Water quality on the seaward 

 side of the Florida Keys exhibits 

 two basic spatial patterns. Along 

 the northeast-southwest axis (Key 

 Largo to Dry Tortugas), changes in 

 water quality are related to the 

 physiography of the islands and the 

 nearby ocean currents. Along the 

 southeast-northwest axis, from the 

 outer shelf break to the island's 

 shoreline, changes in depth, circu- 

 lation, and sediment size largely 

 control variation of the water 

 quality. 



From Key Largo to the Dry Tor- 

 tugas, water variations are related 

 to three of the area's physiographic 

 characteristics: (1) the continuity 

 of the islands; (2) the proximity of 

 the Gulf Loop Current; and (3) the 

 proximity of the Florida Current 

 (Marszalek et al. 1977). The upper 

 Keys, including Plantation, Taver- 

 nier. Upper and Lower Matecumbe 

 Keys, and Key Largo, form a nearly 

 continuous barrier of islands effec- 

 tively isolating the seaward reef 

 tract from the highly variable water 

 quality of Florida Bay (Ginsburg 

 1956, Marszalek et al. 1977). For 

 the middle Keys (Long Key to New- 

 found Harbor Keys), and to a lesser 

 extent the lower Keys, the island 

 barrier is broken up by numerous 

 channels and passes which allow 

 wind-driven and tidal mixing of bay 

 and reef tract waters (Stockman 

 et al. 1967, Marszalek et al. 1977). 

 The Gulf Loop Current's entrainment 

 of continental edge waters from the 

 eastern gulf, like the Florida Bay 

 water, creates a more varied water 

 quality regime than normally ob- 

 served in oceanic waters (Wennekins 

 1959, Marszalek et al. 1977). 



This relatively more variable 

 current dominates the lower Keys' 

 seaward environment as it wraps 

 around the distal islands and joins 

 the Florida Current. In contrast. 



the Florida Current provides a more 

 stabilizing influence on the Keys' 

 seaward waters. In addition, the 

 axis of the current produces spin- 

 off eddies as it converges on the 

 Keys. This sends tropical waters of 

 oceanic salinity into the seaward 

 environment of the upper Keys. The 

 lower Keys and distal islands are 

 comparatively less influenced by the 

 spinoffs. 



The net result of these three 

 physiographic factors is: (1) a 



stable water quality regime seaward 

 of the upper Keys; (2) a great deal 

 of variability in the middle Keys' 

 marine environment; and (3) an envi- 

 ronment in the lower Keys and distal 

 islands that is more moderate than 

 (2) and less stable than (1). Addi- 

 tional confirmation of the influence 

 of these three factors on reef tract 

 water quality comes from Vaughan's 

 (1918) extensive study of reef tract 

 temperatures. Vaughn (1918) shows 

 that Carysfort Reef (Key Largo) has 

 the narrowest temperature range 

 observed. The reduced temperature 

 range infers a more stable environ- 

 ment, reflecting the close proximity 

 of the Florida Current and the 

 spatial separation from the Gulf 

 Loop Current and Florida Bay waters. 

 Maximum mean monthly temperatures 

 for the reef tract occur in the late 

 summer ranging from 28.7°C (84°F) at 

 Fowey Rocks to 31.1°C (88°F) at Sand 

 Key (Key West) . Minimum mean month- 

 ly temperatures occur during January 

 and range from 21.8° to 22.5°C (71° 

 to 73°F). There is a critical dif- 

 ference between low temperatures 

 from one reef to another that is not 

 evident in mean monthly values. 

 Vaughan (1918) examines the coldest 

 ten-day periods and reports that 

 Fowey Rocks, the northern-most reef 

 station, characterized by limited 

 reef growth, exhibits the coldest 

 ten-day period; 2.3°C (8°F) colder 

 than the next lowest value, reported 



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