1. Controls to reduce loadings of primary nutrients to maintain biotic integrity in rivers and streams have not been widely implemented because the relation between nutrients and chlorophyll, and its consequences for higher trophic levels, is confounded in lotic ecosystems by their openness, the variable degree of nutrient limitation and by the effect of physical factors.

2. The relationship between primary nutrients and biotic integrity in rivers and streams was tested using biological, physical and chemical information collected since 1982 from similar locations in streams throughout Ohio using standard procedures.

3. There was a negative correlation between nutrients, especially total phosphorus, and biotic integrity. The deleterious effect of increasing nutrient concentration on fish communities in low order streams was detectable when nutrient concentrations exceeded background conditions (total inorganic nitrogen and phosphorus > 0.61 mg L–1 and 0.06 mg L–1, respectively).

4. These results suggest that the control of release of toxins and oxygen-demanding wastes to rivers is insufficient to protect aquatic life, and confirm the importance of non-point sources of pollution in catchment planning as well as the combined effect of habitat and riparian quality on nutrient assimilation.