Seasonally shifting limitation of stream periphyton: response of algal populations and assemblage biomass and productivity to variation in light, nutrients, and herbivores

Abstract

We determined whether there were seasonal changes in the relative importance of consumers and resources in controlling stream periphyton. Our analysis included effects on algal populations and assemblage biomass and productivity. We used factorial experiments in which we manipulated snail densities, irradiance, and streamwater nutrient concentrations during two seasons, fall and spring, and compared responses with previously published summer findings. Periphyton biomass and productivity were much greater when snails were removed and nutrients and light were elevated during all seasons, indicating that all three factors were limiting or nearly limiting throughout the year. However, the relative importance of factors shifted seasonally. Irradiance limited periphyton biomass in summer and fall but not spring. In contrast, nutrients were more limiting in seasons in which light levels were higher: nutrient addition generally resulted in effects of greater magnitude in fall and spring than in summer. Snail growth was stimulated by enhanced irradiance in summer (p = 0.06) and by nutrient addition in fall, indicating resource limitation of both periphyton and snails. However, top-down control of periphyton by snails was also important: snails maintained low biomass assemblages dominated by only a few grazer-resistant species (e.g, basal cells of Stigeoclonium tenue, Chamaesiphon investiens) during all seasons.