Control of Periphyton Biomass in Laurentian Streams (Québec)

Abstract

Hypotheses concerning the factors controlling periphyton biomass are mostly based on experimental evidence. To examine their application under natural conditions, we sampled periphyton and invertebrate biomass in 12 Laurentian streams (Québec) covering a range of total phosphorus from 5 to 60 μg/L. We sampled at open and shaded sites to explore light limitation by canopy cover. Periphyton biomass measured as chlorophyll a or ash-free dry mass was not related to nutrient concentration or canopy cover. Only current velocity and depth explained a significant but relatively small fraction (10% and 20% respectively) of periphyton variation among sites. Grazer biomass and mean grazer size were positively correlated with phosphorus concentration, which explained 48% and 45% of their variation respectively; canopy cover was not significant. These results indicate that, at least in summer under stable flow, grazer biomass rather than periphyton biomass would increase along a nutrient gradient typical in Laurentian streams. The results suggest top-down control of periphyton biomass.