Spatial and temporal distributions of biogenic fluxes demonstrate that POC and
PIC collected from bottom traps on the slope are derived mainly from the shelf.
The lateral fluxes of POC and PIC leaving the shelf are assumed to be effectively
intercepted by traps on the slope floor. Exports of POC and PIC from the ECS
continental shelf, which are a major goal of the KEEP study, can then be estimated
by integrating the time-weighted mean fluxes from the sea floor over the distance
of slope. Data from traps [T1(T3)-T4-T5-T6] located on a transect over the slope
are used for analyses. In doing so, we first extrapolate the POC and PIC fluxes of
deployed bottom traps (> 50m above the bottom) to expected fluxes measured from
the floor. Linear extrapolation of flux with water depth is applied for each trap
because we found linear relationships between POC(PIC) fluxes and depth in T4
and T5 traps that were deployed successfully for three depths (figure 9). Before making the integration, depth-extrapolated POC flux from each trap site is
subtracted from the vertical contribution (8-34%, see discussion in the temporal
variation) to represent more rigorously for lateral flux of POC. The same factor is
used to correct PIC flux as the mean POC/PIC ratio of all data approximates 1.0.
The integrated area (g C m-1d-1) represents the lateral flux through 1m- wide slope
extending 8 x 104 m from the shelf break to the lower slope. The integrated area
is calculated to be 2.506 x 104 and 4.536 x 104 g C m-1d-1, respectively, for POC
and PIC exports from the southern ECS continental shelf (figure 10).
In order to estimate total exports of POC and PIC from the ECS continental
shelf, the modeled export values on a basis of one meter width from southern ECS
slope are normalized to the latitudinal distance (1000 km) of the whole slope
estimated from the Cheju Island to the northern tip of Taiwan (figure 1). This estimate may introduce a large uncertainty because southern ECS canyon-slope
appears to be a major conduit of particle transport. However, to a first
approximation, total exports of POC and PIC can be estimated as followings:
POCexport = 2.506x104 g C m-1 d-1 x 106 m x 365 d yr-1 =9.15 x 1012 g C yr-1 (1)
PICexport = 4.536x104 g C m-1 d-1 x 106 m x 365 d yr-1 = 16.6 x 1012 g C yr-1 (2)
The calculated values may be regarded as upper bounds of exports from the
ECS shelf, as POC and PIC fluxes measured from the MHC are higher than those
from general slope and NMHC. The POC flux of T5 trap at 960m (water depth
1060m) is also about two times the flux (38±27 mg C m-2 d-1) found by Oguri et al.
[38] from northern ECS slope at 1030m (water depth 1080m). Nevertheless,
Chen and Wang [9] recently reported 8.3±4.2 x 1012 and 22.2±15.0 x 1012 g C yr-1,
respectively, for modern POC and PIC exports from the ECS shelf, derived from
water and nutrient budgets through a box model. Our estimates are very close to
the ranges found by Chen and Wang [9]. These comparable values indicate that
our estimated values are reasonable and the uncertainty may be smaller than
expected.
The primary productivity is spatially and temporally variable on the ECS shelf
[17, 20, 49]. Zhang [49] and Gong et al. [17] reported an average primary
productivity of 438 and 397 mg C m-2 d-1, respectively, in the ECS shelf. Liu et al.
[28] argued that 493 mg C m-2 d-1 may be reasonable for the Kuroshio-influenced
system. If the average value (443 mg C m-2 d-1) is adopted for productivity on the
ECS shelf, then the total POC produced biologically from the whole ECS shelf
would be 146 x 1012 g C yr-1, based on the shelf area of 0.9 x 1012 m2 [9]. Thus,
the estimated annual export of POC from the ECS shelf is equivalent to 6.3% of
annual primary production occurring on the shelf. This value is somewhat
between the shelf export measured from SEEP I (<10%) and from SEEP II (<1%)
areas [3, 13, 40], but very close to the value (6.4%) measured by Falkowski et al.
[14] from the combination of SEEP I&II and to the value (5.7%) developed from
nutrient budgets by Chen and Wang [9] from the ECS shelf.
Following the same method for POC export developed from sediment trap
data, the export of particulate nitrogen from the ECS shelf is 1.15 x 1012 g N yr-1.
The annual primary production on the shelf is equivalent to 25.8 x 1012 g N yr-1 if
Redfield ratio (6.6) is applied for biological production. The exported particulate
nitrogen, therefore, represents only 4.5% of total nitrogen required for sustaining
primary production on the shelf at a steady state. This export ratio is much
smaller than the f-ratios (0.17-0.51) measured on the ECS shelf [10] and 0.15
estimated by Chen and Wang [9]. A large fraction of new production appears to
be buried on the shelf. Therefore, recycled nitrogen may be more important than
allochthonous nitrogen in maintaining relatively high production on the ECS shelf.
4. CONCLUSION
The estimate of particulate carbon exports from the ECS continental shelf is
imperative for constructing carbon budgets in the continental margin of the western
boundary current system. This study illustrates particle and biogenic fluxes in the
southern ECS slope. Particulate carbon collected by sediment traps deployed on the
slope area is derived primarily from the shelf. Exports of particulate carbon from the
ECS shelf were derived from measured particulate carbon fluxes and shelf-wide
extrapolation. Despite the possible uncertainty associated with large spatial and
temporal variations in particle and biogenic fluxes, the exports of particulate organic
and inorganic carbon are estimated to be 9.15 x 1012 g C yr-1 and 16.6 x 1012 g C yr-1,
respectively. Only about 6.3% of carbon and 4.5% of nitrogen fixed biologically on
the ECS shelf are exported to the slope. The similar extent of exports obtained both
from SEEP (6.4% primary production) and KEEP (6.3% primary production) studies
may imply the similar role of continental shelves in regulating carbon cycling and
export in both western boundary current systems.
Acknowledgments. The authors would like to thank the National Science Council, Republic of China for financial support under Contract Nos. NSC
84-2611-M110-006k2, NSC 85-2611-M110-010-k2, 86-2611-M110-002k2 and NSC
88-2611-M110-006k2. The captain, crew and research assistants aboard the R/V
Ocean Researcher I and II are appreciated for technical support and sampling assistance during deployments and recoveries of sediment traps. We are also
grateful for helpful comments from Profs. K. K. Liu, C. L. Wei and S. Lin. This
research is a contribution to the KEEP study, a recognized program of the JGOFS.
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