Abstract
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.
Highlights
Light and nutrients are the main abiotic factors that limit the growth of autotrophic periphyton in stream ecosystems (Rosemond, 1993; Warren et al, 2017)
In the P availability (P-)enriched flumes, the heterogeneous light availability interacted with P addition and resulted in a significantly higher periphyton biomass in the illuminated half of the flume compared to the shaded half, in which periphyton growth was apparently light limited
While P enrichment significantly increased periphyton relative P content, light had no effects on periphyton C:P ratio, which only differed between the two nutrient treatments and not between the differentially illuminated areas
Summary
Light and nutrients are the main abiotic factors that limit the growth of autotrophic periphyton in stream ecosystems (Rosemond, 1993; Warren et al, 2017). Periphyton quality is often quantified as algal content of phosphorus (P) and nitrogen (N) relative to carbon (C), i.e. C:P and C:N stoichiometric ratios (Stelzer & Lamberti, 2002; Fink & von Elert, 2006). Periphyton relative content of nutrients decreases as the ratio of light to nutrient availability increases, consistent with the light:nutrient hypothesis (Sterner et al, 1997; Fanta et al, 2010; Hill et al, 2011). Algal PUFA content increases with P availability, as P is needed for the biosynthesis of PUFArich phospholipids and galactolipids, but high-light availability dilutes algal PUFA content while C is accumulated (Hill et al, 2011)
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