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Observations in a mangrove lined coast in south-western Puerto Rico (rainfall 800-1,000 mm; evaporation 1,900-2,200 mm; mean annual temperature 25º C; and average tidal range 0.3 m) have shown that the degree of structural development of the mangrove forest is closely related to wave exposure. In exposed coastal segments, sand berms formed by wave action within the outer fringe prevent water flow towards the inner forest, resulting in high salinities. The berms are higher in areas where the outer mangrove fringe has been destroyed by storms or other causes. Dead mangrove stands are found behind these areas and salinities reach 75-80‰ We suggest that the mangrove fringe contributes to dissipate wave energy over a broad area which reduces the high of the berm. The presence of absence of "blowholes" in the seagrass Thalassia; bed is an index of the degree of protection that the coast receives. The "thickness" of the mangrove fringe is also related to the degree of shelter: it is "thin" in high energy segments, "thick" in coastal segments subject to intermediate energy and "thin"again in the most sheltered locations. In the outlying cays mangrove development follows a similar pattern: the outer exposed cays (essentially coral islands mantled by coarse sands) are devoid of mangrove cover or have stunted trees (generally Laguncularia; canopy height, 2 m). Islands which are less exposed are colonized by Rhizophora which frequently forms overwashed forests (canopy height, 8-9 m). In the most seltered areas, Rhizophora colonizes the shallow banks, forming islands which soon develop an inner hypersaline lagoon due to the accumulation of material in the outer edges and the accumulation of salt in the interior. The wave energy level reaching a given section of the coast is therefore an important factor which determines the degree of structural development of the mangrove forest. High energy levels are associated with erosion, destruction or deposition of berms which affect mangrove development. Low energy levels, however, are associated with a rapid process of island maturation, aging and death due to rapid salt accumulation