/ Data are presented on the vegetation dynamics of two impounded marshes along the Indian River Lagoon, in east-central Florida, USA. Vegetation in one of the marshes (IRC 12) was totally eliminated by overflooding and by hypersaline conditions (salinities over 100 ppt) that developed there in 1979 after the culvert connecting the marsh with the lagoon was closed. Over 20% recovery of the herbaceous halophytes Salicomia virginica, S. bigelovii, and Batis marltima was observed at that site after the culvert was reopened in 1982, but total cover in the marsh remains well below the original 75%. No recovery of mangroves was observed at this site, The second site (SLC 24), while remaining isotated from the lagoon during much of the study, did not suffer the complete elimination of vegetation experienced at the first site. At this location, mangroves increased in cover and frequency with a concomitant decrease in herbaceous hatophytes.Considerable damage to the vegetation was evident at IRC 12 when the impoundment was closed and flooded for mosquito control in 1986. Although the damage was temporary, its occurrence emphasizes the need of planning and constant monitoring and adjustment of management details as conditions within particular marshes change. Storms and hurricanes may be important in promoting a replacement of black mangroves by red mangroves in closed impoundments because the former cannot tolerate pneumatophore submergence for long periods of time.

The effects of reestablishing a marsh-lagoon connection through culverts upon the fish populations and the physical conditions of two impounded sub-tropical marshes are examined. After install ation of the culverts, mean salinities in the two marshes increased from approximately 13 ppt to 25 ppt and pH from 7.1 to 7.3. The abundance of marsh resident species, particularly those that forage in the upper marsh, decreased significantly after connection, but utilization of the marshes by transient species increased by almost two orders of magnitude. Modification of the marsh hydroperiod seems to be most important in influencing the changes in the patterns offish utilization, abundance, and diversity observed at the study sites.

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