Simultaneous gradients of phosphorus and light were applied in experimental streams to develop quantitative relationships between these two important abiotic variables and the growth and composition of benthic microalgae. Algal biovolume and whole-stream metabolism responded hyperbolically to phosphorus enrichment, increasing approximately two-fold over the 5-300 mg L 21 range of experimental phosphorus concentrations. The saturation threshold for phosphorus effects occurred at 25 mg L 21 of soluble reactive phosphorus (SRP). Light effects were much stronger than those of phosphorus, resulting in a nearly ten-fold increase in algal biovolume over the 10-400 mmol photons m 22 s 21 range of experimental irradiances. Biovolume accrual was light-saturated at 100 mmol photons m 22 s 21 (5 mol photons m 22 d 21 ). Light effects were diminished by low phosphorus concentrations, and phosphorus effects were diminished by low irradiances, but evidence of simultaneous limitation by both phosphorus and light at subsaturating irradiances was weak. Contrary to the light : nutrient hypothesis, algal phosphorus content was not significantly affected by light, even in the lowest SRP treatments. However, algal nitrogen content increased substantially at lower irradiances, and it was very highly correlated with algal chlorophyll a content. Phosphorus enrichment in streams is likely to have its largest effect at concentrations ,25 mg L 21 SRP, but the effect of enrichment is probably minimized when streambed irradiances are kept below 2 mol photons m 22 d 21 by riparian shading or turbidity.

1. This study investigated the combined effects of light and phosphorus on the growth and phosphorus content of periphyton. To investigate the potential for colimitation of algal growth by these two resources, diatom-dominated periphyton communities in large flowthrough laboratory streams were exposed under controlled conditions to simultaneous gradients of light and phosphorus. 2. Periphyton growth rate was predictably light-limited by the subsaturating irradiances (12-88 lmol photons m )2 s )1 ) used in this experiment. However, phosphorus concentration also limited growth rate: growth increased hyperbolically with increasing soluble reactive phosphorus (SRP), reaching a threshold of growth saturation between 22 and 82 lg L )1 . 3. Periphyton phosphorus content was strongly and nonlinearly related with SRP, reaching a maximum at 82 lg L )1 SRP. Contrary to the Light : Nutrient Hypothesis, periphyton phosphorus content did not decrease with increasing light, even at the lowest concentrations of SRP. Periphyton phosphorus was highly correlated with periphyton growth rate (Spearman's q ¼ 0.63, P < 0.005). 4. Multiple regression analysis reinforced evidence of simultaneous light and phosphorus limitation. Both light and periphyton phosphorus content were significant variables in multiple regressions with growth parameters as dependent variables. Light alone accounted for 67% of the variance in periphyton biomass, and the addition of periphyton phosphorus as an additional independent variable increased the total amount of variance explained to 81%. 5. Our results did not support the hypothesis that extra phosphorus is required for photoacclimation to low light levels. Rather, the effect of additional phosphorus may have been to accommodate increased requirements for P-rich ribosomal RNA when growth was stimulated by increased light. The potential colimitation of periphyton growth by phosphorus and light at subsaturating irradiances has important implications in both theoretical and applied aquatic ecology.

1. The light : nutrient hypothesis (LNH) states that algal nutrient content is determined by the balance of light and dissolved nutrients available to algae during growth. Light and phosphorus gradients in both laboratory and natural streams were used to examine the relevance of the LNH to stream periphyton. Controlled gradients of light (12-426 lmol photons m )2 s )1 ) and dissolved reactive phosphorus (DRP, 3-344 lg L )1 ) were applied experimentally to large flow-through laboratory streams, and natural variability in canopy cover and discharge from a wastewater treatment facility created gradients of light (0.4-35 mol photons m )2 day )1 ) and DRP (10-1766 lg L )1 ) in a natural stream. 2. Periphyton phosphorus content was strongly influenced by the light and DRP gradients, ranging from 1.8 to 10.7 lg mg AFDM )1 in the laboratory streams and from 2.3 to 36.9 lg mg AFDM )1 in the natural stream. Phosphorus content decreased with increasing light and increased with increasing water column phosphorus. The simultaneous effects of light and phosphorus were consistent with the LNH that the balance between light and nutrients determines algal nutrient content. 3. In experiments in the laboratory streams, periphyton phosphorus increased hyperbolically with increasing DRP. Uptake then began levelling off around 50 lg L )1 . 4. The relationship between periphyton phosphorus and the light : phosphorus ratio was highly nonlinear in both the laboratory and natural streams, with phosphorus content declining sharply with initial increases in the light : phosphorus ratio, then leveling off at higher values of the ratio. 5. Although light and DRP both affected periphyton phosphorus content, the effects of DRP were much stronger than those of light in both the laboratory and natural streams. DRP explained substantially more of the overall variability in periphyton phosphorus than did light, and light effects were evident only at lower phosphorus concentrations (£25 lg L )1 ) in the laboratory streams. These results suggest that light has a significant negative effect on the food quality of grazers in streams only under a limited set of conditions.