31 



indicates that diatom biomass might be stimulated by nutrient 

 release from macrophyte substrates (Burkholder and Wetzel 1990). 

 The specific objectives of this study are to: 



1 ) obtain information on diatom assemblages in a calibration set of 

 Florida lakes that represent a wide range of macrophyte 

 presence; 



2) identify periphyton and planktonic components of the 

 assemblages; 



3) perform explanatory analyses to determine which variables 

 influence diatom assemblages over the range of macrophyte 

 presence; 



4) construct predictive models that could be used to quantitatively 

 assess historical macrophyte standing crop using percentages, 

 sedimentary concentrations or accumulation rates of diatoms; 

 and 



5) derive a plan to assimilate historic macrophyte inferences into a 

 scheme that permits a more complete assessment of former lake 

 trophic state than models restricted to concentrations of water 

 column nutrients. 



I propose to develop new multivariate models predicting 



historical macrophyte presence in the following manner. For 



explanatory analyses of diatom communities, a cluster analysis will 



be used to identify diatom taxa with similar ecological responses. An 



indirect (unconstrained) ordination method, such as PCA, will be used 



to ordinate taxa in linear combinations that best explain the variance 



between assemblages. The ordination axes will then be correlated 



with environmental variables to determine which variables exert the 



greatest influence on diatom assemblages. Multivariate predictive 



models will be constructed by stepwise linear regression of taxa and 



by the direct ordination method canonical correspondence analysis, 



in which the linear combinations of diatom taxa are constrained in a 



manner best explained by the limnological variables of interest. 



