J... '^'^ '-'- \ ■ ,: 9 9 



used, however, and we assume that plant water content is 90% of 

 fresh weight (Canfield and Duarte 1988), it is possible to multiply 

 mean percent P by the total dry weight of floating-leaved and 

 submerged biomass to obtain an estimate of the kg of P that would 

 be released to the water column assuming 100% decomposition of 

 macrophyte biomass. Dividing this mass of P by the volume of the 

 lake produces a water-column P concentration that represents the 

 macrophyte component of trophic state. This P concentration can be 

 added to water-column total P inferences obtained using equation 

 3.9 to estimate the potential total P content of the water column 

 (WCP, Canfield et al. 1983a). Historical WCP estimates should take 

 macrophyte nutrients into account and thereby provide a more 

 complete assessment of historical trophic state than inferences based 

 solely on water-column total P. 



Whitmore's (1989) model for predicting TSI(TP) from the TROPH 

 1 diatom index is useful for historically assessing changes in lake 

 trophic state, but inferred TSI(TP) values cannot be detransformed to 

 yield the water-column total P inferences necessary to calculate WCP. 

 Two separate equations are used to derive TSI(TP) from total P 

 values, depending on whether a given lake is P-limited or nutrient- 

 balanced (Huber et al. 1982). In historical applications, a lake may 

 have undergone changes in nutrient limitation over time, especially 

 if the lake received agricultural runoff, or if sewage effluent had 

 been directed into the lake. It would be difficult, therefore, to select 

 the appropriate equation to detransform an historic TSI(TP) 

 inference. Equation 3.9 in the present study will provide inferences 

 of log-transformed water-column total P, however, and these 



