MULTIPLE FACTORS LIMIT SEASONAL-VARIATION IN PERIPHYTON IN A FOREST STREAM

This study was conducted to characterize periphyton biomass, productiv ity, and algal species composition, and to examine how these parameter s changed seasonally over the course of two years in Walker Branch, a temperate forest stream. Seasonal variations in several factors potent ially controlling periphyton (irradiance, streamwater nutrient concent rations, temperature, and the incidence of severe storms) were large. However, there was no consistent seasonal variation in herbivore (snai l) density, which was high (> 1000 animals/m2). Regression analysis wa s conducted to determine the influence of potential controlling factor s on periphyton biomass and productivity. Seasonal changes in periphyt on biomass were small and only weakly related to irradiance (-), snail density (-), and temperature (+). Biomass was lowest in early spring and highest in summer in 1989; but during the following year, biomass was similar year-round. Seasonal variation in primary productivity was also relatively small, but was positively related to inorganic nitrog en concentration and was highest during summer. Although there were so me small seasonal changes in algal species composition, more striking was the fact that Stigeoclonium sp. (primarily grazer-resistant basal cells) dominated year-round, forming > 45% of total algal biovolume. T he lack of strong seasonal variation in periphyton biomass and product ivity and the observed dominance by a grazer-resistant alga appeared t o be primarily the result of the high and relatively constant density of snails. The lack of a relationship between periphyton biomass and p roductivity also suggested an overriding effect of snails, which can c onsume increases in productivity when they occur. These findings sugge st that in streams where herbivore density is high, periphyton product ivity and biomass may remain relatively constant, despite seasonal flu ctuations in potentially limiting physical and chemical variables. Alt hough high densities of snails were probably the primary cause of the lack of large seasonal variation in periphyton biomass, productivity, and species composition, strong asynchrony in several growth-limiting factors, such as nutrients and irradiance, may also have limited seaso nal changes in periphyton, implying that multiple factors were importa nt.