14 



oligotrophic lakes because of the importance of macrophyte nutrient 

 loss to epiphytes, whereas host specificity was less pronounced in 

 mesotrophic and eutrophic lakes because of the greater effect of 

 water-column nutrients on periphytic taxa. 



Several studies have demonstrated the importance of diatom 

 growth form to patterns of colonization and physical structure of 

 epiphytic diatom communities. Achnanthes and Cocconeis are 

 solitary cells that lie adnate to the substrate and colonize 

 horizontally, and these genera are usually the initial colonizers on 

 new substrate (Robinson and Rushforth 1987). Later colonizers must 

 contend with space limitations, and genera such as Gomphonema and 

 Cymbella are at an advantage because they grow on long stalks and 

 colonize in a vertical orientation (Roemer et al. 1984, Robinson and 

 Rushforth 1987). This upward expansion of the epiphytic community 

 improves light and nutrient availability for taxa in the higher tiers 

 (Hudson and Legendre 1987), though some adnate forms below such 

 as Cocconeis placentula var. euglypta (Ehr.) CI. exhibit shade 

 tolerance (Robinson and Rushforth 1987). Swift-moving taxa capable 

 of complex movements including Nitzschia and Navicula can be 

 observed within the community matrix (Hudson and Legendre 1987). 

 As the thickness of periphyton on the substrate becomes too great, 

 cells on the outer tiers are subject to loss by grazing or sloughing off 

 by currents (Roemer et al. 1984, Hudson and Legendre 1987). 

 Sloughed off periphytic taxa can become part of the planktonic drift 

 and are then referred to as tychoplanktonic (Lowe 1974). 



