Different groups of marine benthic organisms show contrasting latitudinal patterns of biodiversity. The widely accepted paradigm of increasing biodiversity towards the tropics does not seem to be valid for macroalgal floras of the Southern Hemisphere. We compiled a database summarizing the distributional ranges of macroalgae along the coast of Argentina to test whether biodiversity decreases towards lower latitudes, as in the Pacific coast of South America, and whether breaking points in the geographical distribution can be recognized in one or more areas of the Southwest Atlantic south of 36°S. We found a clear trend of decreasing biodiversity with decreasing latitude. The interpretation of some biodiversity declines is confounded by changes in the intensity of the sampling effort. A 51% reduction in algal species richness between 42 and 41°S coincides with the boundary between the Argentine and Magellanic Zoogeographic Provinces. This sharp breaking point is related to a thermal anomaly caused by long residence times of water masses within San Matías Gulf, suggesting an upper thermal tolerance limit for most Antarctic/sub-Antarctic seaweeds. A further reduction occurs at 38-37°S. This breaking point can be explained by the disappearance of suitable hard substrata, since rocky outcrops give place to wide extensions of sandy beaches. The impoverished algal assemblage inhabiting the northern coast of Argentina is mainly related to the reduction or disappearance of the Antarctic/subAntarctic floristic component. This area is characterised by a predominance of widely distributed species, Chlorophytes and opportunistic filamentous or foliose algae.

Our aim was to analyse changes in species turnover and structure of macrofaunal assemblages associated with intertidal coralline algal turfs at 4 spatial scales along the coast of Argentina (southwestern Atlantic): provinces (~10 6 m), localities (~10 4 m), sites (~10 2 m) and replicate quadrats (~1 to 3 m). Corallina officinalis was by far the dominant algal species in most samples, but C. elongata and Jania rubens var. rubens were also frequent. Frond density was 3 times higher in the southern, cold-temperate Chubut province than in the northern, warm-temperate Buenos Aires province. Macrofaunal species richness, diversity and evenness were also significantly higher in samples from Chubut than in those from Buenos Aires, with 'province' explaining 86 to 98% of the variance in the analytical model. In total, 118 macrofaunal taxa belonging to 11 invertebrate phyla were found. Mytilid bivalves and polychaetes were the most important groups contributing to differences between provinces. Brachidontes rodriguezii was extremely abundant in Buenos Aires province, while Rhynchospio glutaea and Perumytilus purpuratus were dominant in samples from Chubut. Changes in assemblage structure were significant at the scale of provinces, localities and sites in Patagonian localities. Measures of beta diversity showed that taxonomic turnover was correlated with distance between samples at scales of 10 4 m or higher, with the highest at the scale of provinces. Higher biodiversity in the Magellan assemblage than in warmer areas of the northern coast of Argentina may be related to the Pacific origin of the Magellan fauna, which entered the southwestern Atlantic during the Tertiary period. KEY WORDS: Coralline turf · Beta diversity · Latitudinal trends of biodiversity · Spatial scales · Rocky intertidal · Corallina · ArgentinaResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 363: 147-156, 2008 than on European and South American shores (Kelaher et al. 2004). As coralline algae provide habitat for a large number of small invertebrates (Chapman et al. 2005, Bussell et al. 2007) and occur along 1000s of km of shoreline, they may be useful in studies analysing changes in benthic biodiversity and taxonomic turnover at different spatial scales.Species diversity has been partitioned into different components. Beta diversity, i.e. the extent of change in species composition among the samples of a data set (Whittaker 1975, Gray 2000, describes taxonomic turnover in assemblages at various spatial scales within a habitat (Winberg et al. 2007). A vast array of measures to assess beta diversity has been proposed (Koleff et al. 2003), some of which are used to analyse the degree of taxonomic change in the benthic environment (Ellingsen 2002).Latitudinal gradients in species richness are well documented in terrestrial ecosystems (Rosenzweig 1995). In the marine realm, however, the existence of a latitudinal cline for the shallow water fauna is debatable. In the northern hemisphere, the num...

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