Nutrient-diffusing substrates for periphyton were made from clay flower pots (O.D. = 8.8 cm), sealed with plastic petri dishes, and filled with 2% agar and specified nutrients. When placed in water, the nutrients slowly diffuse through the agar and clay walls of the pots, becoming available to organisms colonizing the outer surface. Forty-eight pots, 16 containing 0.1 M KH2PO4, 16 with 0.1 M NaNO3, and 16 with no added nutrients, were placed at 0.5 m depth in Douglas Lake, Michigan. Four pots of each nutrient treatment were sampled for algal periphyton and invertebrates after 7, 14, 25, and 36 days. A total of 72 algal species were enumerated. Of these, Epithemia adnata (Kutz.) Breb., Rhopalodia gibba (Ehr.) O. Mull, and Anabaena sp. experienced strong growth stimulation in response to phosphate addition. No significant effects of nitrate addition were noted. Measures of algal community structure also reflected the impact of phosphate addition. Final algal biomass was enhanced 10-fold, successional change was prolonged, and species diversity declined relative to nitrate and control pots. Chironmids, chydorid Cladocera, and gastropods dominated the invertebrate fauna found on the pots. Densities of the small, algivorous chironomid Corynoneura nr. lobata Edwards were highest on phosphate pots at the end of the study, in apparent repsonse to the increase in periphyton biomass.