Usually the lagoon ecosystem depends upon the interchange between marine and freshwater systems. Land-locked lagoons with temporary connections to the sea are uncommon environments. Frequently marine renewal and the maintenance of a "steady state" depend on a man-made channel. Thus the period of the year when sea-connections occurr and their extent have an important influence on lagoon evolution. St. Andre lagoon belongs to this group of systems and is now recovering from a period in which spring interchanges with the sea either did not exist or were insufficient. The macrobenthos and physical-chemical parameters were studied in 1978-79 and 1982-86. During the latter time fish were also studied. Species colonization comes from two different pools, freshwater and marine. The latter provides sudden and rich spring recruitment. Organic enrichment, anoxia and decline of salinity progressively shift the community towards a more stress-tolerant and limnetic state. In contrast to sand substrates, muddy areas have higher organic contents which constrain the settlement and development of benthic communities. The disruption of the traditional spring opening process leads to an organic enrichment, summer dystrophies and a very striking annual community cycle, with few species surviving during the entire year.