Keywords: Biogeographic region, F r e s h w a t e r seepage, G r o u n d w a t e r salinity, Mangal, Mangrove, Tidal flat h y d r o l o g y AbstractThe regional distribution of m a n g r o v e species along the tropical N W coast of Australia is closely related to climate. In s u b h u m i d regions with rainfall >1 000 m m / y r mangals have 12 species which are recurring and c o m m o n . The climate becomes more arid southwards and species richness decreases: 8 species are c o m m o n in mangals in semiarid regions and 5 species are c o m m o n in arid regions. Rainfall influences the distribution of species across tidal flats because of direct recharge to g r o u n d w a t e r systems and also because of subsurface seepage along the hinterland edge. M u c h of the increase in species diversity is located along high parts of tidal flats and along the edge of the hinterland.At the local level s t r a t i g r a p h y forms an i m p o r t a n t part of the h y d r o l o g y by determining the distribution of aquifers and aquacludes. Fresh water seeps into the tidal lands via buried, discrete aquifers and dilutes the highly hypersaline g r o u n d w a t e r . M a n g r o v e s inhabit these local areas above the less saline groundwater. The influence of rainfall and freshwater seepage on mangals is presented as a unifying theme which helps to explain distribution and diversity of mangroves at both a regional and local level.
A l o n g ria shorelines m a n g r o v e assemblages are closely related to habitat types and there is a recurring pattern in their distribution that is linked to the origin and history of the g e o m o r p h i c units. The coastal g e o m o r p h o l o g y is related to ancestral landforms developed prior to the post-glacial transgression, as well as to tidal levels, aspect, m o d e r n shore-forming processes, and types of h i n t e r l a n d / t i d a l flat contacts. These historical and process patterns develop distinct g e o m o r p h i c units which dictate the distribution of habitats and their mangrove assemblages.M a n g r o v e assemblages are classified according to their habitat: main tidal flat, hinterland fringe, alluvial fan, spit/chenier, tidal creek bank, tidal creek shoal and rocky shore. Within a given habitat there are various physico-chemical gradients which are instrumental in developing zonation within the assemblages. The variety of internal zones within an assemblage is related to the richness of species within the regional species pool which in turn is related to climate.
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