Effects of nutrient enrichment on algal biomass across a natural light gradient

Abstract

Algae are limited by light and nutrients, and the combined effects of light and nutrients on algae are often more important than their independent effects. To investigate the interactive effects of light and nutrients on benthic algae, we measured the effects of nutrient enrichment on chlorophyll-a concentrations across a natural light gradient in a Sierra Nevada subalpine stream using nutrient diffusing substrata (NDS) bioassays during the summer of 1999. Algal chlorophylla concentrations were nitrogen limited across the entire light gradient. Chlorophyll-a concentrations increased with increasing light levels for algae growing on substrata to which nitrogen was added, whereas chlorophyll-a concentrations decreased with increasing light levels for algae growing on substrata receiving phosphorus or no nutrient additions. These results demonstrate that for essential resources such as light and nutrients, the magnitude of the response to enrichment by one resource depends on the relative availability of the other resource.