水研センター研報, 第3号, 27−29, 平成14年

Bull. Fish. Res. Agen., No. 3, 27−29, 2002

短 報

Regeneration of Cd, P, and N on the continental shelfof the East China Sea in summer
Kazuo ABE＊
Abstract Regeneration of three biolimiting constituents, cadmium（Cd）, phosphate（PO４）, andnitrite plus nitrate（NO２＋NO３）, was examined on the continental shelf area of the East China
Sea in the summer of １９９９. Vertical profiles of these constituents showed increase of concentrations toward the sea bottom. The regenerated（molar）ratios to the consumed oxygen（O２）wereestimated to be ０．
００１
３４×１
０−３：０．
００６８４：０．
０９９８：１ by plotting these constituents against theApparent Oxygen Utilization（AOU）, and these ratios generally agreed with a previous study
near the Ryukyu Islands. The apparent increase of Cd, PO４, and NO２＋NO３ in the bottom layerfrom winter to summer was around ０．
０７９nM, ０．
４１μM, and ６．
１μM, respectively, and this wasattributable to the active supply caused by the decomposition of the organic matter in the bottom layer in this season．
Key word: Regeneration, Cd, P, N, East China Sea

In summer and autumn, the thermocline waswell developed in the continental shelf of the EastChina Sea（Kusakabe et al., １９９８）and under thisthermocline, the concentrations of nutrients increased due to the decomposition of organic matter（Watanabe et al., １９９５）. The objective of thisstudy was to examine the regeneration of biolimiting constituents such as cadmium（Cd）, phosphate, and nitrite plus nitrate（NO２＋NO３）at the（PO４）shelf edge of the East China Sea in summer．
Water samplings were carried out vertically atfive stations across the shelf edge of the East
China Sea（DK line）from ０ to ２００ using acidcleaned １０l Niskin bottles aboard the R／V No.３
Kaiyo-Maru of the Nippon Kaiyo Co., LTD., inJuly １９９９（Fig. 1）. The water depths at the stations were ８３６ , １，０２４ , ２８３ , １２７ , and １
１５for DK-０５, ０６, ０７, ０８, and ０９, respectively.
The detailed analytical procedure for Cd was described in Abe（２００１）, and the water samples for
Cd at DK０８ and DK０９ were filtered through a０．
６μm Nuclepore filter. The analysis of PO４,NO２＋NO３ was carried out using a Bran+Luebbe

Fig. 1. Sampling sites in this study.

continuous flow analytical system Model Traccs８００, and dissolved oxygen was analyzed on board

by Winkler titration.
The concentration levels of Cd and nutrients

2002年3月13日受理（Accepted on March 13, 2002）
水産総合研究センター業績 A 第14号（Contribution No.A 14 from Fisheries Research Agency）＊
西海区水産研究所石垣支所 〒907-0451 沖縄県石垣市字浮海大田148-446（Ishigaki Tropical Station, Seikai National Fisheries
Research Institute, 148-446 Fukai-Ota, Ishigaki Okinawa 907-0451, Japan）

ABE

28
tended to increase toward the inner shelf station,and maxima of approximately ０．
２nM for Cd, ０．
８μM
for PO４ and １１μM for NO２＋NO３ were observed
near the bottom at DK０８. Cd was inserted into
the Redfield-type stoichiometric equation, as were
the nutrients as follows（Kudo et al., １９９６；Abe,
２００１）
：
（３８O２
（CH２O）
１０６ NH３）
１６H３PO４Cda＋１＝１
０
６CO２＋１
６NO３＋H３PO４＋１２
２H２O＋aCd

Figure. 2 shows a plot of Cd, PO４, and NO２＋
NO３ against the AOU at stations DK-０８, ０９, and ０７.
The regression lines of the data at DK-０７ in eachpanel were calculated as follows :Cd＝０．
０４
１０＋（０．
００１３４×１０−３）
×AOU （r ＝０．
９７６）



PO４＝０．
１
６４＋（０．
００６８４）
×AOU（r ＝０．
９７１）

NO２＋NO３＝２．
０４＋（０．
０９９８）
×AOU （r ＝０．
９５２）

Good linearity was obtained at DK-０７, and theslopes of these equations denote the regeneration
ratios, so the molar ratio of regenerated Cd, PO４,and NO２＋NO３ to O２ was ０．
００１３４×１０−３：０．
００６８４：０．
０９９８：１. The ratios of the remineralized Cd toP, Cd to N, and N to P were estimated as ０．
１９６, ０．
, and（nMCd／μMphosphate）
０
１３４（nMCd／μMnitrite＋nitrate）
１４．
６（μMnitrite＋nitrate ／μMphosphate ）, respectively. TheRedfield ratio defines the ratio of the regenerated Pand N to the consumed O２ as １：１６：１３８（Redfield
et al., １９６３）, so the ratio noted above was convertible to ０．
１８５×１０−３：０．
９４４：１
３．
８：１
３８, and, as regards P and N, a slightly slower remineralization
was observed. The ratio of remineralization for P
was shown to be about １：１５０ in the equatorialsurface water（Abe, ２００１）, which agreed with theresult of P（１：１４６）in this study. The obtained ratio of Cd to P was very similar to that in a previous report on the remineralization and seawater ratios near the Ryukyu Islands（Abe, ２０００；Abe, submitted）, and the ratio of N／P was similar to theratio of NO３／PO４ in seawater（e.g., Pai and Chen,.
１９９４）
The intercept of the y-axis in Fig. 2 denotes
the preformed fraction, and the preformed Cd, PO４,and NO２＋NO３ were ０．
０４１nM, ０．
１
６４μM, and２．
０４μM at DK-０７, respectively.

A preformed fraction itself indicates an initial concentration when
the water mass is exposed to excess oxygen in thesurface layer. In this study, at DK-０８ and ０９, thepreformed concentration was assumed to indicate
the concentration in winter because the water inwinter was well mixed and the calculated AOU
values were almost ０μM from the surface to thebottom at the station near DK-０８ and ０９（Iseki,. The slopes calculated using the data at DK１９９３）-０７ were applied to the DK-０８ and ０９ plots forthe area near the bottom, and the other layers ineach panel with the assumption that the remineralization rate at DK-０７ was equal to that at the otherFig. 2. Relationship between AOU and Cd, PO４, andNO２＋NO３. The solid three lines are regression
curves at DK-０７. The slope of this line was applied to DK-０８ and ０９ data for the bottom andthe other layers and is shown as a broken line ineach panel. The hatched area shows the winterconcentration level of the bottom（upper）and theother layer（lower）for each constituents.

Regeneration of Cd, P, and N on the continental shelf of the East China Sea in summerstations（Fig. 2）. The y-intercept, namely the concentrations in winter at DK-０８ and ０９ were around, ０．
０．
１１nM and ０．
０
７５nM（Cd）
３４μM and ０．
１６μM, and ４．
（PO４）
３μM and ２．
０μM（NO２＋NO３）fordata near the bottom and for the other layers,respectively. These values generally agree with theconcentrations（around ０．
１nM and ０．
０７nM of Cd,
０．
３μM and ０．
２μM of PO４, and ３．
５μM and１．
８μM of NO２＋NO３ for the deep and surface
layer, respectively）at the near station to DK-０８and ０９ in the winter（Iseki, １９９３；Abe, unpublisheddata）.
The percentage of the regenerated fractions
was calculated by dividing the difference between
the observed and the preformed values by the observed concentration. Approximately ７０〜８０％ of
the apparent regenerated fractions for the threeconstituents were calculated to be near the bottomlayer, which suggested that there was an active
supply caused by the remineralization in the bottom layer in this season. Near the bottom at DK０
８ and ０９, high total carbon concentrations over２．
１ mol／l and low pHs below ８．
０ were also observed in this study, supporting the idea of the decomposition and remineralization of organic matterin the bottom layer. Tsunogai et al.（１９９７）showedthat the continental shelf zone was a large net
source of dissolved inorganic carbon to the ocean,which means that remineralization of biolimiting
constituents also occurs actively in the continental
shelf zone. The apparent increase in the concentrations in the bottom layer of DK-０８ and ０９ fromwinter to summer was around ０．
,
０７９nM（Cd）
０．
４１μM（P）, and ６．
１μM（N（）the regeneration ratio−３
was ０．
. The enhancement in
１９×１
０ ：１：１４．
９）the bottom layer was considered to be the resultof the decomposition of the accumulated biogenic
particulate matter.
In conclusion, the concentrations of Cd, PO４,and NO２＋NO３ in the bottom layer increased at thecontinental shelf stations in the summer, and theseenhancements corresponded to the consumption ofdissolved oxygen, which suggested that the apparent increase was attributable to the decomposition
and remineralization of biogenic organic matter.

29

Acknowledgments
I am deeply appreciative of the assistance withthe sampling that was provided by the officers andcrew of the R／V No. ３ Kaiyo-Maru of Nippon
Kaiyo Co., LTD. I would also like to express
special thanks to Chief Scientist M. Yamada andother scientists of this expedition. This study wassupported by the program for Global Carbon Cycle
and Related Mapping Based on Satellite Imagery, which is funded by the Ministry of（GCMAPS）
Education, Science and Technology of Japan.
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