salinity difference of 8 ppt during 

 August 1973, although temperature 

 varied less than 0.5°C (0.9°F). This 

 form of stratification commonly 

 occurs following a large influx of 

 freshwater runoff, forming a fresh- 

 water lens in the canal. The sudden 

 isolation of the more saline waters 

 by the density gradient creates an 

 anoxic environment, possibly result- 

 ing in fish kills (FDPC 1973, 

 Chesher 1974). 



The relatively deep and shel- 

 tered nature of the canal waters 

 leads to a more moderate temperature 

 range than observed in the Keys' 

 nearshore environment. Temperatures 

 range from 23° to 32.6°C (73° to 

 91°F) although measurements are 

 sparse, particularly for the colder, 

 winter months. Generally tempera- 

 tures decrease with depth due to 

 differential heating. Where inver- 

 sion of this trend occurs, stratifi- 

 cation of the higher temperature 

 waters is usually associated with 

 low dissolved oxygen. 



A by-product of the passage of 

 freshwater runoff over and through 

 the oolitic and coralline limestone 

 formations is the introduction of 

 CaC03 (calcium carbonate) and a 

 consequent increase in pH. This 

 process is reflected in canal pH 

 values ranging from 7.2 to 9.3 

 (FDPC 1973, Chesher 1974). In 



canals where stratification is pre- 

 valent, pH can decrease as much as 

 one unit from surface to bottom 

 (FDPC 1973). 



The macronutrients nitrogen and 

 phosphorus are generally low in man- 

 rnade canals. Dissolved inorganic 

 phosphorus as P averages between 

 0.040 to 0.082 ppm (FDPC 1973, 

 Chesher 1974). USEPA (1975) reports 

 total phosphorus as P ranging from 

 0.01 ppm to 0.06 ppm. Nitrate and 

 total kjeldahl nitrogen as N range 



from undetectable to 0.33 ppm and 

 from 0.19 ppm to 1.48 ppm, respec- 

 tively. Higher nutrient levels are 

 found in canals with increased human 

 activity (marinas, commercial and 

 transportation land use), mangrove 

 peat and mud substrate, and poor 

 circulation (FDPC 1973, Chesher 

 1974, USEPA 1975). 



A distinctly diurnal dissolved 

 oxygen patten is prevalent in man- 

 made canals in the Florida Keys 

 (Chesher 1974, USEPA 1975). Lowest 

 values are generally found shortly 

 after dawn and highest values in mid 

 afternoon. Vertical and horizontal 

 variations in oxygen can result from 

 biological production, restricted 

 flushing, and resuspended organic 

 matter. Variations may also be af- 

 fected by the interactions of these 

 factors with circulation patterns, 

 canal geometry, and anoxic layering 

 (FDPC 1973, Chesher 1974, USEPA 

 1975). A depth profile resulting 

 from this interaction typically 

 resembles a bell shaped curve with 

 lowest oxygen values at the surface 

 and bottom, and highest values at 

 mid-depth. Figure 44 illustrates 



the combined effect of these actions 

 bringing aerated waters in with the 

 tide floating over the anoxic layer, 

 but under a wind-driven layer de- 

 rived from the oxygen depleted water 

 at the canal's dead end (FDRC 

 1973). 



Of the heavy metals monitored, 

 copper, lead, and zinc appear in 

 greatest concentration associated 

 with boat basins/marine and urban 

 stormwater conveyance systems (FDPC 

 1973, Chesher 1974, USEPA 1975). 



Table 22 summarizes the most 

 complete survey of pesticides in 

 Florida Key canal sediments (Chesher 

 1974). Mangrove channels and tribu- 

 taries on Mud and O'Hara Keys pro- 

 vide an approximation of background 



107 



