Species-specific stoichiometric effects of leaf litter on algal growth, production, and cell quotas

Dissolved organic matter and nutrients released from leaf litter are important cross-ecosystem resources supporting freshwater food webs. Dissolved organic matter supports heterotrophic organisms in freshwater communities. However, the role of nutrients released from leaf litter in the autochthonous production in aquatic ecosystems is not well understood. Therefore, we investigated how dissolved nutrients released from leaf litter affect algal growth, biomass production, and cellular elemental ratios. Specifically, we experimentally examined the response of green algae to nitrogen (N) and phosphorus (P) released from leaf litter of 11 temperate tree species and the degree of deficiency of nutrient elements other than N and P relative to algal demand in these litter leachates by supplementing these elements. We found that algal growth did not significantly increase with dissolved N or P in the leaf leachates. In contrast, algal biomass production increased with dissolved N concentration, regardless of amendment. Algal growth and production limitation by deficiency of elements other than N and P was found only in the leachate of Japanese hemlock, indicating that the concentrations or release efficiencies of these elements in this leaf litter were lower than those of N and P relative to algal requirements. More importantly, leaf litter leachates from different tree species altered algal cell quotas and C:N:P ratios, which would affect secondary production. These results suggest that variations in leaf litter leachate stoichiometry caused by vegetation change would affect the abundance and chemical composition of phytoplankton, and thus the trophic transfer efficiency between phytoplankton and herbivorous zooplankton.