Discussion
The effect of leaf litter leachates on the growth, production, and cell
stoichiometry of the green alga Scenedesmus varied significantly
among the leaf litter leachates of different tree species. As nutrient
concentrations were not the same among the culture media, growth rates
in different types of leachates could not be directly compared with each
other. Therefore, we plotted the C biomass accumulated during the 7-day
incubation period against TDN and TDP in the leachate. The result showed
that the accumulated C biomass increased with increasing TDN in the
leachate regardless of the micronutrient amendment, although the
increasing trend was attenuated at higher TDN concentrations. In
addition, C biomass was highly limited in the leachates of Siebold’s
beech, Japanese elm and Japanese hemlock regardless of high TDP
concentrations (species ID 5, 6, 7 in Fig. 2g and h). These results
suggest that the variation in C biomass among the different culture
media from leaf litter leachate was mainly caused by TDN concentration.
In a previous study, Ho et al. (2023) suggested that nitrogen is the
most deficient element in the leaf litter leachates relative to
phytoplankton demand, because the percentage of release relative to leaf
litter elemental content was lowest for TDN compared to DOC and TDP in
all 11 tree species studied. The present study confirmed the validity of
this suggestion for phytoplankton production.
However, the algal growth rate,
estimated from temporal changes in cell abundance, differed from that of
C biomass. In the present study, it ranged from 0.2 to 0.3
d-1in for most of
the leaf litter leachates, except for Japanese elm and Japanese hemlock,
where the growth rate was <0.2 d-1. The
result suggests that the nutrient conditions in the present study were
moderate according to the growth rate of Scenedesmus under
different culture conditions in previous studies . However, the growth
rate was not closely related to either TDN or TDP in leaf litter
leachate, especially when micronutrients were not amended. The result
suggests that the factors regulating growth rate, i.e., cell division
rate, and net C biomass accumulation rate, i.e., organic carbon
production rate, are different in this algal species.
It should be noted that when micronutrients were added, the algal growth
rate significantly increased in the leaf litter leachates of oak,
Japanese elm and Japanese hemlock, although the opposite trend was found
in the leachate of hinoki cypress. The results contrasted with the
response of C biomass, which showed a significant difference between the
culture medium with and without micronutrient amendment only in Japanese
hemlock. These results suggest that the fitness of the present algal
species in the terms of cell division rate is more sensitive to
micronutrient supply than the net production rate. In fact, the algal
growth rate was significantly correlated with TDP but not TDN when
micronutrients were additionally supplied. This fact implies that the
variation of algal growth rate among different types of leaf litter
leachates was mainly regulated by P supply in concert with
micronutrients, whereas that of net production rate was more regulated
by N supply. The relative importance of P rather than N may be related
to the fact that P is an essential element of DNA and RNA and can
strongly limit cell division .
Micronutrients are known to control the enzyme activity that regulates
the N and P use efficiency of phytoplankton . Indeed, in leaf litter
leachates from Japanese red pine and hinoki cypress, a significant
increase in N and P cell quotas was found when micronutrients were
additionally supplied. In these types of leaf litter leachates,
micronutrients may play a role in P and N use efficiency. In addition,
micronutrients appear to play different roles in algal growth and C
biomass accumulation. Several studies have shown that the factors
affecting algal cell division rates are different from those affecting
the photosynthetic production rates . For example, Li et al. showed that
in the dinoflagellate Amphidinium carterae , the cell division
cycle was halted but the photosynthesis continually accumulated C in
cells under P deficiency. On the contrary, Negi et al. found that in the
green alga Chlorella sorokiniana , N deficiency did not change
the cell density, but decreased the photosynthetic rate.
As mentioned above, the addition of micronutrients tended to reduce the
growth rate of Scenedesmus incubated in a culture medium from the
leaf litter leachate of hinoki cypress. The result suggests that the
leaf litter leachate of hinoki cypress may contain excessive amounts of
some micronutrients that are harmful to Scenedesmus . Rueter et
al. showed that Al and Cu tended to reduce the growth rate ofScenedesmus when supplied in excess. Because we added a mixture
of micronutrients, we could not determine which micronutrients and trace
elements were harmful when supplied in excess. However, the present
study indicates that the water-soluble content of micronutrients and
trace elements varies widely among the leaf litter of different tree
species as suggested by Pourhassan et al. .
Studies have shown that decomposition and nutrient release rates
somewhat differ between coniferous and broadleaf tree species . However,
we could not determine whether leaf litter leachates from broadleaf or
coniferous trees had a better nutrient composition for the growth rate
of Scenedesmus due to the large variation of growth rate within
each of these two groups. In addition, we could not find significant
difference in the C biomass production rate of this green alga between
the culture media from broadleaf and coniferous leaf litter leachates
due to the large variations in the rate within these tree species. These
results suggest that the effect of leaf litter on aquatic organisms
cannot be generalized across broad taxonomic tree groups.
Similar to the growth and C biomass production rates, leaf litter
leachates from different tree species had different effects on the C:N:P
ratio of the algae. It is well known that the elemental contents of
algae are important determinants of the quality of food for herbivorous
zooplankton . In this study, the molar C:N and C:P ratios ofScenedesmus varied from 6.6 to 21 and 106 to 600, respectively,
depending on the leaf litter leachate. In general, algal cells with C:N
and C:P ratios close to Redfield ratios are good quality food for
zooplankton growth . In the leaf litter leachates of Siebold’s beech,
Japanese elm, Japanese red pine and hinoki cypress, the molar C:N and
C:P ratios ranged from 6.6 to 12 and 106 to 160, respectively, which are
close to the Redfield ratios. Thus, algae grown in lentic waters
receiving nutrients from leaf litter of these species would be suitable
food for herbivorous zooplankton. However, in the leaf litter leachates
from Japanese maple and Japanese cedar, the C:P ratio ofScenedesmus cells was greater than the threshold elemental ratio
(>300) above which the growth rate of zooplankton such asDaphnia is limited by phosphorus . Urabe and Watanabe also
suggest that algal cells with C:N ratios greater than 15 are suboptimal
food for zooplankton. In this study, the C:N ratio of Scenedesmuscells in the leaf litter leachates of
Japanese maple and Japanese hemlock
was greater than this value. In addition, depending on the leaf litter
leachate of different tree species, algae may alter the cellular content
of other chemical substances, such as fatty acids and amino acids, that
affect food quality for herbivores. Together with such a possibility,
the present results suggest that the mass transfer from algae to
herbivorous zooplankton in a lake may be influenced by the vegetation
composition in the watershed via subsidies of nutrient inputs from leaf
litter.