have an irregular floor of soft carbonate mud, 

 with patches of bedrock exposed by scouring. 

 Moderate to dense stands of Thalassia and 

 Diplanthera cover the mud deltas at each end 

 of the channels and, to some extent, the chan- 

 nels themselves where sediments are suffi- 

 ciently deep to afford them attachment. Nu- 

 merous "grass" ledges are formed along the 

 channel banks where undercutting removes 

 the soft sediments, leaving a dense mat of 

 overhanging Thalassia rhizomes. 



In addition to passing through the channels, 

 water also is exchanged across the surfaces 

 of the banks, but the dense cover of epiphyte- 

 laden Thalassia restricts this flow and acts 

 • as an efficient baffle and filtering system. 

 Although tidal water extends into the lake 

 {McCallum and Stockman, 1964), poor flushing 

 is indicated by abrupt differences in water 

 clarity and salinity between the lake and the 

 ocean water to the south. Strong winds and 



seasonal changes in ocean level cause the 

 largest fluctuations in the lake's water level 

 (Ginsburg, 1956). Maximum observed differ- 

 ence in lake level was 38 cm. 



We measured salinities and tempera- 

 tures of the lake water at monthly intervals 

 from November 1964 to January 1968 (table 1). 

 Highest salinity (49.6 p.p.t.) was recorded 

 in July 1965; lowest salinity (27.8 p.p.t.), 

 in September 1966. McCallum and Stockman 

 (1964) reported that in Florida Bay, "...fluc- 

 tuations in the amount of fresh- water run- 

 off from the mainland produce seasonal and 

 annual fluctuations in salinity." The lack of 

 rainfall in the summer of 1965, and, con- 

 versely, the abundance of rainfall in the sum- 

 mer of 1966, were primarily responsible for 

 the difference in the two salinity values. Sur- 

 face temperature in the lake ranged from 

 32.2° C. in September 1965 to 16.6° C. in 

 December 1966. 



Table 1. — Salinity and temperature of surface waters in Porpoise Lake, 

 November 1964 to January 1968 



- = No data 



