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1004 Yamamuro and Koike
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Flesh gxy Weight $ rnw2)
Fig. 6. Am lmonium flux and uptake rate of PON vs.
flesh dry weight of Corbicula japonica determined by continuous
incubation experiments (0) and in situ measurements
(0). Explanation of arrows given in text.
do not have enough data to determine whether
higher DIN flux is coherent to in situ measurement
or is included in the variation of the
metabolic response of the clams. However,
considering the long-term nitrogen budget of
the clam population, we speculate that the result
of the continuous incubation method is
more reliable.
From results of the continuous incubation
method in Fig. 6, the regression lines between
the ammonium excretion rate (E: mg-atoms
N m--2 h-l) or the PON uptake rate ((1: mgatoms
N m-2 h-l) and the biomass of C. japonica
( W: g m-2) were calculated as
E = 0.003W + 0.085 (r2 = 0.26, P < 0.5),
and
U = 0.014W - 0.046 (r2 = 0.93, P < 0.05).
Substituting the mean biomass of C. japonica
(34.2 g m-2) in these equations gives the mean
a------ -.L-:* ____-- ---A ____ 1 t-. /-- .‘,,,..“.‘,, .-.,-.
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4.5 mg-atoms N m-2 d-l for ammonium efflux
and 10.4 mg-atoms N m-2 d-l for PON uptake.
The assimilation efficiency for nitrogen by
C. japonica was estimated to be 56% (Table
1). Although -80% of AE was reported for
other bivalve species using pure phytoplankton
as food (Foster-Smith 1975; Griffiths
1980a), the AE obtained from measurements
in the natural habitat of those bivalves was
between 40 and 70% (Griffiths 1980b; Berry
and Schleyer 1983). Thus, our result that 44%
of the PON taken up (equivalent to 4.6 mgatoms
N m-2 d-l) is egested as feces seems
appropriate. Therefore, the remaining portion
(1.3 mg-atoms N m-2 d-l) would be used for
growth of C. japonica.
Based on the previous examination during
35 d of growth from July to August (Shimane
Prefectural Fish. Exp. Sta. 1984), C. japonica
became 1.29 times bigger for the size class of
< 17-mm shell length, 1.08 times for 17-20-
mm, 1.03 times for 20-23-mm, and 1.02 times
>23-mm in terms of flesh dry weight. If we
use the natural population obtained from the
in situ experiment, the size distribution of C.
japonica at the study site was 18.9 g m-2 for
the size class of < 17-mm shell length, 7.16 g
m-2 for 17-20 -mm, 4.35 g m-2 for 20-23-mm,
and 3.78 g m-2 for >23-mm shell length in
terms of flesh dry weight. Thus, growth of C.
japonica at the study site during 35 d in summer
can be estimated as 6.3 g m-2. Using the
nitrogen content of the flesh of C. japonica,
7.0 mg-atoms N g-l (Nakamura et al. 1988),
we estimate the growth of C. japonica in terms
of nitrogen to be 1.3 mg-atoms N m-2 d-l.
This estimate agrees well with the growth rate
of 1.3 mg-atoms N m-2 d-l determined from
estimates based on results from the continuous
incubation in our study, suggesting high credibility
for the continuous incubation method.
Figure 7 summarizes the possible nitrogen
budget of C. japonica at the study site in summer.
Ammonium excretion, 4.5 mg-atoms N
m-2 d- l, contributes half of the sediment ef-
flux. Murphy and Kremer (1985) estimated in
situ ammonium excretion by Mercenaria mercenaria
to be 4.6 mg-atoms N m-2 d-l. Doering
et al. (1987) also showed the enhancement
of sediment ammonium efflux from 3.4 to 5 .S
mg-atoms N rnA2 d- l with M. mercenaria added
to an experimental aquarium. These efflux