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that although some macrophytes may be neutral hosts, others affect 

 epiphytic production or community composition. Cattaneo and Kalff 

 (1981) replied that epiphytic biomass was mostly related to the 

 surface area of the substrate on which the epiphytes grow, and that 

 water chemistry exhibits a greater influence than macrophytes on 

 epiphytic production. 



Recent studies by Burkholder and Wetzel (1990) seem to offer a 

 more definitive explanation of macrophyte influence on epiphytes. 

 They measured alkaline phosphatase (APA), an enzyme that 

 catalyzes hydrolysis of organic P compounds to release 

 orthophosphate, in epiphyton growing on natural and artificial 

 plants. They observed, as did Cattaneo and Kalff (1979), that APA 

 concentrations were higher in epiphyton growing on artificial 

 substrates than they were in epiphyton growing on macrophytes. 

 Burkholder and Wetzel concluded that epiphyton on artificial 

 substrates are P-limited and synthesize APA to provide a P source, 

 although the epiphyton growing on macrophytes were not P-limited 

 because of nutrient release by the macrophytes. 



Epiphyton can in turn exert effects that influence the growth of 

 their macrophyte hosts. Nutrients are usually in abundant supply to 

 rooted macrophytes, and macrophytes generally do not seem to 

 compete with epiphyton for this resource. When epiphyton biomass 

 is high, however, epiphyton may shade their macrophyte hosts 

 (Eminson and Moss 1980). Filbin and Barko (1985) observed that 

 epiphytic biomass in Eau Galle Reservoir, Wisconsin comprised up to 

 33% of the macrophyte and epiphyte biomass, and they concluded 

 that epiphyton may have limited macrophyte growth by light 



