Beach Morphodynamics and Clam (Donax Hanleyanus) Densities in Buenos Aires, Argentina
Keywords:Donax hanleyanus, densities, population structure, mobility, beach morphodynamic, grain size
The studied coastal area is located along the Atlantic Ocean, between the localities of Punta Rasa and Villa Gesell. Twelve monitoring sites were established along the shore. Two morphologic types of beach profiles were observed. One type located northward composed by fine sand, has a linear configuration profile with low slopes (0.028) and narrow beach (40-70 m width). The other beach type is composed by coarser sand with higher slopes (0.014) and wider beaches (40-80 m), usually presenting a stable berm, and one or two tidal berms. The mean grain size of the beach sediments where the clam Donax hanleyanus occurs varies from fine sand (2.10-2.80 phi) for the north sector (A) to medium to coarse sand (0.70-1.90 phi) in the south sector (B). Donax hanleyanus is found in the lower third of the intertidal zone in the transverse profile. A permanent lack of individuals was observed in the northern part of sector A due to the fresh water influence of the Rio de La Plata estuary. The mean density observed was about 400 individuals per square meter and the highest densities were recorded for Mar del Tuyu with 2250 individuals per square meter. The grain size of the sediments analyzed was not correlated with the occurrence of Donax hanleyanus, although it was observed that the presence of coarser sheets with mean grain size of 0.71 phi and standard deviation of 1.07 affect the burrowing capacity of Donax. The life span of Donax honleyanus is greater in the south where they can live up to three years. The growth rates are similar for both areas although in the south a decrease in the growth rates was observed during winter. It was observed that the grain size and the morphological changes did not condition the presence of Donax but affected the mobility of Donax hanleyanus along the transverse beach profile.