| Light and nutrients are essential resources necessary for the growth and survival of primary producers; and algal biomass, elemental ratios, and community composition are affected by both the absolute and relative levels of light and nutrients. In addition, changes in algal food quantity and quality can affect the feeding behavior of algal consumers. Algal and grazer responses to light and nutrient availabilities are further influenced by the relative openness of the system to grazer immigration, particularly when algal resistance to grazing changes with ontogeny. To examine the combined effects of light and nutrients on algae and invertebrate grazers, and the influence of system openness on these interactions, I conducted three field experiments in Convict Creek, a subalpine stream in the Eastern Sierra of California.; First, I conducted a field experiment to determine which factors limited algal biomass. Algal biomass was limited by nitrogen, and the magnitude of algal nitrogen limitation depended on light levels. When nitrogen levels were high, algal biomass increased as light levels increased. At ambient nitrogen levels, algal biomass decreased with increasing light levels.; Next, in two field experiments using artificial stream channels, I investigated the effects of light and nutrients on algal biomass, elemental composition, and community composition, and the effects of algal biomass and quality on invertebrate grazers in systems with restricted and open grazer immigration. The effects of light and nutrients on algal biomass and elemental composition were interactive. Invertebrate grazers responded more strongly to algal biomass than quality, but invertebrate responses depended on both system openness and grazer taxa. Filamentous algae were abundant in high light channels when grazer immigration was restricted, and baetid mayfly responses suggested that much of the filamentous biomass was unavailable. Chironomids responded positively to increased algal biomass. When channels were open to grazer immigration, filamentous algal blooms did not occur, and both baetids and chironomids responded positively to increasing light and nutrient levels, suggesting top down regulation of algal biomass in open systems. |
