Introduction
Dissolved organic and inorganic matter released from forest leaf litter is an important cross-ecosystem resource that influences the communities in freshwater ecosystems . Numerous studies have shown that, in lakes and ponds, terrigenous dissolved organic carbon (DOC) can enhance heterotrophic microbial production and reduce underwater light intensity, thereby reducing the algal growth and primary production rates . However, because leaf litter contains various elements other than C, including nitrogen (N) and phosphorus (P), it is likely to promote autochthonous production through algal growth in aquatic ecosystems where leaf litter is introduced. Indeed, recent studies have shown that leaf litter stimulates algal growth and increases energy transfer from autochthonous organic matter to higher trophic levels through the grazing food chain in lakes .
However, the effects of leaf litter on aquatic organisms through the release of nutrient elements may vary among tree species because the elemental content of leaves is highly species-specific . In addition, the water-soluble fractions of nutrient elements in leaf litter also vary among tree species . In fact, our previous study showed that the concentrations of dissolved N and P in leaf litter leachates varied widely among 11 tree species because the release efficiencies of these nutrients differed among tree species (Ho et al. 2023). These results suggest that the production of autochthonous organic matter is affected by the change and succession of the surrounding terrestrial vegetation through the changes in the inputs of various dissolved nutrient elements from the leaf litter. To test this possibility, it is essential to investigate the response of algae to leaf litter leachates from different tree species.
In freshwater ecosystems, the algal production rate of is mainly limited by the supply rate of dissolved P, although at certain times it may be limited by N or co-limited by N and P . In addition, essential micronutrients, such as iron, are known to affect algal production and growth rates in aquatic ecosystems , because these trace metals are essential core elements of mechanoenzymes that play various functional roles in cell metabolism and catabolism . Therefore, if the relative and absolute levels of these micronutrients released from leaf litter differ among different tree species, algal production and growth rate may change depending on the dominant tree species growing around the freshwater ecosystems.
In autotrophs such as algae, the biomass production is related to its photosynthetic activities, cell division rates, i.e., growth rate, and cell quotas, i.e., cell-specific C, N, and P contents, are affected by various cell processes other than photosynthesis . Accordingly, the nutrient elements that affect algal production rates are not necessarily the same as those that affect growth rates . More importantly, the growth of herbivorous zooplankton is influenced by the cellular N and P contents relative to the C content in the algae . This fact suggests that not only the production rate of autochthonous organic matter but also its mass transfer to higher trophic levels may be affected by changes in the dominant tree species growing around the freshwater ecosystems via macro- and micronutrients supply from fallen and submerged leaf litter. However, although several studies have shown that leaf litter can stimulate algal growth and increase energy transfer to zooplankton , no study has yet examined whether such effects differ among leaf litter from different tree species.
In the present study, therefore, we investigated whether the algal response is different depending on the tree species providing the leaf litter to freshwater ecosystems. For this purpose, we prepared culture media using leaf litter leachates from 11 common tree species in the plain and mountainous regions of northeastern Japan. Then, we investigated the cell growth rate, carbon biomass production, and the C, N, and P cell quotas of a green alga Scenedesmus obliquus . We used Scenedesmus because this group of green algae is common freshwater phytoplankton and has been used in various experimental studies . Our specific objectives are to clarify (1) if N and P nutrient supply relative to algal requirements for growth and production differ among the leaf litter leachates of the 11 tree species, (2) if the leaf litter leachates provide sufficient micronutrients, and (3) if the cell C, N, and P quotas of the algal species vary depending on the type of leaf litter leachates. Based on these results, we discuss the possibility that the quantity and quality of autochthonous organic matter produced by algae and its mass transfer to higher trophic levels in aquatic ecosystems are influenced by the changes in tree species composition in the surrounding forests.