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The southeast South American shelf marine ecosystem extends over the entire continental shelf off the eastern shores of Argentina, Uruguay, and southeastern Brazil (23° to 55°S). The region south of 40°S is usually known as Patagonian Continental Shelf of Argentina (PCSA). The intertidal environments of the Patagonian shore are among the most diverse in South America in terms of their environmental heterogeneity. The productivity distribution over the continental shelf is also heterogeneous and is concentrated on the marine frontal zones. The Patagonian coast is one of the richest and most productive temperate marine ecosystems in the world. The sea near the Patagonian coast is one of the ocean regions in the world in which smaller changes to its physical variables have been observed. There has been a sea-level rise in the region offshore of Patagonia, but the change has been more moderate than the global average. Patagonian coasts of Argentina have only 18 cities, many of them growing rapidly. The population density in this area is below 4 inhabitants/km2. Despite increasing industrial activities in coastal cities in last decades, quality of coastal waters remains high. Argentina has extensive legislation to control marine pollution and protect biodiversity. Despite the advances of the last decades, more efforts are needed for the effective conservation of marine ecosystems on the Patagonian coast.

We conducted a comprehensive survey of existing knowledge about exotic marine organisms introduced to the southwestern Atlantic Ocean, including coastal and shelf areas of Uruguay and Argentina. This domain is equivalent to the so-called Patagonian Shelf Large Marine Ecosystem, and corresponds to two biogeographic provinces: warmtemperate (Argentine Province) and cold temperate (Magellanic Province). The search included species that can be confidently categorized as introduced (31) and cryptogenic species (46). We present a comprehensive picture of recorded introductions (the first for this region) and some prominent ecological consequences. Most coastal ecosystems between the La Plata River estuary and central Patagonia have already been modified, or are expected to be so in the short term. Five recent, human-mediated biological invasions have already had a significant ecological impact. A barnacle (Balanus glandula) belt has developed on all rocky shores where none was present 30 years ago, a macro-fouler (Limnoperna fortunei) and a reef-builder (Ficopomatus enigmaticus) have strongly modified estuarine ecosystems, Pacific oyster (Crassostrea gigas) reefs are expanding in shallow bays at a fast rate, and kelp (Undaria pinnatifida) is rapidly modifying nearshore benthic communities along the coasts of central Patagonia. At this point, it is uncertain how many of the cordgrasses (Spartina spp.) found in coastal salt marshes correspond to exotic forms. Only exposed sandy beaches appear to be free from the pervasive ecological impact of invasion by exotic species. Poor knowledge of the regional biota makes it difficult to track invasions.