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Abstract THE DYNAMIC HYDROLOGY OF AN OVERWASHED MANGROVE ISLAND Daniel W. Urish, Raymond M. Wright and Wilfrid
Rodriguez The tidally induced hydrodynamics in an overwashed mangrove island,
in conjunction with the topography, greatly affect the ecosystem
and vitality of the resident mangrove systems. This is evident from
a 20 year study on the small archipelago of Twin Cays lying along
the Belizean Barrier Reef. This study concentrates on the interior
hydrology of West Island of Twin Cays, a 21.5 ha island characterized
by a vigorous fringe of red mangrove (Rhizophora mangle L.)
and a depressed central portion. The dominant water feature is a
large shallow pond interconnected by channels in direct communication
with the surrounding reef lagoon. Sparse dwarf red mangrove dominant
in this pond. Poor flushing creates water temperatures ranging from
25o C in the winter to 40o C in the summer.
High evapo-transpiration creates a hypersaline condition of 45 ppt
salinity in summer. In winter with the infusion of fresh rain water,
the pond changes to a brackish water of less than 5 ppt . Additionally,
of primary significance to the vitality of the mangrove ecosystem
is the duration of the flooding-exposure cycle, viz. the hydroperiod.
The temporal and spatial characteristics of the interior flow system
were investigated using extensive field measurement and the employment
of animated computer graphics produced from dye flow studies. While
for the past 8000 years the mangrove growth has managed to keep
up with rising sea level, the future is in doubt because of anticipated
greatly increasing sea level rise rates. The islands of Twin Cays,
with its history of comprehensive observational research, remains
an important location for observing and measuring changes in the
mangrove systems as they occur in a world of dramatic coastal change.
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