Knowing the study area. The city of Rio Grande with an area of 3.338, 356 km2 located in the South Coastal Plain of the RS, with ones of its main centered activity economic in the familiar agriculture represented by the cultures of the onion, and in the agriculture of great scale, the irrigated rice, presenting a great contrast in its agrarian structure. The Plain, in a general way, and the entornos of the estuary of the Lagoon of the Ducks in particular are extremely plain areas, with variations very subtle and located of topography, however of basic importance in the structure of ecosystems. The great vulnerability appears from there of these areas in probable scenes of rise of the level of the sea, since they shelter rich ecosystems very, of the great biodiversity, extremely sensible the changes, beyond being small farms of economic activities and leisure of most important in the state and exactly national context. In the south extremity of Brazil the dominant feio is the complex to lagunar Duck, with a surface of 10.227 km2, the Lagoon of the Ducks is classified as the biggest lagoon of the type ‘ ‘ estrangulado’ ‘ of the world (KJERFVE apud ASMUS, 1998), being distinguished in such a way for its extension as for its partner-economic importance which had the functions played for this system, the estuarina area of the south of the Lagoon of the Ducks represents 971km2 10% of the total of the Lagoon approximately. In entorno of the estuary an agriculture of small familiar scale directed toward a production of hortigranjeiros is practised where the main culture is of the onion introduced in Brazil for the Portuguese..