Results
Photosynthetic active radiation (PAR) in the water column was lower in
the sections with larger shaded areas throughout the experiment (Fig.
2a). Water temperature varied from 18 to 25ºC during the experiment but
showed no notable differences in mean values of the water columns or the
vertical profiles among the four treatments of the two ponds regardless
of degree of the shading (Fig. S1(a), S2). In all treatments, pH values
gradually decreased towards the end of experiment and were higher in
pond 218 (Fig. S1(b)). Dissolved oxygen (DO) concentration varied among
the treatments and between the ponds, but were within the range of 5 to
12 mg L-1 (Fig. S1(c)).
Phytoplankton biomass (mg C L-1) correlated
significantly with chlorophyll a (µg L-1)
(r =0.702, p <0.001), varied temporally (Fig. S3),
and was generally higher in the no shade treatments, followed by the low
shade treatments in both Pond 217 and 218 (Fig. 2b). Zooplankton biomass
also varied temporally (Fig. S4) and was generally lower in low shade
treatments compared with other treatments (Fig. 2b). To remove effects
of the initial conditions, we calculated mean phytoplankton (P )
and zooplankton biomasses (H ) in samples collected during the
period from June 10 to August 28. Phytoplankton biomass was lower in
Pond 218 regardless of the treatments, but such a notable difference
between the ponds was not found in zooplankton biomass. Accordingly, no
significant relationship was found between the mean values of
phytoplankton and zooplankton biomass (Fig. 2a).
Both for zooplankton and phytoplankton, the community compositions were
similar among the four treatments within the same pond (PERMANOVA,F =0.993, p =0.42 for algae; F =1.23, p =0.34
for zooplankton) but significantly differed between the two ponds
(F =1.59, p =0.017 for algae; F =3.82, p =0.047
for zooplankton). In zooplankton communities, copepods predominated in
pond 217, while large cladocerans including Daphnia occurred
abundantly in pond 218 (Fig. S5). In phytoplankton communities,
Euglenophyceae and Chrysophyceae occurred abundantly in pond 217, and
Euglenophyceae and Dinoflagellata dominated in pond 218 (Fig. S6). In
all treatments, cyanobacteria biomass was less than 10%. According to
previous knowledge (Lampert and Sommer 2007), we defined that
phytoplankton smaller than 30 µm for the major axis of the cell or
colony were edible. Then, we estimated the ratio of edible phytoplankton
biomass to total phytoplankton biomass as a fraction of the edible
phytoplankton (αedi ). It varied from near zero to
almost one in all the treatments of both ponds (Fig. S3). Seston carbon
to phosphorus ratio varied from 90 to 310 (Fig. S7) and was higher for
treatments with less shade in pond 217, while in pond 218 seston carbon
to phosphorus ratio did not vary among the treatments (Fig. 3b).
Chlorophyll a specific daily production rate estimated from the
P-I curve (Fig. S8) varied temporally depending on weather conditions
but was, in general, higher in treatments with less shade (Fig. 3c).
Daily primary production rates also varied and were higher in treatments
with less shade in pond 217, although among the treatments in pond 218
the levels were similar (Fig. S4).
Observations of fish abundance in each treatment section, determined
using minnow traps, showed that banded killifish (Fundulus
diaphanus ) and fathead minnow (Pimephales promelas ) were present
(Fig. S9). Both fish species were collected on all sampling dates in
pond 217 but were not caught after June 21 in pond 218 (Fig. S4). Thus,
mean abundance of these fish species (θ ) was higher in pond 217
than in pond 218 (Fig. 3d). In the former pond, fish abundance also
varied among the treatments, and was greater in no shade treatments than
in any of other treatments. Neither mean of zooplankton biomass
(r = 0.310, p = 0.45) nor mean of specific production rate
(μ ) (r = 0.247, p = 0.56) were significantly
related to mean fish abundance (θ ).
Throughout the study period, the mass ratio of zooplankton and
phytoplankton varied temporally (Fig. S4). Among the treatments, the
temporal mean of this ratio (H/P ratio) was highest in the mid
shade treatment and lowest in the low shade treatment in both ponds
(Fig. 3). However, the H/P mass ratio was higher in pond 218 than
in pond 217. A significant relationship was not detected between theH/P mass ratio and mean PAR in the water column (Fig. 2a;r = 0.155, p = 0.714), mean frequency of edible
phytoplankton (αedi ) (Fig. 3a; r = 0.241,p = 0.565), mean seston carbon to phosphorus ratio
(αnut ) (Fig. 3b; r = −0.265, p =
0.523), and mean specific production rate (μ ) (Fig. 3c; r= 0.081, p = 0.849), whilst a significantly negative relationship
was detected between the H/P mass ratio and mean fish abundance
(θ ) (Fig. 3d; r = −0.818, p = 0.013).
We fitted the H/P mass ratio by αedi ,αnut , μ , and θ among treatments in
the two ponds using a multiple regression linear model. The variance
inflation factors (VIFs) for these explanatory variables ranged from
1.05 to 2.38, indicating a low probability of multicollinearity among
explanatory variables. Moreover, an analysis with the generalized linear
model showed that the model including all of these parameters had the
lowest value of Akaike’s Information criterion (Table S2), indicating
that it was the best model. The multiple regression analysis revealed
that all of these four variables were indeed significant, as evidenced
by the 95% confidence intervals (CIs) that were smaller or larger than
zero, and explained 94% of variance in the H/P ratio (Table 1).
In addition, partial regression analysis showed that all the partial
correlation coefficients of these factors were significant (Fig. 4),
indicating that effects of these explanatory variables on the H/Pmass ratio were independent of each other. More importantly, the
regression coefficient was significantly smaller than zero for seston
carbon to phosphorus ratio (αnut ) while it did
not significantly differ from one for edible phytoplankton frequency
(αedi ) and specific production rate (μ ),
and was smaller than one but larger than zero for fish abundance
(θ ). To examine the effect sizes of these explanatory variables
on H/P mass ratio, we estimated standardized regression
coefficients. The absolute value of the coefficients showed that the
effect size on the H/P mass ratio was highest for θ ,
followed by αnut (Table 1).