Chesher (1974) reports concen- 

 trations of lead, mercury, and chro- 

 mium in natural mangrove channels in 

 the lower Keys (Mud Key and O'Hara 

 Key) to be at or below the back- 

 ground conditions established by 

 Manker (1975). Copper levels, rang- 

 ing from 5.0 to 2.6 ppm dry weight 

 are also at background levels. Zinc 

 concentrations ranging from 10 to 

 26.1 ppm dry weight, are much great- 

 er than those reported by Manker 

 (1975), but are at the lower end of 

 the scale of the values reported by 

 Chesher (1974) from elsewhere in the 

 Keys. No explanation is given to 

 explain the discrepancy. 



Several chlorinated hydrocar- 

 bons are found in the natural man- 

 grove channels of the lower Keys 

 (Chesher 1974). Although in most 

 cases the values are at or below 

 those measured in man-made canals, 

 p,p'DDT at Mud Key is greater than 

 or equal to concentrations in many 

 of the man-made canals. This chlo- 

 rinated hydrocarbon was commonly 

 used to control mosquitos in the 

 Keys until 1968. Because of Mud 

 Key's relatively remote location 

 along the northern outer banks of 

 the lower Keys, the presence of this 

 chemical indicates wide spread geo- 

 graphical contamination and long 

 term persistence. 



Inland and coastal canals re- 

 present one of the more dramatic 

 signatures of man's presence in the 

 Florida Keys. From Key West to Key 

 Largo more than 320 canals permeate 

 the islands. Water quality within 

 the canals, most critically dis- 

 solved oxygen, is controlled by the 

 canal's orientation, dimensions, 



canal walls slope, entrance depth, 

 number of entrances, adjacent land 

 use, and substrate (Chesher 1974). 

 Of these, the canal's orientation 

 and substrate appear to be the most 

 important. 



Substrate in the Florida Keys 

 canals may be: (1) mangrove peat or 

 fine calcareous muds; or (2) Miami 

 Oolite or Key Largo Limestone. 

 Organic material and turbidity (re- 

 sulting from resuspension of fines) 

 often result in the vertical dis- 

 solved oxygen stratification in man- 

 grove peat/mud canals exceeding 2 m 

 (6 ft) in depth (FDPC 1973, Chesher 

 1974, USEPA 1975). This stratifi- 

 cation promotes the isolation of the 

 lower layer of water from tide and 

 wind forced flushing of the canal 

 waters. Indigenous (mangrove peat) 

 and imported (seagrasses, runoff) 

 organic material accumulate in the 

 stagnant bottom layer contributing 

 more, via decomposition, to the 

 oxygen demand and deficit. Canals 

 with rock substrate (Miami Oolite or 

 Key Largo Limestone) do not exhibit 

 anoxic layers until the canal depths 

 exceed around 3 m (10 ft) or greater 

 (Chesher 1974). 



Second in importance to bottom 

 substrate as a control of the ca- 

 nal's water quality is the canal's 

 orientation to the prevailing winds 

 and tides. Elongation parallel to 

 the prevailing wind direction (E-W 

 or NW-SE) maximizes wind forced 

 movement of water. If the canal's 

 mouth is downwind more effective 

 flushing occurs, whereas openings 

 upwind can result in debris build-up 

 and horizontal stratification of 

 water quality from the canal's end 

 to the mouth (FDPC 1973). Multiple 

 openings create a pass-through situ- 

 ation promoting flushing (USEPA 

 1975). 



Salinities range from 27 ppt to 

 41 ppt, and may exhibit vertical 

 increases averaging as much as 2 ppt 

 from surface to bottom. Sombrero 

 Marina (Vaca Key), receiving runoff 

 from residential, commercial, and 

 transportation (U.S. Highway 1) 

 areas, shows a surface to bottom 



106 



