The influence of a washout in the spatio-temporal distribution of the isopod Tholozodium rhombofrontalis
ABSTRACT The permanent freshwater discharges in sandy beaches, known as washouts, promotes changes in different spatial and temporal scales on the environmental characteristics, and consequently on macrobenthic populations. The aim of this study was to evaluate the changes of abundance, distribution and population structure of the isopod Tholozodium rhombofrontalis (Giambiagi, 1922) in a permanent washout. Seasonal samplings were realized in the Barrancos beach, Pontal do Paraná city, state of Paraná, southern Brazil, in three sites: on the washout (inside washout), intermediate (30 meters away from the washout), and distant (150 meters away from the washout), during four seasons of the years 2012 and 2013. The beach was morphodynamically characterized as an intermediate beach, composed of fine and well sorted sediments. At the washout site the topographic profiles were smoother and longer, and the water table shallowed and fresh. In contrast, in the distant site the profiles were steeper and shorter, and the water table deeper and saline. The intermediate site presented characteristics similar to that of the washout. There was a spatial and temporal variability in the abundance and distribution of the isopod, being the greater abundances in the intermediate and washout sites, with peaks in the spring and summer seasons. These variations were related to the presence of brackish water and high values of chlorophyll a. The juveniles proportion were the higher of all classes, and remained temporally regular, indicating a continuous recruitment. The proportions of females remained similar temporally confirming the population stability. The proportion of males was low at all sites and during all samplings; however, the low representativeness of this class did not negatively affect the establishment of the population. The ovigerous females were present in all sites and in all samplings, with highest proportions in the winter, pointing to a possible preference for a colder season for reproduction. The presence of a permanent washout provided to this beach conditions of estuarine salinities, which possibly favored the establishment of the population.