce the salinity of interstitial waters and affect benthic biodiversity. Large freshwater gradients are promoted by river mouths and also change beach morphodynamic and sediment organic contents, which are covariates that have hindered the understanding of salinity-specific effects on benthic species in previous studies. Here, we aimed to assess how salinity reduction affects macrobenthic communities at small spatial scales to control the effects of environmental covariates. We assessed the macrofaunal spatial changes across few-meters gradients of freshwater influence (30 m) that were promoted by small streams (~2 m wide) within three subtropical beaches in southeastern Brazil. Our results showed that salinity was the only environmental factor that explained the macrofaunal variations across such small-scale gradients. We noted that salinity reductions decrease the overall macrobenthic abundance and richness in the first 15 m close to streams. Such variations are associated with changes in polychaete abundance (mainly Scolelepis squamata), which form larger patches with 1200-2800 individuals/m² only at sites with high salinity (greater than 25). We also found that a salinity reduction from 33 to 20 may promote a decrease of 85% in polychaete abundance near the streams. Nevertheless, salinity reductions did not affect crustacea abundance across the gradient. In the current global change context, these results suggest that freshwater input has great potential to impact polychaete patches, reduce macrobenthic biomass and secondary production and thus threaten important beach functions and services, such as bioturbation and food provision for the top predators.