and Syringodium, which grow best in 20-40 

 but Diplanthera and Ruppia are euryhaline to 

 about 50% o , so they often take over where the 

 others are unable to survive (Fig. 26B). In 

 Florida estuaries, the sea grasses penetrate gen- 

 erally to about 7 ft (2.1 m) except where the 

 water is exceptionally clear as in parts of Pen- 

 sacola Bay where penetration attains about 12 ft 

 (3.7 m) . Offshore, however, Thalassia grows in 

 dense beds to 60 ft (18 m) (Dawes, 1967), and 



Halophila baillonis to 240 ft (73 m) (Dawes and 

 Van Breedveld, 1969). 



Algae are frequently the most conspicuous 

 flora in shallow water, and they exist below 

 depths tolerated by sea grasses in our estuaries 

 (Fig. 26A) . Dawes and Van Breedveld (1969) 

 found them to 240 ft (73 m) off Tampa Bay. The 

 red algae Acanthophora, Agardhiella, Gracilaria, 

 Hypnea, and Laurencia and the brown algae 

 Dictyota and Sargassam are often abundant and 



-10 



ALGAE 



THALASSIA 



DIPLANTHERA 



^yj^^MJ B^ 



DIPLANTHERA 



1 



^ RUPPIA 



) 



THALASSIA 



SYRINGODIUM 



B 



THALASSIA 



SYRINGODIUM 



DIPLANTHERA 



RUPPIA 



SPARSE 



ABUNDANT 



SPARSE 



ABUNDANT 



ABUNDANT 



ABUNDANT 



10 



15 



20 



SALINITY p.p.l. 



25 



30 



35 



40 



Figure 26. — Schematic drawings of (A) the zonation of sea grasses in shallow water in Boca Ciega Bay just north 

 of the Bayway to St. Petersburg Beach and in Tampa Bay just south of Bayboro Harbor, St. Petersburg, and 

 (B) salinity preferences and tolerances of sea grasses. MHHW = mean higher high water; MLHW = mean 

 lower high water; MHLW = mean higher low water; MLLW = mean lower low water. (Modified from Phillips, 

 1960b; Moore, 1963.) 



50 



