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After a massive mortality of the dominant species (the clam Mesodesma mactroides) occurred in 1995, changes in the intertidal community in sandy beaches of Argentina were monitored. Eight sampling stations were established in a 40 km open stretch and samples were taken every October each year up to 2001. Biomass, density, size frequency distribution and mean growth rates for the most abundant species were analyzed. During the mortality event, only the benthic stages of M. mactroides were affected, their total mean biomass diminishing from 1,399 g to 2 g per running meter beach. Post-mortality recruitment was normal and the growth rates for the youngest cohorts were similar to those previously reported. After a two years’ lag, the wedge clam Donax hanleyanus replaced M. mactroides as the dominant species, increasing from 6.0 g/m up to 24.3 g/m. However, dominance replacement did not restore the productivity of the intertidal macrobenthic assemblage and, despite the increment of D. hanleyanus stocks, the community total biomass remained < 1% of the pre-mortality levels. Since 1998 on, M. mactroides and D. hanleyanus showed several peaks in abundance. Disturbance, mainly due to non-regulated fisheries, has been delaying the community recovery. Taking into account the interactions among species and human activities, the present individual resourcebased management should be replaced by an integrated systembased management program including both conservation and tourism requirements.

We investigated the temporal and spatial changes in the floristic composition and abundance in sand dunes along a coastal strip in the province of Buenos Aires (Argentina). Firstly, we analysed changes over a span of 70 years, comparing 41 of our own inventories carried out in 14 beach resorts in 2005-2006 with 18 inventories made in the first half of the XXth century (1930)(1931)(1932)(1933)(1934)(1935)(1936)(1937)(1938)(1939)(1940) in the same study area. We grouped sampled plots into four categories for comparison, taking into account the time period (1930-1940 or 2005-2006) and location at each end of the surveyed coastal strip in the northern or southern 'tourist' sectors. Secondly, we analysed the effect of afforestation with exotic trees on extant sand dune vegetation for the period 2005-2006. A total of 74 plant inventories were carried out in afforested and natural dunes at the same resorts. In both comparisons we contrasted plant richness, percentage plant cover, similarity and diversity. A cluster analysis was used to classify categories taking into account plant abundance and composition. Significant differences in total diversity were observed between sampling years, suggesting an increase in diversity at the present time due to urbanization and recreation related activities. Nevertheless, no single exotic plant species was clearly dominant across the sampled sites, suggesting that these coastal areas are, up to now, resistant to alien invasion. Diversity indexes discriminated by plant groups indicating different habitat conditions and exotics were shown to be more sensitive to existing changes than to total richness and cover.