Population structure, growth and production of the surf clam Donax hanleyanus (Bivalvia: Donacidae) from northern Argentinean beaches

The surf clams Mesodesma mactroides Reeve, 1854 and Donax hanleyanus Philippi, 1847 are the two dominating species in macrobenthic communities of sandy beaches off northern Argentina, with the latter now surpassing M. mactroides populations in abundance and biomass. Before stock decimation caused by exploitation (during the 1940s and 1950s) and mass mortality events (1995, 1999 and 2007) M. mactroides was the prominent primary consumer in the intertidal ecosystem and an important economic resource in Argentina. Since D. hanleyanus was not commercially fished and not affected by mass mortality events, it took over as the dominant species, but did never reach the former abundance of M. mactroides. Currently abundance and biomass of both surf clams are a multiple smaller than those of forty years ago, indicating the conservation status of D. hanleyanus and M. mactroides as endangered. Therefore the aim of this study is to analyse the population dynamics (population structure, growth and reproductive biology) of D. hanleyanus and M. mactroides, and to compare the results with historical data in order to detect possible differences within surf clam populations forty years ago and at present.

Lake level reconstructions at Laguna Potrok Aike (southern Patagonia, Argentina) for the last 49 ka

Four seismic surveys and a stratigraphic record from southernmost Patagonia (Argentina) based on 51 AMS-14C dates obtained in the framework of ICDP expedition 5022 "Potrok Aike Maar Lake Sediment Archive Drilling Project" (PASADO) provide a database to compare the 106 m composite profile from the lake centre with piston cores from the littoral and outcrops in the catchment area. Based on event correlation using distinct volcanic ash layers with unique geochemical composition and optically stimulated luminescence (OSL) dates on feldspars, sediment records are firmly linked. This approach allows to match the sediment record with water levels during the past ca. 49 ka providing evidence for lake level variations. Reconstructed lake levels were 20 m higher than today during the last Glacial until the early Holocene. With the migration of the Southern Hemispheric Westerlies over this site the lake level dropped ca. 55 m for a period of two millennia. Thereupon the water balance was more positive again causing a stepwise rise of the lake level until the maximum was reached during the Little Ice Age with a subsequent lowering since the 20th century. We suggest that the mid- to late-Holocene lake level variation is caused by intensity changes of the Southern Hemispheric Westerlies.

Microsedimentological characterization as indicators of sedimentary processes and climate changes during Lateglacial at Laguna Potrok Aike, Santa Cruz, Argentina

Palaeoclimatic and paleoenvironmental high latitude records in the Southern Hemisphere are scarce compared to the northern counterpart. However, understanding global evolution of environmental systems during sudden climate changes is inseparable from an equivalent knowledge of both Hemispheres. In this context, a high-resolution study of lacustrine sediments from Laguna Potrok Aike, Santa Cruz province, Patagonia, Argentina, was conducted for the Lateglacial period using concurrent X-Ray Fluorescence (XRF) and Scanning electron microscope analyses. Peaks of Ca/Si and Mn, and occurrences of the green alga Phacotus lenticularis have been interpreted as variations in ventilation of the water column from 13.6 to 11.1 ka cal. BP. During this interval, mild climate conditions during the Younger Dryas are characterized by relatively weak westerlies favouring the formation of a stratified water body as indicated by preserved manganese and Ca/Si peaks and high Total Organic Carbon (TOC) values. In this environment, water in the epilimnion can reach sufficiently high temperature to allow P. lenticularis to grow. Colder conditions are marked by peaks in Ca without P. lenticularis and occur during the Antarctic Cold Reversal (ACR). In this Lateglacial interval, micropumices were also detected in large amount. Image analysis of thin sections allowed the counting and size measurement of detrital particles and micropumices separately. Micropumices significantly influence the iron and titanium content, hence preventing to use them as proxies of detrital input in this interval.