Variation of reactivity of particulate and sedimentary

A cta Oceanol ogica Si nica , Vol 22, No 4, pp 557~ 568, 2003

ISSN 0253-505X

Published by China Ocean Press

Variation of reactivity of particulate and sedimentary organic matter along the Zhujiang River Estuary

*

Chen Jianf ang 1, 2 , Jin Haiyan1, 2 , Yin Kedong 3 , L i Yan 1, 2 ( Recei ved Ap ri l 6, 2003; accep ted Oc tober 12, 2003)

Abstract

T o investigate organic matter sour ce and r eactivity in t he Zhujiang River ( Pearl Riv er )

Estuar y and its adjacent ar eas, par ticulate organic carbon ( P OC) , particulate hydrolysable amino acids ( PHA A) , and Chl a dur ing two cruises in July 1999 and July 2000 w er e measur ed T he highest P OC and PHAA concentration w as obser ved in the waters w ith max imum Chl a. T he spectr a distribution, r elative content ( dry w eig ht in millig ram per gram) , PHAA-C% P OC and other indicato rs such as the r at ios of amino acids vs amino sugars ( AA/ A S) and g lucosamine vs g alactosamine ( Glum/ G al) suggested that part iculate amino acids in the water column and sediments in the Zhujiang R iver Estuary w ere mainly deriv ed from biog enic processes rather than transported from terrestr ial erosion In inner estuar y where high turbidit y w as often observ able, organic matter w as mainly contributed by re- suspension of bo tto m sediments w ith rev ealed zooplankton, microbial r ew orked characteristics, which sugg est that t hese org anic matters were r elativ ely old

In the estuarine brackish r eg ion, or ganic matter in w ater

column is mainly contr ibuted by relatively fresh, easily degradable phytoplankton deriv ed org anic matter During physical- biolog ical processes w ithin the eastuary , organic matter derived from phytoplankton w as subjected to alteration by zooplankton grazing and bacterial rewo rking Key words Org anic matter, reactiv ity, amino acids, the Zhujiang R iver Estuary

IN TR OD U CTIO N

Est uarine and cont inent al margin sediments are import ant depositories of carbon and associated elements because of higher suspended matt er inputs of t errest rial sources On the ot her hand, enhanced nut rient loading due t o urbanizat ion may enhance t he primary product ivit y of coast al waters, thus added marine organic carbon deposit s in t he sediments It is est imated t hat * This research w as support ed by the Jacky Club of Hong K ong ( prepp Project ) , NK BRSF Project under cont ract N o. G 200078500 and N at ional N at ural Science Foun dation of China under cont ract No 40106004 1

Key Laborat ory of Submarine G eosciences of St at e O ceanic Administration, Hangzhou 310012, China. E - mail:

jf chen @ sio. zj. edu. cn 2 Laboratory of R egional Oceanography, S econd Inst it ut e of O ceanography, St at e O ceanic A edministration, H angzhou, 310012, China 3 Department of Biology, Hong K ong U niversity of S cience an d Technology, Hong K ong

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Chen Jianfang et al

about 83% of organic carbon accumulat ing in modern marine sediments is buried along cont-i nental marg ins, m ainly in deltaic and shelf areas ( Benner, 1982)

So t he coast al seas and con-

t inent al margins may be a signif icant sink for anthropogenic CO2 ( Walsh, 1991) However, estuaries are located in t he most complex environments on eart h in which freshw at er discharge and t idal f orcing are changing on time scales of seconds to seasons and t urbulent m ix ing of f resh w ater and salt w at er can generat e abrupt changes in temperat ure, salinity, pH and nut rients along the estuary Up to now , the knowledge of dynam ics and f at e of terrest rial or marine org anic matt er in est uaries is st ill very lim it ed ( Hedg es et al . , 1997; Hedges and Keil, 1999) T he formation and degradat ion of org anic matt er in an estuary are an int egrative process of est uarine ecosystem dynamics T he reactivity of organic m at ter mainly depends on it s sources and in t urn it largely det ermines t he possibility and t he rate of nut rient s regenerat ion from organic mat ter ( It tekkot, 1988; Nguyen and Harvey, 1997)

So characterizing react ivity of organic

mat ter f rom riverside fresh w at ers to of fshore oceanic w at ers is import ant for understanding the ecosystem dynamics and sediment ary carbon accumulat ion in the estuary T he Zhujiang River is the second larg est river in China in t erms of water discharge, it ent ers into the Sout h China Sea t hrough eight distribut aries T he discharge in w et season ( April to September) account s for 80% of t he tot al annual discharge In sum mer, due to large river runoff , t he mean f low pat tern inside such an est uary has a strong net seaw ard surf ace flow of fresh w at er and a st rong inflow of heavier brackish w at er at dept h, which results in a strong st rat ification A number of surveys show s t hat t he nit rat e concent rat ion across t he est uary is g enerally hig h, w it h a max imum values over 100 mol/ L in surface waters, t he rat io of N top is up t o 200 1, which is one order hig her t han t he Redf ield rat io ( Zhang et al , 1999; Yin et al , 2000)

Alt houg h some marine chemist ry studies have been carried out such as nut r-i

ent s and t race met al distribut ion ( Wang and Peng, 1996; Zhang et al , 1999) , carbon budget ( H an, 1991) , nut rients dynamics and lim it at ion ( Yin et al , 2000) , only very limit ed dat a on spat ial distribut ion and fate of org anic carbon have been available in the estuary In this paper, we present results f rom tw o cruises in July 1999 and July 2000 in the Zhujiang River Est uary Water, particulate and sediment samples were t aken and analyzed for Chl a, POC and part iculate hydrolysable am ino acid T his paper w ill focus on organic matt er dist r-i bution and its react ivit y variabilit y in t he Z hujiang River Estuary, dy nam ic of amino acids ( AAs) in the estuary will be report ed elsew here M AT ERIALS A ND M ETHOD S

T wo cruises w ere conduct ed on board H aiji an 74 during July 17 to 28, 1999 and July 22 to 28, 2000 in the Zhujiang River Est uary, sampling w as conduct ed in neap t ide periods T he sampling locat ions w ere desig ned to cover t he riverine plume, t he estuarine plume t o the coastal plume and oceanic w at ers More det ails are show n in Fig 1 A CT D ( Seabi rd ) and a YSI 6600 ( Y SI R , USA) w ere used to measure in situ w at er dept h, salinit y, temperature, dissolved ox ygen, pH as w ell as turbidit y U pper ( 0 5 m below

Variat ion of reactivity of organic mat ter along t he Zhujiang River Estuary

Fig 1

559

Sampling st at ions in t he Zhujiang River Est uary Gray circles stand f or stat ions sampling

during July 1999, gray squares mean stations sam pling during July 2000, and black circles ( 1, C1, C2) stand f or stat ions in w hich samples w ere t aken in bot h 1999 and 2000

surface) and lower ( 1 m above sediments) w ater samples were t aken w it h a 2 5 L Go- f lo sampler Nut rient s and Chl a w ere analyzed on board according t o Chinese Nat ional St andard M et hods ( GB 17378- 1998)

Concent rat ions of suspended sediment ( SS) were w eight ed aft er

filt ered by 0 45 m cellulose f ilt er F or POC and PHAA analysis, about 1 000 m L and about 200 mL w ater samples were f i-l t ered by a pre- combusted 47 m m What man GF / F filt er on a millipore glass f ilt rat ion set , t hen the filters were st ored in frozen and dark Sediment samples were taken w ith a mult-i corer Sub- sampling ( in 2 cm slices) w as carried out im mediately aft er the mult-i corer was brought onto t he deck T hen all samples including filters and sediment s w ere f reeze- dried in a shorebased laboratory T he dried sediments w ere homog enously grounded before analysis POC and sedimentary organic carbon w ere det ermined by Shimadzu T OC 5000A T OC analy zer ( using a Solid Sample Module SSM - 5000A) and a Carlo Erba EA1106 CH N analyzer respectively aft er removing of carbonate on t he f ilters w it h H Cl T he resullt w rer calibrat ed by acetanilide and org anic carbon sediment reference GBW07314, w hich has been used as national st andard Amino acids analysis follow ed a met hod described previously ( Chen et al , 2000)

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Chen Jianfang et al

Briefly, sediment samples, f ilt ers or filtered w aters w ere hydrolyzed w it h 5 mL 6 mol/ L HCl for 22 h under argon An aliquot of hy drolysate w as dried at 40

under argon in order to re-

move rem aining HCl T he residue w as dissolved w it h 10 l 0 1 mol/ L H Cl, derived w ith fluorescence reagent ( AQC, Wat ersR ) and t hen analyzed on Wat ers 600E H PL C system according to t he procedures developed by Cohen and Dennis ( 1993) . T he system included t he Waters 600E solvent delivery unit , ACCQ- T AG column for amino acids separat ion, 474 f luorescence det ect or, 717 auto- sampler and M illennium 32 chromatography workst at ion T w o additional secondary amino acids ( - alanine, - aminobutyric acid, sigma) and tw o amino sugars ( glucosamine, g alact osamine, sigma ) were added into a mixture am ino acid st andard sample ( P ierce, U SA) w hich cont ained 17 individual prim ary amino acids and am monium, t hus the new amino acid st andard mixt ure has cont ained 19 amino acids and 2 am ino sugars T he int ernal st andard of - aminobut yric acid w as added during t reating and analyzing t he standard and samples Amino acids and amino sugars w ere ident ified f rom retent ion t ime and their amount w ere calculated by comparing st andard peaks and samples peaks, w hich w ere correct ed wit h an internal standard Relative error in duplicate analysis is less than 5% for total amino acids, less than 10% for total amino sugars, and less t han 10% for individual amino acids and amino sugars R ESU LTS AN D D IS CU SSIO N

Sp at ial distr ibut ion and sources of POC f rom f r esh w ater s to oceanic waters F ig ure 2 shows mixing diagrams of POC, PHAA and Chl a vs salinit y and graphs of org anic matt er vs Chl a At the freshwater end- members, concentrations of POC and PHAA in surface layer vary f rom 0 32 t o 1 15 mg/ L and 0 23 t o 0 95 mg/ L respectively Higher values occur in mid- salinity w at ers ( F ig 2a) . POC and PHAA concentrations are generally low er in bott om layer than t hose in surf ace layer in waters of low t o mediate salinit y, while POC and PHAA are higher in oceanic bott om w at ers, w hich indicat es t hat t hose higher concentration organic mat ter in m id- salinit y w at ers w ill sett le w hen surf ace w at ers move seaw ard T he diluted curves of POC and PHAA against salinit y are rat her complicated, show ing that physical mix ing is not a dominant f act or cont rolling POC and amino acids contents in t he est uary POC and PHAA dist ribut ion are quite similar t o that of Chl a, t he regressions of POC vs. Chl a and PHAA vs Chl a are all sig nificant ( Fig 2e) , indicating that most of t he particulate organic mat ter of t he Z hujiang River Estuary is derived from plankt on Generally, t he distribut ion patt erns of PHAA relat ive content ( dry wt in milli grarn per g rorm) against salinity and Chl a are signif icantly linear in surface w at ers, suggest ing t hat PHAA relat ive cont ent in bulk part iculat e mat ter increases from fresh w aters to oceanic waters T he higher Chl a concentration is in the w aters, t he hig her relative PH AA content in t he bulk part iculat e mat ters ( F ig 2d) . Much low er PHAA relat ive cont ent in bott om water ( Fig 2f) suggests decomposition of organic matt er during set tling or resuspension of sediment s w ith lower PHAA relative cont ent

Variat ion of reactivity of organic mat ter along t he Zhujiang River Estuary

Fig 2

561

T he mix ing digrams of POC, PHA A and Chl a vs. salin it y ( a-c) and graphs of organ ic mat t er vs.

Chl a ( d-f ) based on all data set of cruises 1999 and 2000 R egression is signif icant w hen R 2 > 0 122, n = 32,

= 0 05

Reactiv ity of p art iculat e organic mat ter in the the Zhuj iang River Estuary T he Zhujiang River discharges an average amount of 85 M / a of sediment ( Zhang et al , 1999) . How much organic matt er in t he t he Zhujiang is derived f rom these suspended matt er? T able 1 show s t he relat ive cont ent of amino acids in dry w eight in milligram per gram in dif ferent sam ple sets As easily degradable organic matt er, more AAs in bulk mat erial means more fresh organic m at ter in bulk particulate mat ter T he amino acid cont ent of Chinese loess is very low , usually less t han 0 15 mg/ g ( L iu, 1985)

L ow amino acid cont ent ( 0 80 mg / g, see

PRD-soil in T able 1) is also measured in soils f rom t he suburb of Guang zhou in the Zhujiang River Delta area ( Wang et al , 1991) . In the Zhujiang River Estuary, amino acid relat ive cont ent in particulate mat t er is usually one to t wo orders of magnit ude higher t han t hat in soils

Chen Jianfang et al

562

from t he Zhujiang River Delt a, even in the w aters w ith max imum t urbidity. T his suggests t hat soil is not likely a major source of amino acids in the estuary However, it should be noted t hat upst ream of the river can produce amino acid via plankton grow t h w hich may discharge int o the estuary af terw ards Zhang et al ( 1992) found t hat w hen the t urbidity of t he Huang he River w as much higher and nearly no f resh plankton in w et seasons, PHAA relat ive content varied from 1 2 to 1 5 mg/ g ( dry weig ht ) and w as quite sim ilar to t hat in the soil But t he value increased up to 6 9 to 44 4 mg/ g ( dry w eight ) in dry season w hen higher Chl a w as m easured T his support s our previous suggest ion t hat riverine or estuarine biological production accounts for most of the PHAA in w at er column in the est uary Amino acid relat ive content in sediments of t he Pearl River Estuary ranges f rom 0 9 to 2 9 mg/ g It only represents 1% of that in biogenic particulate m at ter and less than 10% of t hat in t he t urbidit y max imum part iculat e matt er T his dramat ic decrease in the amino acid relat ive content in t he est uary indicates that most of t he PHAA have been lost before it is ult imately preserved in t he sediments Orgonic mat ter is easily deg raded into amino acids w hen it is freshly produced T he percent age of amino acids in bulk fresh org anic material is higher t han t hat in old organic matt er ( Cow ie and H edges, 1994; Chen et al , 1999) . T hus, the percent age of organic carbon presented in amino acids ( here combined as PHAA- COC) could be a sig nificant indicat or for organic mat ter react ivit y T able 2 show s PHAA- COC variations in samples from t he Zhujiang River Est uary and in the material f rom dif ferent sources PHAA- COC in turbid or biogenic part iculat e mat ter of our sample set can be com parable w it h t hat in previous reports about other large rivers ( Itt ekkot , 1988) or plankton ( Cowie and Hedges, 1992) . PHAA- COC in terrestrial mat eriaTable 1

Amino acid relative contents ( dry w eight ) in part iculat e mat ter, sedim ents from

Zhujiang River Estuary and a comparison of it s cont ent s w it h dif ferent sources Chines

Pacific

PR D

loess

sediments

soil

Bio PM- up

Turb PM-up

Zhujiang River Estuary PM- bot tom

sedim ents

A verage ( mg/ g)

0 08

0 5

0 8

88 0

27 8

29 9

1 5

M aximum ( mg/ g)

0 15

0 7

2 8

138 0

48 8

74 9

2 9

M inimum ( mg/ g) 0 04 0 4 0 4 62 0 9 8 17 9 0 9 N ot es: Bio PM means biogenic part iculat e mat ter wit h Chl a> 5 g/ L and SS < 10 mg/ L, T urb PM means turbidity part iculat e mat ter w ith SS > 20 mg/ L U p and bot tom means sampling at 0 5 m below t he surf ace and 1 m above t he bott om D at a of Chinese loess ( Liu et al , 1985) , soil f rom the Zhujiang River Delt a ( PRD ) in Guangdong province ( Wang et al , 1991) and sediment s f rom deep cent ral Pacific O cean ( Chen, un published dat a) . Tabl e 2

Comparison of amino acids derived particulate organic carbon ( PHA A- CPOC) bet w een Zhujiang River Estuary part iculat e matt er and pot ent ial amino acids source mat erials

Higher

PRD

Y R PM

YR PM

plants

soil

( w et season)

( dry season)

A verage

3 6

3 4

8 5

25 0

M aximum

6 8

7 4

12 5

M inimum

2 6

1 4

6 5

Plankton

Zhujiang River Estuary Turb PM

Bio PM

42 0

22 3

40 0

33 0

58 0

36 3

52 0

18 0

33 0

14 3

25 0

N otes: Higher plant s dat a f rom Jennerjahn and Itt ekkot ( 1997) . Dat a of t he Zhujiang River Delt a soil ( PRD soil) f rom W ang et al . ( 1991 ) , dat a of t he Huanghe River part iculat e mat ter ( YR PM ) in wet and dry season f rom Zhang et al . ( 1992) , plankton data from Cowie and Hedges ( 1992) , Turb PM and Bio PM mean turbidity part iculat e matt er an d biogenic part iculat e mat t er respect ively, see Table 1

Variat ion of reactivity of organic mat ter along t he Zhujiang River Estuary

563

ls, such as hig her plants ( Jennerjahn and It tekkot, 1997) and soil ( Wang et al , 1991) , is very low PHAA- COC in riverine suspended m at ter f rom the Huanghe River in w et season is much lower t han that in the dry season ( Zhang et al , 1992) Based on the comparisons above, it can be f urt her deduced t hat PHAA in t he Zhujiang River Est uary is derived f rom riverine or est uarine biolog ical product ion processes, thus, it is quit e react ive and easily deg radable A mi no acids sp ectr a and comp osi tional modif icat ion in the Zhuji ang R iver Est uary T he spectra of PHAA are shown in Fig 3 T he dominant AAs are, in a descending order, g ly cine, glut am ic acid, proline, aspart ic acids, alanine, serine, leucine, isoleucine, lysine and phenylalanine T he mole percent ages of these amino acids are all great er t han 5% T he AAs spect ra of PHAA in t he Zhujiang River Estuary are quite similar to those on plankton sample, sediment trap materials and the previous report s on plankton ( Cow ie and Hedg es, 1992; Jennerjahn and Itt ekkot, 1997) , but different f rom that in soil ( Wang et al , 1991) , w hich indicates that plankton is t he dominant source of organic matt er How ever, there are st ill signif-i cant differences among the average spectra of biogenic part icles, t urbidit y particles and bent hic sediments ( T able 3)

T he most pronounced feat ure is that t here are hig her g lut amic acid

( Glu) and proline ( P ro) in biogenic particles t han those in turbid part icles and benthic sed-i ment s, w hile non- protein amino acids, such as - alanine ( b- Ala) and - am inobuty ric acid ( gAba) , are higher in benthic sediments than t hose in other tw o sources Non- prot ein amino acids such as - alanine is absent in living organisms, theref ore, hig her - alanine indicates active modif icat ion of t he released AAs from phyt oplankton

Fig 3

Amino acids spectra of part iculat e matt er in t he Zhujiang River Est uary and a comparison w it h those in plankt on,

set tl ing particulate mat t er and soil Plankton represent s in sit u culture bloom sample in t he Zhujiang R iver Est uary in July 1999, SCS t rap mat erial represent s sediment trapmaterial in t he Sout h China S ea ( Chen et al , 1999) and soil represent s surface soil from t he Zhujiang River Delt a ( W ang et al , 1991) .

T he rat io of amino acids to amino sugars ( AA to AS) is a useful proxy for diff erentiat ing organic m at ter derived f rom phy toplankt on and zooplankton ( Deg ens and Mopper, 1976; Itt ekkot et al , 1984)

Am ino sug ars are mainly derived f rom cut in mat erial w hich is a major

component of zooplankt on w it h high depeletion in phyt oplankton If organic matt er is dominat-

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564

ed by phytoplankton- derived m aterial, the ratio of AA to AS w ill be hig h Wit h an increasing input of zooplankton- derived organic mat ter, t he rat io decreases T he rat io of glucosamine to g alactoam ine ( Glum to Gal) is helpful in differentiat ing bact erial biom ass from organic matt er ( Itt ekkot , 1984; H aake et al , 1993) T able 4 shows a comparison of Glum/ Gal of diff erent mat erials Bacterial cell w alls have nearly equal amounts of t he tw o amino sugars, hence, the rat io is usually no more than 1~ 3 in bacteria ( Kandler, 1979) . In the subarct ic ocean w here bacterial activity was much less t han t ropical oceans, t he rat io is up to 20 in sedim ent t rap mat erial ( H aake et al , 1993) . In cont rast, glucosam ine is far more abundant than galactoamine in zooplankt on ( M ller et al , 1986) . T he rat io of ( Asp+ Glu) t o ( b- Ala+ g- Aba) ( combined as AA- p/ AA- np) could be used as an indicat or of degradat ion st ages of organic matt er ( Itt ekkot et al , 1984, Cow ie and Hedges, 1994) . Non- protein am ino acids, such as -alanine and - aminobut yric acid, are absent in living cells as they are decomposit ion products of aspartic acid and glut am ic acid T hus, t he rat io of parent to daug ht er could be a good indicator of organic m at ter react ivity Table 3

A verage amino acids spect ra ( mole percnetages) in biogenic particles, t urbid part icles and sediment s in t he Zhujiang River Est uary and adjacent areas

ASP SER G LU G LY HIS AR G TH R A LA PR O b-A LA g-ABA

CY S TY R VAL M ET LY S L IE LEU PH E

Biogenic particles 10 2 5 9 13 2 12 4 1 0 4 6 7 0 0 3 11 2 3 8

0 2

0 1 0 3 6 9 0 2 4 8 5 1 7 6 3 9

T urbid part icles

9 7 5 4 8 9 11 1 0 5 3 4 6 3 0 0 9 4

4 4

0 8

0 0 0 0 6 4 0 1 3 8 4 5 6 2 3 5

S ediment s

10 6 6 3 9 4 16 0 0 8 4 8 7 1 0 9 0 9

7 1

2 0

0 3 1 8 6 4 0 2 3 9 3 6 5 7 3 2

N otes: Biogenic particles repesent samples of C hl a> 5 g/ L and SS < 10 mg/ L, and turbid part icles repesent samples of S S > 20 mg/ L

F ig ure 4 show s t hat AA/ AS and Glum/ Gal increase along the est uary Decreasing trend w as observed f rom biog enic matt er ( St as F1, F2, 31) , turbidit y mat ter ( St as 1, 3) to sed-i ment s ( St as 1- C3) , suggesting t hat phyt oplankt on- derived organic mat ter account f or the most part of part iculat e biogenic matt er in oceanic end- member, w hile organic matt er preserved in sediments is that modified by zooplankton grazing and bacterial reworking As w e ment ioned before, relative cont ent of amino acids in dry weig ht stand for freshness of organic matt er in bulk mat erial In biogenic particulate matt er ( St as F1, F 2) , AAs reachs as high as 200 mg/ g , it drops dow n t o 10~ 20 mg/ g in t urbidit y part iculat e samples, and in sediments it is only less t han 2 mg/ g So organic mat ter preserved in sediments is relat ive old Tabl e 4

R at io of G lum/ Gal in diff erent materials

Bact eria

Soil

Y R PM

SCS-trap mat erial

St n p t rap material

Foraminif era

G lum / G al

1 3

1 5

1 6

3 3

10 5

6 8

Ref erence

1

2

3

4

5

6

Y R PM represents River part iculat e mat t er, SCS-t rap mat erial represent s sediment t rap mat erial from t he Sout h China S ea, and Stn P t rap material represent s sediment t rap material f rom S ta. P in subarct ic Pacific Ocean 1 indicat es K andler ( 1979) , 2 in dicates D egens et al ( 1976) , 3 indicat es Zhang et al ( 1992 ) , 4 indicat es Chen et al ( 1999 ) , 5 indicat es Haake et al ( 1993) and 6 indicat es Chen et al ( 2000)

Variat ion of reactivity of organic mat ter along t he Zhujiang River Estuary

Fig 4

565

Part iculat e amino acids relat ive cont ent s ( A As) and ratios of G lum / G al, AA / AS along t he estuary and a com parison wit h those in t he Zhujiang River Est uary

( 1- up means part iculat e matt er at 0 5 m water dept h, th e ot hers are the same mean; surface sedim ents: 0~ 2 cm)

T he t op t wo graphs of F ig 5 show t he salinit y distribution in surface and bot tom layers in the Zhujiang River Est uary during sampling period According t o t he salinity gradient , t he est uary could be divided int o tw o parts: w est ern w at ers and east ern w at ers Western w at ers are more fresher w aters and turbider than east ern one, due to t hree t ributaries pouring into w est ern part w hile t here is no bigger river discharg ing into east part Ot her 6 g raphs of Fig 5 show the dist ribut ing diff erences of AA/ AS, AA- p/ AA - np and Glum/ Gal betw een t he surf ace and the bot tom layer of t he Zhujiang River Est uary T here are sig nificant diff erences bet ween west and east , surf ace and bott om, inner and out er of t he est uary In general, ratios of AA t o AS, Glum to Gal and AA- p t o AA- np increase w ith the salinit y g radient increasing w hich ind-i cat e that the f reshness of part iculat e organic mat t er increases as one moves downstream int o the coast al plume In t he bott om waters, phytoplankton produced amino acids beyond the estuarine plume mig ht sink and could be t ransport ed back w it h the salt wedge, being modif ied by zoop-l

Chen Jianfang et al

566

Fig 5

Amino acids related rat ios and organic mat rix spat ial variat ion along t he PR E Left represens surface layer and right represent s bott om layer.

Variat ion of reactivity of organic mat ter along t he Zhujiang River Estuary

567

ankton grazing and bact eria rew orking Re- suspension of sediment s mig ht have cont ributed to bulk am ino acids in w aters w it h higher turbidit y CO NCLU SION S

Biological processes coupled w ith t he physical processes such as water masses circulation cont rol t he product ion, degradat ion and t ransformat ion of part iculat e organic matt er of the Z hujiang River Estuary When f resh w ater f low s int o the estuary, suspended solids gradully decrease along the est uarine- coastal plume, w hich improves light penet rat ion correspondingly and benefits for phytoplankton blooms However t he low nutrients of oceanic w at ecs limit phyt oplankt on blooms As a result , the region of Chl a max imum occurs at an intermediate salinity w aters of t he coast al plume Biological processes st art ing from phytoplanton growt h dominat e product ion of part iculat e org anic matt er in the estuary On t he other hand, higher PH AA concent rat ion near t he fresh w ater is largely at tribut ed to higher SS of t urbidit y w aters, w hose rel ative content is quite low

In est uarine physica-l biolog ical processes, organic mat ter derived

from phyt oplankt on is subject to modification of zooplankton g razing and bacterial reworking as shown by the rat ios of AA t o AS, AA- p to AA- np and Glum to Gal Most of organic matt er is deg raded and t ransformed w it hin the estuary and only a minor part, mainly modif ied by zooplankton grazing and bacterial rew orking , w ould ult imat ely be preserved in sediment s A ck now ledgements

T he main analysis wor k w as carried out in Hong K ong U niversity of Science and T ech-

nology ( HKU ST ) . T he autho rs ar e indebted to P rof. Chen Jay- Chong for his invitations T hanks should also be g iven to our co lleag ues in the Second Institute of O ceanogr aphy and staffs in HKU ST REFEREN CES Benner R ( 1982) Burial of organic carbon and pyrit e surfur in t he modern ocean: it s geochemical and environment al signifi cance A mer ican Jour nal of Sci ence, 282, 451~ 473. Ch en Jianf ang, Zhou Huaiyang, J in Haiyan ( 2000) A mino acids and t heir biogeochemical implicat ions in sediment s from t he Changjiang Est uary and its adjacent area A cta O ceanologi ca Si nica ( in Chinese) , 22, ( Suppl)

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