Download - ASLO

More documents

Recommendations

Info

1000 Yamamuro and Koike Rubber stoppers Fig. 4. Diagram of setup used for in situ measurements of benthic metabolism at Torigasaki station in summer 1996. water passed through sediment core C90-n for 1 h, A the inner area of the sediment core (20 cm2), and F’O the flux obtained by a control core. To examine the in situ effect of the clams, the benthic chamber (Fig. 4) was placed on the lake bottom over sediment with and without clams where the sediment for continuous flow incubation was taken. Before the experiment, the position of the propeller inside the chamber was adjusted so that circulation was strong enough to completely mix a drop of dye within a few seconds, but not strong enough to cause visible resuspension of surface sediment inside the chamber. The area of bottom covered by the chamber was 78.5 cm2 and the volume of water encapsulated - 1,300 ml. Measurements were carried out on undisturbed sediment on 5 (site A, B) and 10 (site A’, B’) September 1990 and on the sediment without C. japonica, prepared as previously described, on 5 (site C) and 10 (site C’) September 1990. The chamber (Fig. 4), without the rubber top and stoppers, was gently placed in the sediment by a SCUBA diver. After a few minutes, the rubber top and stoppers were plugged, and 30 ml of initial water sample were taken with a plastic syringe from the plastic tube at the side of the chamber. After 1 h, another sample was taken in the same way as the initial sample, and the clams inside the chamber were collected to measure their biomass. Water samples were immediately filtered through precombusted Whatman GF/F glass-fiber filters and the filters were stored in N,N-dimethylformamide to extract Chl a and pheopigment. Filtrates were frozen for later DIN, DON, and phosphate analyses. C. japonica was dried at 60°C and its dry weight determined. Analytical methods-DIN and phosphate of the water samples were analyzed with a Technicon AutoAnalyzer with the Technicon industrial method (154-7 1 W/B for ammonium, 158-71Wfornitrateandnitrite,and 155-71W for phosphate). Total dissolved nitrogen concentration was analyzed by a high-temperature combustion method with a Yanaco TN-7 total nitrogen analyzer (Yanaco, Kyoto) following the procedure of Koike and Tupas (1993). DON
Table 1. Nitrogen content (wt/wt) of particulate organic materials (POM) and egested feces of Corbiculajaponica, and nitrogen assimilation efficiency (AE) of C. japonica. Nitrogen metabolism of a bivalve 1001 N content (%) Samples Mean SD n ;: Aug, bottom water POM 3.01 0.49 3 Feces 0.95 0.10 2 69 Aug, surface water POM 2.63 0.20 3 Feces 1.13 0.92 2 58 Sep, bottom water POM 1.30 1 Feces 0.76 0.14 5 42 concentration was calculated by subtracting the amounts of DIN from the concentration of total dissolved nitrogen. Particulate organic N (PON) was measured with a CHN analyzer (Yanagimoto MT-II). Concentrations of Chl a and pheopigment in the extracts were determined by fluorometry as described by Strickland and Parsons (1972) with a fluorometer (Turner Designs 1OR) equipped with color filters (5-60 for excitation and 2-64 for emission). The sample volume from the benthic chamber was insufficient for PON analysis, so PON for in situ incubations was estimated from the concentration of Chl a and pheopigment, assuming that C. japonica does not select for living or dead materials because the retention of particles is controlled mechanically through the filtration of bivalves (Jorgensen 1983; LaBarbera 1984). The equation used for the conversion was PON (mg-atoms N liter-l) = 0.239 x Chl a (mg liter-‘) + 0.794 x pheopigment (mg liter-l) + 3.59 (r2 = 0.57, P < O.OOl), obtained from analysis of 26 samples of overlying water at the study site in summer (Yamamuro 199 1). Results Nitrogen assimilation eficiency-The nitrogen content of suspended particles in the bottom water (3 cm above the sediment) showed I I I I I I 1600 2100 800 1400 1800 2230 Sampling time Fig. 5. Temporal change of the difference in concentrations of measured materials [ammonium, PON, DON, nitrate and nitrite (NOX), and phosphate] between the outflow of the incubated sediment core (C90-2) and the reservoir. significant variation (1.3-3.0% in Table 1). Calculated nitrogen assimilation efficiency (AE) ranged from 42 to 69% and higher values of AE tended to correspond to higher nitrogen contents of food-suspended particles (Table 1). Nitrogen metabolism measured by continuous flow incubation -Concentrations of most of the dissolved and particulate materials became roughly constant after 19 h of incubation (Fig. 5). DON in the outflow water never attained steady state, and the difference of nitrate and(or) nitrite concentrations between the water in the reservoir and outflow water from the sediment cores was almost negligible, so we did not calculate the flux of these components. The calculated ammonium and reactive phosphate flux from sediment showed positive values with a range of 0.14-0.25 mg-atoms N m-2 h-l and 9.5-31 pg-atoms P m-2 h-l (Table 2). On the other hand, PON transferred from the overlying water to the sediment at a rate of 0.3-0.6 mg-atoms N m-2 h-l. The ammonium-N : phosphate-P ratio (atom) of the flux of ammonium and phosphate calculated from their means varied from 6 to 15. Variation of Chl a content in the top 1 cm of undisturbed sediments (C90- 1 to C90-4) was 59-70 pg (20 cm)-2 (Table 3), while that of disturbed sediments was 53-75 pg (20 cm)-2. A gastropod, Assiminea lutea japonica, was the only benthic animal other than C. japonica (Table 3). This species is very small (~5 mm in shell height), so we could not separate its
Page 1 and 2: Limnol. Oceanogr., 38(5), 1993, 997
Page 3: Nitrogen metabolism of a bivalve 99
Page 7 and 8: Nitrogen metabolism of a bivalve 10
Page 9 and 10: Nitrogen metabolism of a bivalve 10
Page 11: Nitrogen metabolism of a bivalve 10

Download - ASLO

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?