Light intensity regulates growth and reproduction of a snail grazer (Gyraulus chinensis) through changes in the quality and biomass of stream periphyton

Summary 1. The light : nutrient hypothesis (LNH) proposes that herbivore growth rates are maximised at intermediate light-to-nutrient ratios. A reduction to light intensity (i.e. decreased light-to-nutrient ratio) should lead to reduced food availability for herbivores while excessive light intensity in oligotrophic environments (i.e. increased light-to-nutrient ratios) should increase the C : N and C : P ratios of producers. However, this hypothesis has not yet been supported by studies on stream ecosystems. 2. We tested the LNH by experimental application of controlled natural gradients in light intensity to oligotrophic laboratory channels that included periphyton and the freshwater snail Gyraulus chinensis. 3. The results in this oligotrophic environment indicate that light regulated the flow of matter between trophic levels and grazer reproduction by controlling C : P ratios of the producers.