Heavy metal contamination in coastal ecosystems can significantly impact biological activity, metal retention, and biogeochemical cycling. This study examined the levels of arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) in sediments and leaves of two mangrove ecosystems in Puerto Rico: La Parguera and Laguna Grande. We also evaluated ratios of metal concentrations in plants and soils, i.e., bioconcentration factors (BCF) and differences in metal concentrations between green and senescent (yellow) leaves, i.e., retention percentages (RT%). Levels of metals at both sites were low relative to previous mangrove studies, but were markedly higher at the Laguna Grande site due to anthropogenic influences. Relationships between metals, between metals and sediment carbon, and metal retention and bioavailability also differed between the sites.  Results suggest that differences in relationships between metals and between metals and carbon may be useful for identifying sources and effects of metals. Further, metals with high retention by plant percentages, such as zinc and lead at La Parguera, suggest that plant-mediated stabilization processes can reduce immediate bioavailability but may pose latent risks under changing environmental conditions. Conversely, metals like cadmium and nickel, with low retention and high bioavailability at Laguna Grande, indicate greater potential for biological uptake and ecosystem stress. These findings underscore the critical role of site-specific environmental factors, such as metal loading, plant density, sediment chemistry, and hydrodynamics, in mediating metal behavior. Further research is needed to explore the direct physiological effects of metal exposure on plants and their implications for carbon storage and ecosystem health in mangrove-dominated systems.