91 



1981, Brenner et al. 1990, Whitmore 1989). The majority of Florida 

 lakes are soft-water, acidic and low in alkalinity (Brenner et al. 

 1990). Nevertheless, many lakes on phosphatic sands or carbonate- 

 rich bedrock are naturally high in productivity and dissolved solutes 

 (Canfield 1981). Because the present survey spanned a wide range 

 of limnological conditions, trophic state, pH and specific conductance 

 formed a dominant environmental gradient that emerged as the 

 principal determinant of diatom community composition. 



Jackson and Charles (1988) observed the effect of a similar 

 environmental gradient on macrophyte distribution in the 

 Adirondack lakes of New York. They found that alkalinity, pH and 

 ionic composition were interrelated factors that determined the 

 distribution and species composition of aquatic vegetation. Jackson 

 and Charles stated: 



We conclude that the chemical gradient underlying 

 compositional variation among our Adirondack softwater 

 sites is the tail end of a broad pH complex-gradient that 

 extends to highly alkaline waters. At the scale of 

 environmental variation observed in Adirondack lakes, 

 the main factors associated with vegetation variation are 

 pH, alkalinity, Ca, Mg, and perhaps Al. In regions where 

 the gradient is broader, or at least where the hardwater 

 portion is represented, conductivity and trophic status 

 become more important (Jackson and Charles 1988, p. 

 1456-1457). 



Because Florida lakes exhibit a wide scale of variation from 



softwater to hardwater conditions, the resulting chemical gradient 



determined diatom community composition in the same manner that 



the chemical gradient in Adirondacks lakes determined macrophyte 



composition. The fact that diatom community composition was 



