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.