The purpose of this project was to study the dynamics of the periphyton community with relation to nutrients in the Maple River, Emmet County, Michigan. The project included 4 parts: (1) seasonal variation of periphyton community structure (species composition, biomass, and physiognomy) and periphyton response to nutrient enrichment in 2 sites in the Maple River, (2) the effects of macro-invertebrate colonization (mainly filter-feeders) on algal responses to nutrients, (3) the relative importance of nutrients and grazing on the periphyton community structure, and (4) the independent and interactive effects of ammonium, nitrate, and phosphate on periphyton community structure. The experiments were conducted in both natural and laboratory streams. Algal community structure and the effects of nutrient enrichment on algal growth varied seasonally and spatially. In general, algal growth was limited by availability of phosphorus in the summer. Nitrogen might be a secondarily limiting nutrient for algal growth. However, the effects of nitrogen on periphyton varied with nitrogen forms. Algal community growth can be significantly stimulated by either ammonium or nitrate enrichment under the phosphate enriched conditions. Algal species, however, showed different responses to nitrate, ammonium, or nitrate plus ammonium as inorganic nitrogen sources. Both Gomphonema parvulum and Nitzschia palea strongly preferred ammonium as an inorganic nitrogen source and had no significant response to nitrate enrichment. Achnanthes minutissima had no preference. Algal responses to nutrient addition can be masked by invertebrate colonization (hydrophsychids) if the response is only measured as chlorophyll a. Enhancement of algal biomass by nutrient loading can also be masked by grazing activities.