Dissolved organic matter and nutrients released from forest leaf litter are important cross-ecosystem resources that support freshwater ecosystems. Dissolved organic matter released from leaf litter is one of the allochthonous sources for heterotrophic organisms in freshwater communities. However, the role of macro- and micronutrients released from leaf litter in producing autochthonous organic matter in aquatic ecosystems is not necessarily well understood. In this study, therefore, we investigated how dissolved nutrients released from leaf litter affect the algal growth, biomass production, and cell elemental quotas. Specifically, we focused on the responses of the green algae to TDN, TDP, and micronutrients released from the leaf litter of 11 temperate tree species. We found that the algal growth rate increased with TDP when micronutrients were amended. In contrast, the algal biomass production increased with TDN, regardless of micronutrient amendment. Micronutrient limitation of algal growth rate was found in the leaf litter leachates from oak, Japanese elm, and Japanese hemlock. However, algal biomass production was limited by micronutrients only in the leaf litter leachate from Japanese hemlock. 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 forest vegetation change or succession affect the quantity and quality of autochthonous organic matter and thus the mass transfer efficiency in the aquatic community by changing the stoichiometry of nutrient input from the leaf litter.

Dissolved organic matter and inorganic nutrients released from forest leaf litter through leaching are the important energy and nutrient sources that support the production of aquatic food webs. Litter leachate-derived dissolved organic carbon (DOC) is a critical energy source for aquatic heterotrophic microbes, and inorganic nitrogen and phosphorus can enhance primary production. In this study, we experimentally measured the release efficiencies and amounts of dissolved organic DOC, total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP) of the leaf litter from 11 temperate tree species by soaking the leaf litter in water for 28 days. We found that the maximal release efficiency (% of element released per estimated mass of the element) was the highest for P and lowest for N. These efficiencies were species-specific. Additionally, the DOC:TDP and TDN:TDP ratios varied among the leachates of different leaf litter species and were considerably lower than the C:P and N:P ratios in leaf litter biomass; the DOC:TDN ratio was higher than the C:N ratio in leaf litter biomass. These results suggested that the ratios of organic carbon to nutrients dissolved into water cannot be fully elucidated using the elemental ratios of leaf litter mass. Based on these findings, we concluded that changes in the vegetations with different leaf litter stoichiometry can alter the relative importance of detrital and grazing food chains in aquatic ecosystems.

The biomass ratio of herbivores to producers reflects the structure of a community. Four primary factors have been proposed to affect this ratio, including production rate, defense traits, and nutrient contents of producers as well as predation by carnivores. However, the relative importance of these factors across natural communities is elusive, in part because of the lack of a framework for quantitatively comparing their effect sizes. Here, we develop a framework based on Lotka-Volterra equations for examining the relative importance among these factors in determining the biomass ratio. We further utilize it to analyze plankton communities in experimental ponds with different carnivore (fish) abundance and light input. We found that all four factors contributed significantly to the biomass ratio, but carnivore abundance had the largest effect size, followed by the stoichiometric nutrient content. The present framework is useful for quantifying relative roles of these factors shaping terrestrial and aquatic communities.