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.

摘要 沙滩中的永久性淡水径流（又称冲沟（washouts））会在不同时空尺度上改变环境特征，进而对大型底栖动物种群产生影响。本研究旨在评估永久性冲沟中等足类动物菱形额圆壳水虱（Tholozodium rhombofrontalis, Giambiagi, 1922）的丰度、分布及种群结构变化。研究于2012至2013年的四个季节，在巴西南部巴拉那州巴拉那蓬塔尔市的巴兰科斯沙滩设置三个采样点开展季节性采样：冲沟内（冲沟内部）、中间带（距冲沟30米处）与远带（距冲沟150米处）。该沙滩在地貌动力特征上属于过渡型沙滩，沉积物为分选良好的细颗粒泥沙。冲沟采样点的地形剖面更为平缓且更长，地下水位更浅且水质为淡水；与之相反，远带采样点的地形剖面更陡峭且更短，地下水位更深且水质为咸水。中间带采样点的特征与冲沟采样点相近。该等足类动物的丰度与分布存在时空异质性，中间带与冲沟采样点的丰度更高，并在春季与夏季达到峰值，这一变化与半咸水环境及高叶绿素a含量密切相关。幼体在所有种群类群中占比最高，且其占比随时间保持稳定，表明种群存在持续的补充过程。雌性个体占比随时间无显著波动，证实了种群的稳定性。所有采样点与采样时段内雄性个体占比均较低，但该类群的低占比并未对种群的存续产生负面影响。所有采样点与采样时段均发现抱卵雌性个体，其占比在冬季达到最高，表明该物种可能偏好低温季节进行繁殖。永久性冲沟的存在为该沙滩提供了河口盐度的水环境，这可能有助于该种群的建立与存续。