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.

List of phytoplankton, zooplankton and zoobenthos species abundances

We studied the effects of changed quality of inflow water of aquaculture ponds on three aquatic communities, phytoplankton, zooplankton and zoobenthos, during two seasons of rearing common carp (Cyprinus carpio). The new water source coming from a deep tube well was markedly different in water chemistry from the surface water sources previously used to maintain the investigated fish ponds. Ponds supplied by the tube well water were characterized by lower oxygen and water hardness, and higher total ammonia and conductivity reaching subsaline conditions. Multivariate analysis (co-inertia) revealed that all investigated groups, except Mollusca (zoobenthos), decreased in species richness, abundance and biomass due to changed water chemistry, but differed in the level of susceptibility to stressors. Assemblages of Rotifera and Cladocera were the most affected showing a sharp decline in density and number of species since 29 out of 44 species disappeared from the ponds. The abundance of Copepoda (Cyclopoida) was relatively high although significantly lower in new environmental conditions (P<0.05), with adults being more tolerant to changed inflow water than larvae. Phytoplankton, except Bacillariophyta, had a highest potential to replace previous species with newcomers more adapted to changed inflow water, providing 36 immigrant species while 49 became extinct. Although mainly influenced by fish predation, Chironomidae (zoobenthos) were undoubtedly affected by changed water chemistry, decreasing from 11 to only 3 species. These results suggest that this pattern was a result of the shift from freshwater to subsaline conditions.