Life in the surf-zone: Variations of faunal assemblage structure in temperate sandy beaches of the Southwestern Atlantic coast

Abstract

This study explores the changes of biological descriptors -richness, diversity and abundance-of faunal surf-zone assemblage along a temperate coastal fringe. Three mesotidal sandy beaches with different morphodynamic states, adjacent to an estuarine zone, were seasonally sampled during a year. Zooplankton, zoobenthos and fish samples were taken in the surf-zone and a set of potential explanatory environmental variables were measured and/or quantified. Generalized linear models (GLM) were employed to compare environmental and biological variables between beaches and seasons. Beaches varied from tide-dominated flat to tide-modified reflective to intermediate. The assemblage included 108 taxa, of which few were numerically dominant. Richness of each biological group was similar along the shore and higher values of α-diversity were associated with minimum values of total abundance. β-diversity was mainly a consequence of species turnover, mostly due to changes in zoobenthic and zooplanktonic taxa. Results of GLM test showed that seasons and beaches have an interdependent effect on the abundance of the surf-zone assemblage. All biological groups showed marked seasonal variability in its occurrence, possibly associated with environmental variables, such as temperature and photosynthetic pigment concentrations. Zooplankton and fish groups reached maximum abundance, during autumn and spring respectively, in tide-modified beaches where the surf-zone was well developed and waves reached higher heights. Zoobenthic abundance peaked in beaches with narrower surf-zone and lower wave energy, during autumn, winter and spring, which could be advantageous for those taxa that escape from predators by burying or for species that actively swim near the seafloor. The copepod Acartia tonsa, the clupeidae fish Ramnogaster arcuata and the mysids Arthromysis magellanica and Neomysis americana all typical species of the neighboring estuary, were the main responsibles for these distribution patterns. The connection of the surf-zone with the adjacent estuarine area is also evident by its role in enriching the dissolved organic matter of surf-waters.