2.4 Data analysis
All statistical analyses were conducted in SPSS 17.0. Specifically, for
the short-term depuration experiments, a paired t-test was used to
compare the mean values of the food carbon AE measured under the
different
depuration
times (24 h and 36 h), and an independent t-test was used to compare
mean values among different depuration times in the same experiment or
among experiments with different food rations at the same time. For the
long-term depuration experiments, analysis of covariance (ANCOVA) was
used to compare the release rates of fish body carbon under different
daily food rations. Bivariate correlation with the Pearson correlation
coefficient was used to examine the correlation of the proportions of
DOC, CO2, and PC with depuration time in the long-term
depuration experiments.
3 Results
3.1 Recovery of 14C released in seawater
The results showed that the 14C released from the food
and fish body sources could be sufficiently recovered. For the food
carbon,F14C ’release/F14C releasewere 105%, 108% and 112% at food rations of 2.2%, 3.2% and 5.7% of
the fish DW, respectively. For the fish body carbon,B14C ’release/B14C releasewere 89% and 105% at the daily food rations of 2.2% and 4.3% of the
fish DW, respectively.
3.2 Food carbon assimilation efficiency and release
The amount of ingested carbon retained in the marine medaka decreased
quickly
(0.07–0.15
h-1) during the first
4
h, and then the decrease slowed to lower rates (0.02–0.03
h-1) until the end of the depuration (Figure 3).
During
the depuration from 24 h to 36 h, there were no significant changes in
the carbon retained in fish at any of the three food rations (t-test,p > 0.10) (Figure 3). The food carbon AE decreased
with increasing food rations (Figure S1). Based on the experiments with
different food rations, the carbon AEs in the fish were 30%–49%
(38.9% on average) and 25%–47% (33.7% on average) corresponding to
depuration times of 24 h and 36 h, respectively; the difference was not
statistically significant (paired
t-test, p >
0.05).
Figure 3 .
Retention
of ingested carbon in marine medaka during the 36-h depuration.
Ingested
food rations are expressed in percentages of fish dry weight (DW). Data
are the mean ± SD (n = 3–5). The
error bars represent the standard deviations.
Note the natural logarithm scale in
the left subfigure.
The proportions of carbon released as DOC, CO2, and PC
varied during the depuration (Figure 4). During the first 2 h, most of
the released carbon was DOC (55%–60%), and the proportion of DOC
decreased during the first 16 h (to
39%–55%).
In contrast, the proportion of CO2 increased (from
25%–32% to 40%–54%) during
the first 16 h. The contribution of PC peaked during the first 4 h (up
to 15%–25%); thereafter, it decreased quickly to the lowest values
(to less than 8%) during the depuration from 8–16 h, and remained
constant until the end of the depuration
(Figure 4a, b, c).
The release rates of
DOC,
CO2, and PC from ingested food decreased
with the depuration time. With the
food ration of 2.2% fish DW, the release rates of DOC,
CO2, and PC decreased from 0.61, 0.32, and 0.13 µg C/mg
DW/h at the beginning to 0.03, 0.05 and
<
0.01 µg C/mg DW/h during the depuration from 8–16 h, respectively. With
the increase in food rations, all release rates of DOC,
CO2, and PC were increased proportionally (Figure 4d, e,
f). However, no significant differences in the release rates of PC among
the different food rations were observed after 8 h of depuration
(t-test, p > 0.1), and the release rates of PC at
all three food rations remained constant (< 0.01 µg C/mg DW/h)
after 16 h of depuration (Figure 4d, e, f) (t-test, p> 0.1).
Figure 4 . Relative
contributions (a, b, c) and release rates (d, e, f ) of
different forms of carbon at different stages of the 36-h depuration
under different food rations. Data are the mean ± SD (n =3–5). The
error bars represent the standard deviations. DOC, dissolved organic
carbon; CO2, carbon dioxide; PC, particulate carbon.
Ingested food rations are expressed in percentages of fish dry weight
(DW)
During the whole depuration period, most of the unassimilated food
carbon was released into seawater in the forms of DOC
(48%–59%)
and CO2 (30%–40%), and only 11%–13% of the
released carbon was PC in fecal pellets (Figure 5a).
Taking the ingested food carbon as 100%, for the 36-h depuration,
25%–45%
(34.8% on average),
18%–29%
(22.9% on average), and 7%–8% (7.6% on average) of the food carbon
were released as DOC, CO2, and PC, respectively (Figure
5b). For the 24-h depuration, the released DOC, CO2, and
PC accounted for 26%–42% (32.7% on average), 18%–25% (20.9% on
average), and 7%–8% (7.5% on average) of the food carbon,
respectively.
A
substantial proportion (46%–49%) of the released DOC was COC. The
ratio did not vary significantly at different stages of the 36-h
depuration, or with different food rations (Figure S2).