Use of size spectra and empirical models to evaluate trophic relationshipsin streams

We measured the biomass size distributions of algae, protozoa, and inverteb rates in several streams of Eastern Ontario and Western Quebec and related assemblage biomass to nutrient (nitrogen and phosphorus) concentrations in the water. Size spectra and measurement of periphytic chlorophyll were then combined with existing empirical models to estimate primary production, in vertebrate production, and grazer removal, to examine herbivory in these na tural assemblages. In general, biomass of organisms increased with nutrient s but the response of invertebrates was stronger than that of algae and pro tozoans. Secondary production (range 1.7%-4.2%) and algal removal by grazer s (range 62%-175%) were high relative to primary production. This suggests that grazers exert top-down control on algae in these streams and that incr eases in nutrient inputs to oligo- and mesotrophic streams may benefit cons umers more than primary producers.