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This dissertation explores the hydrologic controls on salinity within mangroves and lagoons at sites in Florida and Mexico. The main objective of this research is to better understand hydrologic controls on mangrove ecosystem structure and develop ideas that will be useful to land managers attempting to regulate and conserve these critical habitats. This study was conducted at sites in Ft. Pierce, FL and Costalegre on the central Pacific coast of Mexico. We examined controls on water levels and salinity in a mangrove on a carbonate barrier island along the Indian River Lagoon, east-central Florida. Spectral analysis of water levels showed that mangrove groundwater levels are not tidally influenced. Salinities vary spatially, with values of ~10 in upland environments to ~75 psu in irregularly-flushed mangroves. Water chemistry indicates that water salinities are largely controlled by enrichment due to evapotranspiration. An electrical resistivity survey showed that the freshwater lens is restricted to uplands and that hypersaline waters extend deeply below the mangrove. These results indicate that evapotranspiration lowers water levels in the mangrove, which causes Indian River Lagoon water to flow into the mangrove where it evapoconcentrates and descends, forming a thick layer of high-salinity water below the mangrove. Spatial variability of terrain conductivity in the Ft. Pierce mangrove varied under two hydrologic management regimes, breached rotational impoundment management and rotational impoundment management. The difference in coefficient of variation (CV) between the breached RIM and RIM data was calculated to examine spatial variability in both the shallow and deep layers. A null-hypothesis model was employed to examine the statistical significance of the CV results. The average water levels were -0.06 m amsl and 0.49 m amsl during the breached-RIM and RIM regimes, respectively. The average shallow (EM31) layer terrain conductivity shifted slightly from 1868 mS m -1 to 1825 mS m-1 after the alteration in management regime, yet the standard deviation of these averages decrease from 656 mS m-1 to 216 mS m-1. The average deep (EM34) layer terrain conductivities were 328 mS m-1 and 255 mS m-1 during the breached-RIM and RIM regimes, respectively. The temporal CVs were 0.23 and -0.04 for the shallow and deep layers, respectively. The null-hypothesis model for the shallow layer illustrates that the difference in spatial structure is statistically significant. The deep layer CV was not statistically significant. These results indicate that the transition from breached RIM to RIM resulted in changes to both the physical and chemical hydrologic character of the impoundment, especially in the shallow layer. The second study sites were three mangrove communities along the central Pacific Mexican coast. Salinities varied by water type, with values of ~9 in La Manzanilla, ~17 in La Vena, ~33 in Barra de Navidad, ~0.4 in the fresh waters, and ~34 in the seawater. Sodium and Chloride concentrations and isotopic signatures, as well as salinity, were used as tracers in mass-balance mixing models to quantify estimates of relative fresh-water and seawater contributions to each site. La Manzanilla, a basin mangrove, had mean fresh-water contribution estimates of 63-84%. La Vena, a riverine mangrove, had fresh-water estimates of 39-51%. Barra de Navidad, a fringe mangrove, had low fresh-water contributions of 0-5%. These results illustrates that the role groundwater plays in mangrove hydrodynamics is dependent on the site hydrogeomorphology.