Card Sound Ecosystem - Is partly enclosed by the Florida Keys to the east and southeast and the mainland on the western side. It opens mainly to the north into Biscayne Bay and to the south into Barnes Sound. It is shallow with a range of between 3 and 12 feet in depth. The salinity levels currently found are generally higher than those found in the ocean. The western portion has extensive mud flats near mean sea level and a few low sandy islands directly off the eastern edge of the site. The major plant species found on the mud flats are turtle grass (Thalassia testudinum), manatee grass (Syringodium filiforme) and shoal grass (Halodule wrightii). During the wet season, many varieties of floating or submerged algae are present in great quantities. This ecosystem is a valuable nursery for numerous forms of animal life, as a result of the protection it affords. The Sound is just outside the EMB site, however, it is integrally linked to the biomechanics of the Bank itself.
Saline Mangrove Ecosystem - A luxuriant mangrove community that consists of dense stands of both red and white mangroves is found along the Card Sound shoreline, within this ecosystem. The saline mangrove zone vegetation is dominated by red and white mangroves with an occasional black mangrove and a mixture of halophytic plants. Black mangroves do not extend inland across wide tidal zones. Scrub red mangroves grow inland as far as four miles. Heavy concentrations of mangrove are also found along the banks of the four historic tidal 'rivers' and found on inland 'tree islands' with tidal connections. Tides provide saline water for these areas. The interior saline marshes further inland frequently hold a lens of fresh to brackish water, especially during the rainy season. The best developed mangrove stands are found along the shoreline or mangrove flats which are alternately watered by saline water and rain water runoff. Inland, as the water becomes brackish and then fresh, the red mangroves along the banks of the historic tidal 'rivers' are replaced by buttonwood and then swamp hardwoods.
Freshwater Marsh Ecosystem - A freshwater coastal marsh ecosystem is essentially a freshwater impoundment due in part to the gradual nature of the slope and partly due to the baffling nature of the supported vegetation. During the rainy season, the surface runoff sheet flows into the adjacent saline mangrove zones. This freshwater marsh historically was connected to the vast Everglades system by the transverse glades. In some areas the historic freshwater influences were sufficiently great that freshwater and brackish prairies extended very near to the shoreline (Thorhaug et.al., 1976). The majority of the freshwater now supplied to the EMB marshes is supplied by rainfall and ground water. The bedrock limestone is covered by marl or calcite and peat.
The flora of the freshwater marsh is varied. Sawgrass occurs as a component to a greater or lesser degree throughout this system. Within the flat basins, where the overlying peat is 2 to 5 feet deep, sawgrass will be the dominant vegetation. In the shallow marl soils, spike rush gains in dominance. The historic tidal rivers that originate within this zone are forested with freshwater swamp canopy vegetation in their upper reaches. As these approach the coast they transition into buttonwood and then red mangroves dominate nearer the sound.
There are numerous 'tree islands' within the freshwater marsh. The term 'tree island' is used to describe clusters of woody or sub-tropical climax vegetation that appear dotted throughout the freshwater marsh region. These 'islands' contrast distinctly from the herbaceous marsh species that surround them. They occur on a base of organic soil at slightly higher elevations than the surrounding marshes. They may differ throughout the site, but are either sub-tropical hardwood hammocks, buttonwood stands or sweetbay domes. These tree islands have adapted to flooding for short periods of time, but will not survive flooding lasting several months.