Effects of nutrients and light on periphytic biomass and nutrient stoichiometry in a tropical black-water aquatic ecosystem

This article aims to test the light-nutrient hypothesis (LNH) in a periphytic community in a tropical black-water lake. Individual and interactive effects of light and nutrient availability were assessed with periphyton biomass accrual, nutrient content, and nutrient stoichiometry. We performed a manipulative field experiment with a 4 × 2 factorial design. We used nutrient diffusing substrates to produce four different nutrients treatments: Control (no nutrient added), nitrogen amended (N), phosphorus amended (P) and combined N and P amendment (NP). Two light levels were also considered: high light (near surface water) and low light (near bottom water). Light and nutrients individually and interactively caused significant changes in aggregate periphyton community properties. Total and autotrophic biomasses were significantly higher in high light conditions and in nutrient enriched treatments. Autotrophic biomass was significantly higher in N enriched treatment whereas total biomass was mainly affected by the joint addition of N and P. At lower light availability periphyton growth was limited, even in enriched treatments. Light also strongly affected periphyton nutrient content. Periphyton C, N and P in general increased when subjected to high light conditions. As predicted by the LNH, light promoted an increase in periphyton C:P ratios in P deprived treatments, but an opposite effect was observed on C:N ratios, especially in N-enriched treatments. This experiment revealed that light availability strongly limits the propagation of nutrient effects on periphyton growth. Such complex interdependencies on basal resources affect the proportion of autotrophic to total periphytic biomass that can be an important mechanism to explain variation in the nutrient stoichiometry of periphyton in nature.