Urbanization effects on sandy beach macrofauna along an estuarine gradient

The rapid urban population growth and coastal infrastructure development worldwide add dimensionally and complexly to the management process in sandy shores, and therefore the development of beach quality indices that take into account the increasing role of urbanization are particularly relevant. This work analyses the potential of human population density (HPD), nighttime lights and vegetation cover as indicators of urban impact on sandy beach biodiversity. Each indicator was tested against a large-scale 2-yr study of macrofaunal richness and physical characteristics along a strong environmental gradient defined by the widest estuary of the world, the Rio de la Plata. A negative linear relationship between HPD and the number of species was found. The amount of light recorded at night peaked at the most urbanized area in the coast, Montevideo city, and showed a negative relationship with macrofaunal richness. Vegetation cover, expressed through a normalized difference vegetation index, showed a weak positive relationship with species richness. Multiple linear regressions, combining urban indicators and physical characteristics as explanatory variables of macrofaunal richness, showed the best results when HPD and salinity range were combined. A regression tree explained 65% of deviance and had a structure coherent with previous results. Salinity was the dominant ecological driver: beaches with salinity >= 27.2 showed higher species richness, while beaches with less salinity and higher nighttime lights showed the lowest species richness. Random Forests selected salinity (mean and range) and HPD as the most informative variables to discriminate groups of beaches according to their macrobenthic richness. These results reflect that the salinity gradient is a macroscale driver that shapes species richness patterns along this coast, whereas the effects of urbanization are confined within the dominant large-scale environmental gradient. The identification of suitable urban indicators provided in this work constitutes the first step onto the development of more rigorous approaches to assess this globally relevant and long-lasting stressor.