| Summary: | Litterfall, wood-biomass accumulation, phenology, seedling establishment and survival, surface and interstitial salinity, pH and oxidation-reduction potential, and peat accumulation and substrate subsidence of three mangrove forests dominated by R. mangle exposed to different hydrological regimes were monitored every two weeks from October, 1985 to March, 1987 at Hutchinson Island, Florida. The sites studied were: (1) a managed impounded mangrove forest that is artificially flooded with seawater during the summer for mosquito-control purposes, (2) an unmanaged (not artificially flooded) mangrove impoundment, and (3) a fringe mangrove forest exposed to daily tidal exchange. The purpose of the study was to determine the effect of mid- to long-term (four or more months) sustained shallow flooding on the structure and function of forests of R. mangle, and to compare the results with: (a) nearby forest of R. mangle exposed to tidal exchange, and (b) documented responses of other mangrove forests subjected to changes in their hydroperiod.Estimated net above-ground primary production was 4,335 gm$\sp{-2}\cdot$yr, 4,016 gm$\sp{-2}\cdot$yr, and 5,384 $\sp{-2}\cdot$yr at the managed impoundment, unmanaged impoundment, and fringe forest, respectively. These values are in the upper range of the reported production for mangrove forests. It is suggested that the seasonal draw-down of the water level in the managed impoundment, with the subsequent exposure of the substrate results in an "energy subsidy" in the form of an in situ release of nutrients resulting from the oxidation of the accumulated organic matter and a reduction in the interstitial salinity.The structure and function of the mangrove forest inside the managed impoundment contrast with documented reports of other mangrove forests affected by impounding; while widespread mortality is the norm in impounded mangrove forests, the mangroves inside the managed impoundment seem to be thriving. This difference seems to result from two main factors: (1) the above-mentioned "energy subsidies," and (2) the fact that the present mangrove forest inside the managed impoundment is in a stage of secondary succession, and not a mature forest.
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