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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



southwestern Gulf coast (1.11 and 3). The 

 swamps occur either in lagoons or on outer coasts 

 that lack beaches or cliffs. It is along the beach- 

 less and cliffless coasts, in quiet shallow waters, that 

 the unique mangrove ridge and lagoon are found. 

 Davis (1940) reports the growth on Florida as one 

 of the greatest known. The tropical and near- 

 tropical mangrove forests of the main biogenous 

 environment are dominated by the red mangrove 

 {Rhizophora mangle) and the white buttonwood or 

 white mangrove (Laguncularia racemosa) . Inland 

 from the widely flooded zone, the black or honey 

 mangrove {Avicennia nitida) grows. The latter 

 outruns the other mangroves into the marginal 

 tropical regions north of the main biogenous 

 environment. The black mangrove grows as far 

 northwest as the Chandeleur Islands of Louisiana 

 off the eastern part of Mississippi delta and in 

 spots in the Laguna Madre near the mouth of 

 Rio Grande. In the mangrove forests of southern 

 Florida numerous other trees and plants grow 

 with the mangroves (Davis 1940). 



The fact that red mangroves build out the shores 

 on which they grow has long been known to 

 geologists (Vaughan 1909). The abundant roots 

 and the manner of seeding on shoals by the floating 

 of well-sheathed seedlings aids these trees in 

 occupying marginal marine and lagoonal areas in 

 protected waters (Davis 1940). The black man- 

 grove, however, seeds immediately under its 

 branches, and tends to grow toward land from a 

 shoreline fringe, rather than outward. 



The mangrove barrier ridge and coastal lagoon. — 

 Chart 1113 shows an extremely irregular outer 

 shoreline beginning at the north with the Ten 

 Thousand Islands archipelago. This belt of 

 islands starts at the northwest in the coastal 

 lagoon behind the Cape Romano barrier spit. It 

 then curves to the southeast to end at Lopez 

 River. From Lopez River southeastward to Cape 

 Sable the mangrove swamp of the outer coast is 

 mapped as being much more compact than in the 

 Ten Thousand Islands, and is smoother, but far 

 from regular. It is broken by transverse marshy 

 channels and has, in the northwestern part, an 

 outer line of islets and small peninsulas. From 

 3 to 8 miles inland, there is a zone of highly 

 irregular, more or less intercommunicating swampy 

 lagoons and channels running roughly parallel 

 with the outer coast. Between the inner lagoons 

 and the outer coast, there is a broad belt of man- 



grove swamp which is the ridge. Davis shows 

 that the height of the ridge should be a function 

 of both tidal range and the slope of the bottom 

 and adjacent land surface across which the man- 

 grove belt originally spread. 



The entire coast southeast of Cape Romano 

 (4.1) is composed of mangrove swamps and la- 

 goons except for the sandy barrier islands, spits 

 and beaches of the Cape Romano barrier at the 

 northwest and of Cape Sable at the southeast. 

 The delineation of shorelines for the mangrove 

 forest is difficult (McCurdy 1947) because of the 

 indefiniteness of shoreline position for a marine 

 swamp, especially where the tidal range, as here, 

 varies from about 2 to 4 feet. East of Cape 

 Sable, there is a mangrove belt along the north 

 side of Florida Bay. 



The mangrove peat rests on limestone rock, 

 marl, or shell beds (Davis 1940). The peat sec- 

 tion varies from about 5 to 14 feet. Except where 

 it descends into depressions in the karst, Davis 

 thinks that the general average thickness is about 

 7.5 feet below mean low water. This would place 

 the base of the peat at an average of 8 feet, or 

 slightly more, below mean sea level. The red 

 mangrove seats itself in as much as 2 feet of water, 

 the roots spreading outward somewhat. The 

 seedlings float and ground in a few inches of water. 

 There was in many cores taken by Davis through 

 the peat, an alternation of peat and marl, with an 

 upper marl bed a foot or two thick present in 

 most of the area. The roots of the present swamp 

 trees penetrate this upper marl but without peaty 

 development in it as yet. Alternations of marl 

 and peat in a core may or may not indicate a ver- 

 tical oscillation of sea level, as they, certainly in 

 some cases, have been due to compaction or minor 

 horizontal shoreline changes under essential still- 

 stand of the sea. 



The history of the formation of the mangrove 

 barrier ridge and lagoon may be somewhat as 

 follows: On a broad, well-protected tropical to 

 subtropical shoal coast, especially, as m Florida, 

 where the wind is dominantly offshore but swell 

 and some on-shore wave movement is present, ad- 

 vance of the mangrove forest is assured. The 

 elongated, winged, pod-shaped seedlings ground 

 and take root at any depth down to 6 to 8 inches 

 of water. Root growth may extend as far offshore 

 as a 2-foot depth at low tide (Davis 1940). The 

 dense growth of roots, trunks, and associated veg- 



