Effects of phosphorus enrichment on the spatial heterogeneity of stream periphyton under uneven light conditions

Spatial variations in light and nutrient availability usually lead to a heterogeneous distribution of periphyton biomass within streams, but the effects of nutrient enrichment on periphyton heterogeneity are still poorly understood. We tested phosphorus enrichment effects on the heterogeneity of periphyton quantity and nutritional quality in an environment with uneven light conditions. Natural periphyton was grown at either high or low P supply in circular running water flumes, each of which was half shaded and half in light. After 3 weeks, periphyton biomass was significantly more heterogeneous at high P than at low P supply, as algal production increased with light only in the P-enriched treatment. However, periphyton C:P ratio was equally homogeneous in both P treatments, despite the light differences within each experimental flume. Although this outcome seems to contrast with the light:nutrient hypothesis, it is likely that high-quantity periphyton patches acquired more P and depleted the available P for the low-quantity patches, leading to an even C:P ratio within each flume. Therefore, in an environment with heterogeneous light availability, nutrient enrichment has the potential to increase the heterogeneity of periphyton quantity without affecting the heterogeneity of periphyton quality, due to periphyton patches not being independent of one another.