Many natural streams are net heterotrophic, so I propose that trophic state be divided into autotrophic and
heterotrophic state. This division allows consideration of the influence of external carbon sources as well as nutrients
such as nitrogen and phosphorus. Empirical results suggest that phosphorus and nitrogen are the most important
nutrients regulating autotrophic state in flowing waters and that benthic algal biomass is positively correlated to
gross primary production in streams. Reference (minimally influenced by human activities) nutrient concentrations
and correlations of nutrients with algal biomass are used to characterize reference distributions of stream autotrophic
state. Only when reference nutrient concentrations are in the upper one third of those expected in the United States,
is maximum benthic chlorophyll projected to exceed 100 mg m!2 (a concentration commonly used to indicate
nuisance levels) "30% of the time. Average reference nutrient concentrations lead to sestonic chlorophyll concentrations
above those considered typical of eutrophic lakes ("8 mg m!3
) less than half the time. Preliminary analysis
suggests that autotrophic state is variable in small pristine streams because it is influenced by canopy cover (light),
but heterotrophic state is less variable because it can be based on allochthonous or autochthonous production.
Nitrogen and phosphorus enrichment can influence both heterotrophic and autotrophic state, and these effects could
cascade to animal communities. Stoichiometry should be considered because carbon, nitrogen, and phosphorus are
all involved in trophic state. The proposed definition of trophic state offers a starting conceptual framework for
such considerations.