Modelling large-scale effects of estuarine and morphodynamic gradients on distribution and abundance of the sandy beach isopod Excirolana armata

Understanding the relationships between beach morphodynamics and macrofauna assemblages has been critical in theoretical evolution of sandy beach ecology. However, macroscale studies that consider the concurrent effects of large-scale estuarine and morphodynamic gradients have been exceptional. The present study evaluates the combined effects of large-scale (>400 km) salinity and morphodynamics on the distribution of Excirolana armata. Along a salinity gradient (from 0.10 to 34.30) generated by the Río de la Plata estuary (SW Atlantic Ocean), sixteen Uruguayan sandy beaches were analyzed over a two-year period. A conditional two-step procedure using a General Additive Model (2-step GAM) was performed in order to model occurrence (1st-step) and abundance (2nd-step) of E. armata, in relation to salinity, grain size, sand moisture, compaction and organic matter of the sand, slope, beach and swash width, and water and sediment temperature. The 1st-step GAM retained 6 physical descriptors in the model (decreasing order): mean grain size, organic matter, salinity, beach width, sand moisture and water temperature. The 2nd-step GAM showed that mean grain size, salinity, sand moisture, beach width, sand compaction and organic matter were the most important explanatory variables (in decreasing order) of abundance. Mean grain size was the main predictor in both models, suggesting an important substrate specificity of E. armata towards smallest grain sizes. A meta-analysis concerning large-scale variation in abundance of E. armata in sandy beaches of the Rio Grande ecoregion (28°S–35°S) of the Warm-Temperate Southwestern Atlantic Province gave compelling evidence of this high-substrate specificity of the E. armata. Salinity was also a key factor shaping patterns in occurrence and abundance, which increased from low to intermediate/high salinities (>20), reinforcing the notion of E. armata as a marine species with relatively high tolerance to estuarine conditions. The potential role that internal brooding gives for protection of offspring to osmotic stress is discussed.