mg/l of dissolved oxygen over the 

 reef tract off Key Largo, Planta- 

 tion, Tavernier, and Elliott Keys. 

 All values were measured from 0900 

 to 1800, during the summer months, 

 and therefore represent the higher 

 end of the diurnal and seasonal 

 range of dissolved oxygen values. 

 The majority of oxygen concentra- 

 tions fall below 6.0 mg/l with less 

 than 20% exceeding 7.0 mg/l. From 

 the nearshore stations, monitored 

 concurrently, a wider range (0.9 

 mg/1 to 9.5 mg/l) was reported with 

 25% of the values below 6.0 mg/l and 

 less than 20% exceeding 7.0 mg/l. 

 Other authors (Howard and Faulk 

 1968, Howard et al. 1970, Chesher 

 1974) have observed similar wide 

 ranges of dissolved oxygen in the 

 nearshore and lagoonal environments. 

 Howard et al. (1970) report that 

 dissolved oxygen varies from 3.9 

 mg/l to 11.7 mg/l in Coupon Bight, 

 between Newfound Harbor Keys and 

 Big Pine Key. There the diurnal 

 pattern, which varies within several 

 subenvironments of the bight, char- 

 acteristically peaks at approximate- 

 ly 1500 and is most pronounced (0.7 

 mg/l/hr) within the seagrass ( Tha- 

 lassia testudium ) sub-environment. 

 The lowest (3.9 mg/l) and highest 

 values (11.7 mg/l) are associated 

 with areas where circulation is more 

 restricted. Narrower diurnal ranges 

 are related to environments with 

 better tidal-flushing (channel, open 

 bay, and open-rock shore environ- 

 ments). In mangrove channels and 

 tributaries of the lower Keys (Chan- 

 nel, Boca Chica, Mud, O'Hara, and 

 Sugarloaf Keys) and the upper Keys 

 (Key Largo), dissolved oxygen varies 

 from 2.7 mg/l (North Creek, Key 

 Largo) to 8.0 mg/l (O'Hara Key) 

 (Chesher 1974). Very little oxygen 

 stratification exists in these fair- 

 ly shallow natural channels ranging 

 from an average of 5.15 mg/l at the 

 surface to 4.97 mg/l at the bottom. 



These rather uniform depth profiles 

 of oxygen concentration are in stark 

 contrast with vertical stratifica- 

 tion that is evident in man-made 

 canals. In the upper Keys, values 

 range from 5.92 mg/l at the surface 

 to 2.63 mg/l at the bottom; in the 

 middle Keys, 5.58 mg/l to 4.27 mg/l; 

 and in the lower Keys, 6.03 mg/l to 

 4.74 mg/l (Chesher 1974). 



Phosphorus in the surface wa- 

 ters (upper 50 m or 164 ft) of the 

 Florida Straits range from undetect- 

 able to 1.22 microgram (ug) atoms 

 per liter as P, averaging 0.07 ug 

 atoms per liter (USDC 1974). Inside 

 the reef tract off Elliott Key, to- 

 tal and inorganic phosphorus ranges 

 from 0.15 to 0.25 ug atoms/I, and 

 undetectable to 0.1 ug atoms/I, 

 respectively (Jones 1963). No dis- 

 cernible diurnal pattern exists, and 

 no constant ratio between total and 

 inorganic phosphorus is apparent. 



Like phosphorus, nitrate-nitrite 

 is present in very low concentra- 

 tions throughout the year. Nitrate 

 as N and nitrite as N ranged from 

 2.0 to 9.0 ug atoms/I and 0.05 to 

 0.25 ug atoms/I, respectively (Jones 

 1963). 



A similar trend of low nutrient 

 levels is indicated for the mangrove 

 channels and tributaries in the 

 lower Keys and Key Largo (Chesher 

 1974). Ort hop hosp hate as P averaged 

 0.05 ppm, ranging from 0.02 ppm to 

 0.15 ppm. A general decrease in con- 

 centration occurs from surface to 

 bottom. Nitrate as N ranges from 

 0.05 ppm to 0.15 ppm. Both nitrogen 

 and phosphorus data from this study 

 are based on few measurements within 

 a restricted temporal and spatial 

 framework and therefore may not 

 represent a true picture of the 

 nutrient patterns along the Florida 

 Keys nearshore environment. 



104 



