areas is greatest in the bare mud 

 bottom areas of the inner shelf in 

 Hawk Channel. Ginsburg (1956) 



observes white water within the 

 channel during periods of sustained 

 winds of 24 km/hr (15 mph) or great- 

 er, as does Chew (1954) off Key 

 West. Griffin's (1974) extensive 



study of temporal and spatial vari- 

 ation of turbidity associated with 

 dredging activities off Key Largo 

 reports a similar pattern. Along 

 the inner shelf (Hawk Channel), 

 turbidities peak most intensively 

 during the winter months when pre- 

 vailing northeasterly winds blow 

 down the channel instead of across 

 it. Griffin (1974) reports mean 



suspended sediment concentrations 

 increasing from the shelf break 

 (0.50 optical mg/l) through the out- 

 er reefs (0.80 optical mg/l) to the 

 inner reefs (4.4 optical mg/l). 

 Turbidities resulting from "hard- 

 rock dredging" off Key Largo and 

 measured adjacent to the dredging 

 activity range from 18 mg/l (0.7 m 

 or 2.4 ft outside a functioning tur- 

 bidity diaper) to 212 mg/l (over the 

 edge of the diaper at a leak). Down 

 current from the diaper, turbidities 

 ranged from 22 mg/l to greater than 

 40 mg/l. Apparently the containment 

 procedure, even when malfunctioning, 

 substantially reduces the peak tur- 

 bidity levels (Griffin 1974). Plume 

 direction is, as expected, sensitive 

 to prevailing currents and rarely 

 exceeds the limits of an area 

 extending 0.3 nautical miles along- 

 shore and 0.33 nautical miles off- 

 shore. No detectable impact is 

 noted on a patch reef 0.48 nautical 

 miles to the NNE of the dredging 

 activity. 



Variation of dissolved oxygen 

 across the reef tract is controlled 

 by temperature, salinity, turbu- 

 lence, and bio-chemical processes. 

 As illustrated in Figure 43, turbu- 

 lence generally decreases while tem- 



perature and salinity variation and 

 the concentration of bio-chemical 

 processes per unit volume increases 

 with decreasing depth (Enos 1977). 

 The net effect is generally to in- 

 crease dissolved oxygen variation 

 from the reef to shore. 



Dissolved oxygen in the upper 

 50 m (164 ft) of the Straits of 

 Florida ranges from 3.7 mg/l to 5.4 

 mg/l with an annual mean value of 

 4.6 mg/l (USDC 1974). Jaap and 

 Wheaton (1975) report surface dis- 

 solved oxygen values ranging from 

 5.4 mg/l to 8.2 mg/l over East and 

 West Sambo reefs (outer reefs off 

 Key West). A wider range 4.2 mg/l 

 to 8.6 mg/l is observed near the 

 bottom. Manker (1975) reports a 

 more restricted range of 6.2 mg/l to 

 8.6 mg/l, averaging 6.3 mg/l over 

 the outer reefs off the upper Keys. 

 Farther shoreward over the patch 

 reefs, dissolved oxygen variation 

 increases (Jones 1963). In a patch 

 reef environment seaward of Elliott 

 Key, dissolved oxygen ranges from 

 3.8 mg/l to 5.9 mg/l. Diurnal dis- 

 solved oxygen on the patch reef 

 varies seasonally, resulting in a 

 daily fluctuation of 0.08 mg/l in 

 winter (95% to 120% saturation) and 

 a 1.6 mg/l daily range during the 

 summer (90% to 125%). The maximum 

 daily values occur from 1400 to 

 1600; the minimum values occur be- 

 tween 0300 and 0800 (Jones 1963). A 

 similar trend of slight supersatu- 

 ration during the day and slight 

 undersaturation at night occurs in 

 the shallow reef waters of the Dry 

 Tortugas (McClendon 1918). Jones 

 (1963 notes that minimum oxygen 

 values, observed in the early morn- 

 ing hours, reach a seasonal minimum 

 during late summer and early fall 

 when water temperatures andcommuni- 

 ty respiration are highest. The 

 opposite response takes place during 

 winter months. Manker (1975) re- 

 ports a range of 6.2 mg/l to 8.6 



103 



