A. 2004. Effects of macrograzers and light on periphyton stoichiometry. Á/ Oikos 106: 93 Á/104.Ecological stoichiometry describes the biochemical constraints of trophic interactions emerging from the different nutrient content and nutrient demand of producers and consumers, respectively. Most research on this topic originates from well-mixed pelagic food webs, whereas the idea has received far less attention in spatially structured habitats. Here, we test how light as well as grazing and nutrient regeneration by consumers affects growth and biomass of benthic primary producers. In the first laboratory experiment, we manipulated grazer presence (two different snail species plus ungrazed control), in the second experiment we factorially combined manipulation of grazer presence and light intensity. We monitored snail and periphyton biomass as well as dissolved and particulate nutrients (nitrogen and phosphorus) over time. Grazers significantly reduced algal biomass in both experiments. Grazers affected periphyton nutrient content depending on the prevailing nutrient limitation and their own body stoichiometry. In the nitrogen (N-) limited first experiment, grazers increased N both in the periphyton and in the water column. The effect was stronger for grazers with lower N-content. In the phosphorus (P-) limited second experiment, grazers increased the Pcontent of the periphyton, but the grazer with lower N-content had additionally positive effects on algal N. Light reduction did not affect periphyton biomass, but increased chlorophyll-, N-and P-content of the periphyton. These experiments revealed that the indirect effects of grazers on periphyton were bound by stoichiometric constraints of nutrient incorporation and excretion.

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