Storm-related strandings of mollusks on the northeast coast of Buenos Aires, Argentina

Abstract

The purpose of the paper is to characterize mollusk stranding on the northern coast of Buenos Aires, and to determine the mechanisms of alteration induced by storm surges on the infralittoral macroinvertebrates as well as on the morphosedimentary feature of the beach. The most common living organisms stranded on the beach were Adelomelon brasiliana and their free giant egg capsules, Amiantis purpurata, Atrina seminuda, Buccinanops duartei, Buccinanops gradatum, Buccinanops monilifer, Donax hanleyanus, Mesodesma mactroides, Mytilus edulis platensis, Tivella isabelleana, and Zidona dufresnei. Shells of Mactra isabelleana and Glycymeris longior were also found. Three mechanisms were responsible for strandings on the emerged beach: (i) storm surges related to a decrease in the wave-cut level, (ii) swell conditions with infralittoral organism remobilization in the following 7 days, and (iii) shell bar migration. The high abundance of shells found at the emerged beach is due to Mactra isabelleana, although no living individuals were found during the 6 years of survey. The predominance of G. longior shells resulted from their shape, thickness, and hardness, enriching the beach sediments. This is in contrast to other species (D. hanleyanus, Mytilus edulis platensis, and Mesodesma mactroides) whose shells cannot endure exposure to the transport and weather conditions in the swash zone and on the emerged beach. The intact condition of shells immediately after the storm indicate that they were transported from greater depths where the death of the organisms occured. This is indicated by the simultaneous presence of live stranded individuals and empty shells of the same species (T. isabelleana, Amiantis purpurata, Adelomelon brasiliana, Z. dufresnei, B. gradatum, and Mytilus edulis platensis). Between 35% and 65% of the sand are shell fragments, mostly of deeper water species. The variation in mean grain size, sorting, and skewness among samplings is attributed to shell fragmentation, while the mode remained invariable. Most of the intact shells supplied by the storm were reworked in the swash zone. The percentage of bioclasts remained invariable for 5 months and fell within the interval of 0-0.5 phi.