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1004 Yamamuro and Koike 09 0 0 0 0.0 ! I I 0 0 0 0 0 0 0 0.0 I’ I I 20 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-. /-- .‘,,,..“.‘,, .-.,-. 50 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 efflux. 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
Nitrogen metabolism of a bivalve 1005 rates caused by filter-feeding bivalves were of the same order as our measurements. In the previous studies discussing the effect of filter-feeding bivalves on biogeochemical cycling .of bioelements, the uptake rate of suspended particles is estimated based on batchtype measurement of filtration rate. Such methods seem to overestimate the uptake rate. Jordan and Valiela (1982) estimated 3 times larger uptake than used, and attributed the discrepancy to the difference between PON concentration in the water used for the filtration experiment and the concentration in the habitat of the animals. Doering and Oviatt (1986) compared the filtration rate of hf. mercenaria obtained by a batch-type experiment and a 14C tracer technique in a mesocosm (13 m3) with sediment and concluded that the former estimate could give a rate > 10 times larger than the latter. We believe that our continuous incubation method has less artificial effect on the uptake rate of bivalves and that the direct measurement of uptake rate leaves less ambiguity on estimates compared to converting filtration rate to uptake rate. This could be why our results showed a reasonable nitrogen budget. Efect of bivalves on nitrogen cycling- Among the forms of dissolved nitrogen excreted by bivalves, ammonium is considered to be a major component followed by DON (Bayne et al. 1976). Our results also showed the significant effect of bivalves on nitrogen cycling through ammonium excretion, while the effects on DON release were unclear. DON flux from the sediment and overlying water measured in situ (Table 5) showed both negative and positive values despite the presence of clams. This finding suggests that organisms other than bivalves are involved in the exchange of DON and that the temporal change of such activities is immense. Previous studies of bivalves have focused on their contribution either to the removal of primary producers or to stimulating primary production through the supply of inorganic nutrients by excretion. Because C. japonica inhabits the coast densely at depths
Page 1 and 2: Limnol. Oceanogr., 38(5), 1993, 997
Page 3 and 4: Nitrogen metabolism of a bivalve 99
Page 5 and 6: Table 1. Nitrogen content (wt/wt) o
Page 7: Nitrogen metabolism of a bivalve 10
Page 11: Nitrogen metabolism of a bivalve 10

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