The El Nino phenomenon refers to a warming of the tropical pacific basin that occurs at irregular intervals of 3 - 7 years. The 1982-1983 and 1997-1998 El Nino were the strongest episodes recorded since 1950, and their meteorological impacts were felt around the world. High rainfall associated with El Nino increase freshwater discharge in southern Brazil, which can change salinity and circulation patterns in the estuaries of this region. Based on biological and meteorological long-term dataset (1979-1984 and 1996-2001) encompassing the 1982-1983 and 1997-1998 El Nino events, we evaluated the general hypothesis that El Nino-induced hydrological changes is a major driving force controlling the inter-annual and inter-decadal variation in the structure and dynamics of shallow-waters fishes in the Patos Lagoon estuary. Patos Lagoon is located off the southern Brazilian coast and represents one of the largest coastal lagoons in the world. Patos Lagoon and the adjacent coastal area support one of the most important fisheries in the warm-temperate southwestern Atlantic. About 3,500 artisanal and 3,000 industrial fishermen are temporarily or permanently involving in fishing activities in this region.

The effects of both El Nino events results in cascades of cause and effect over a broad hierarchy of spatial scales ranging from regional macroclimatic patterns to responses of individual organisms to variation in microhabitats. High precipitation associated with both 1982-1983 and 1997-1998 El Nino events significantly affected the hydrology, water quality, and fish assemblages of the Patos Lagoon estuary. Some of the strongest assemblage patterns were clearly caused by species' direct responses to changes in salinity and freshwater outflow, both inside and outside the estuary. The observed increase in freshwater fishes within the estuary was perhaps predictable. However, the reduced abundance of several important estuarine dependent and marine species with broad physiological tolerances was not predicted. The complex life cycles of these species involves movement among multiple coastal habitats during specific stages of development. The greater freshwater outflow during El Nino probably resulted in greater water current velocity and vertical mixing at the estuary mouth, and reduced salinity in the marine coastal area. These changes in and around the mouth of the estuary may have transported many estuarine residents and freshwater species out the estuary, and obstructed the movement of early life stages of estuarine dependent fishes into the estuary. El Nino-associated changes in freshwater discharge from rivers probably affected estuarine productivity as well, with indirect consequences for fish population and assemblage dynamics.

The detailed analysis of the 1997-1998 event showed that El Nino-induced assemblage changes were not highly persistent. Relative abundance of many marine and estuarine dominant fishes increased to pre-El Nino levels within 3 - 6 months of the end of the El Nino period. Based on estuarine productivity and hydrological conditions found after the El Nino onset, alternative hypotheses were suggested to explain the rapid recovery of the fish estuarine assemblage.

Our study reinforces the general viewpoint that recruitment in most marine coastal fish populations is strongly influenced by interannual climatic and oceanographic variation. The ability to predict population and assemblage dynamics in estuarine systems like Patos Lagoon is strongly contingent on capabilities to forecast El Nino phenomena.

The current PhD dissertation produced six published papers (except one that is currently in revision) in regional and international scientific journals. Email: amgarcia@mikrus.com.br