Latitudinal gradients in macroalgal biodiversity in the Southwest Atlantic between 36 and 55°S

Abstract

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.

Intertidal seaweeds from North Atlantic Patagonian coasts, Argentina

“…The proportion of the three groups follows the trend observed by Liuzzi et al (2011) and by Miloslavich et al (2011) for the Argentinean coast. The total number of Rhodophyta, Chlorophyta and Phaeophyceae taxa found in the three sites represented an 11%, 22% and 20% of the total species of each group registered for the Patagonian shelf subregion of South America (Miloslavich et al 2011).…”

“…Recent biogeographic studies along the Argentine coast have demonstrated that most of the seaweed studies were conducted at high latitudes (Liuzzi et al 2011;Wieters et al 2012), mainly in the provinces of Chubut, Santa Cruz and Tierra del Fuego (Mendoza 1999;Asensi and Küpper 2012;Boraso 2013). According to these studies the floristic information about the coasts located in latitudes lower than 42° S is sparse.…”

“…According to these studies the floristic information about the coasts located in latitudes lower than 42° S is sparse. The lower sampling effort on inferior latitudes may be in part related to the decreasing trend of seaweed biodiversity with decreasing latitude (Liuzzi et al 2011). According to this study, the main points of reduction of seaweed biodiversity coincide with the boundary between two biogeographic provinces, the Argentinean province, which extends from the mouth of La Plata River (35° S) to a transition zone between 41-44° S, and the Magellanic province, that extends from Valdés Peninsula to the southern extreme of the continent (Balech and Ehrlich 2008).…”

“…According to this study, the main points of reduction of seaweed biodiversity coincide with the boundary between two biogeographic provinces, the Argentinean province, which extends from the mouth of La Plata River (35° S) to a transition zone between 41-44° S, and the Magellanic province, that extends from Valdés Peninsula to the southern extreme of the continent (Balech and Ehrlich 2008). The coast of Buenos Aires Province has therefore been denoted as a region of impoverished algal assemblages (Liuzzi et al 2011 …”

Areas of endemism in the Southwestern Atlantic Ocean based on the distribution of benthic hydroids (Cnidaria: Hydrozoa)

“…Our analysis showed areas of disjunction (sets 5, 6, 13, 14, Figure 1, and set 5, Figure 2) in the SWAO, in contrast with inshore/offshore areas and latitudinal gradients of biodiversity already documented for the marine realm in biogeographic studies (e.g., Searless 1984; Calder 1992; Liuzzi et al 2011;Fautin et al 2013). Areas of disjunction, recognized by EA, are common in nature (Casagranda et al 2012), and can be related to several ecological and historical aspects of the SWAO.…”