Marine Cheìnistry, 43 (1993) 263-276 263 0304-4203/93/%06.000 1993 - Elsevier Science Publishers B.V. All rights reserved

Isotopic and biochemical composition of particulate organic matter in a shallow water (Great Ouse, North Sea, England)

R. Fichez*'a,P. Dennisb, M.F. Fontaine', T.D. Jickellsb 'Centre ORSTOM de Tahiti, B.P. 529, Papeete, Tahiti, French Polynesia University of East Aiiglia, School of Eizviroimzental Sciences, NR4 7TJ Norwich, UK 'Centre d'Océanologie de Marseille, Station Marine &Endourne, Rue de la Batterie des Lions, I3007 Marseille, France

(Received 8 April 1992; revision accepted 22 January 1993)

Abstract

The biogeochemistry of particulate organic matter was studied in the Great Ouse estuary draining to the North Sea embayement known as the Wash from March 1990 to January 1991. Eleven locations were sampled monthly on a 50 km transect across the shallow estuary from the tidal weir to the middle of the Wash. Particulate organic carbon (POC) and total , and analyses were combined with the determination of stable carbon isotopes. d3C often increased from -30%0 in the river to -22%0 in the tidal freshwater reach. The mixing zone between fresh and marine tidal waters displayed only a slight increase in d3C to -19%0. The change in d3Cvalues in the freshwater tidal reach demonstrated that mixing of riverbome and marine suspended POC was not the only process affecting the carbon stable isotope composition. Complementary sources, interfering considerably with the two end-member sources, may be identified as autocthonous primary production and resuspension of sediment that may be transported upstream. The respective importance of these sources is subject to seasonal variation. From March to August, high concentrations in carbohydrate and protein through the whole estuary indicate that despite turbidity significant primary production occurred. The proportional importance of the uncharacterized fraction of POC, which is considered as complex organic matter, was high from September to January and low from March to August. During most of the year, the biochemical compositions of particulate organic matter in the turbidity maximum and the rest of the estuary were similar. This contradicted the principle that owing to the long residence times of particles degradation processes largely dominate the production processes within the turbidity maximum. The occurence of significant in situ production in such shallow water may partially compensate for the degradation of suspended particulate organics, resulting in a complex relationship between the biogeochemical cycling and the fate of nutrients.

Introduction et al., 1991). of riverine and coastal environments is a direct consequence of Estuaries are complex mixing zones displaying the increase in nutrient loading and therefore the several interrelated physical, chemical or biolo- fate of biologically important elements going gical interfaces, where biogeochemical processes through estuaries must be clarified. The first significantly affect the fate of riverborne material aim of this study was to determine the changes (Kemp et al., 1982; Saliot et al., 1984; Relexans et in the biochemical composition of particulate al., 1988). Oceanographers are concerned with organic matter in an estuary through a seasonal determining the impact of riverborne pollutants cycle. on coastal and offshore marine ecosystems Carbon stable isotope analysis has been exten- (Gerlach, 1988; Brockman et al., 1988; Jickells sively used in estuaries as a tracer of the major

I. sources of organic carbon (Haines and Mon- I * Corresponding author. tague, 1979; Hughes and Sherr, 1983; Gearing 9 f 4 SEV. I99

O 10000396 ~i 42334 we i B &A '63 et al.. 1984: Sinienstad and Wissniar. 1985; Saliot Sea. Determination of the major components et al.. 1988: Lucotte. 1989). Stable carbon iso- of organic matter (POC. and total protein, car- topes have also been used to quantify the mixing bohydrate and lipid) were combined with 6I3C processes between continental and marine par- analysis of POC from the river to the marine ticulate organic carbon (POC) in estuaries end-member of the estuary from March 1990 to where river and ocean end-members were the January 1991. There are evident limitations in only sources (Fontugne and Jouanneau. 19871. measuring the bulk composition of stable car- However. these mixing models are open to bon isotopes and biochemical constituents that criticism. main11 because of the doubtful are discussed in this paper. Nevertheless. seaso- accuracy of' the 6°C estimate for each source, nal coverage of these parameters allows us to h possible changes in the fil3C of the tranported assess the influence of kariable freshwater input organic material. and interference from over- to the estuary on POM composition and flus and looked sources of organics (Fry and Sherr. to infer the potential significance of the diverse 1984: Benner et al.. 1987). 6°C studies in an sources of POM. Stable carbon isotopes are estuary may not accurately resolve all potential especially well adapted to the study of the sources but the) do provide clues about the Great Ouse because of the marked differences respective influence of the main sources. the between niarine and terrestrial organic matter seasonal \ariation in export of continental or 13 C in temperate areas. The present work was estuarine organic material and the nature of the part of a general research programme sources that combine with the continental and (JONUS) developed to estimate the flux and marine end-members. Even though 6°C is not fate of nutrients through the major estuaries of an unequivocal tracer of the origin of par- southeast England and their impact on the ticulate organic matter in most estuarine and North Sea. Particulate organic matter and dis- coastal environments it can yield valuable infor- solved inorganic nutrients are obviously related mation when combined nith other analytical through production and degradation processes investigations. Most of these multiple approaches and studying POM thus represented an indispen- deal with siniultaneous analyses of se\ eral stable sable step in the elucidation of the cycling of isotopes (Peters et al., 1978: S\ieenei et al.. 1980; nutrients in this estuarine environment. Macko. 1983). Combined anlaysis of stable carbon isotopes and biochemical composition of particulate organic matter (POM)has usually RIaterials and methods inlolved chloropigments. POC or particulate organic nitrogen determination (Fontugne and Sirc ticscriptinn Jouanneau. 1987: Ember et al.. 1987: Lucotte. 1989: Harden and Williams. 1989: Matson and The geomorphology and the main characteris- Brinson, 1990) \{hile further investigation on the tics of the Great Ouse estuari (Fig. 1). which is the detailed characterization of biochemical com- niain freshwater input to the Wish embaycnicnt. pounds is rare (Benner et al., 1987: Saliot et al., has been described in previous works (Gould et al.. 1988). This is a rather surprising gap owing 1986: Fichez et al.. 1992). The total catchment of to the conspicuous complementary nature of the river system is 8380 km2 and the average flow both approaches in biogeochemical process rate is 38.5 ni-3 s-' thus making its freshwater floh in\ estigations. ofthe same magnitude as the associated freshwater The stud) presented in this paper WL~Saimed at flows from the Nene. Welland and Witham. Den- determining the origin and fate of particulate ver Sluice is a tidal Meir separating the river from organic matter in a shallow i{ater estuarq, the the tidal fresh\$ater reach. Downstream of Demer Great Ouse. on the British of the North Sluice a narrow (<70 m v, ide) and shallow ( 1-7 ni R. Fichez et al.lMarine Chemistry 43 (1993) 263-276 265

Sampling and sample preparation

Water was collected at 1 m depth at 11 sam- pling locations (TO1-TO11) on a 50 km long transect along the estuary from Denver Sluice (Toll) to the Roaring Middle buoy (TO1) in the centre of the Wash. A monthly survey was done during high water at neap from March 1990 to February 1991. Ø Water samples were filtered in the laboratory on precleaned (500"C, 4 h) Whatman GF/C glass fibre filters (Whatman International, Maidstone, 53'01 THE WASH @ UK) under moderate vacuum within a few hours of collection. Loaded filters were dried (5OoC, 48 h) and stored in the deep freeze prior to analysis. Salinity was measured at each sampling location. INGS IYHN 10

15 km 613 c analysis o Mass spectrometry of carbon stable isotopes is

WER SLUICE performed on CO2 gas samples. Two different methods for producing CO2 from organic car- 53'3C I I bon are classically described in the literature, 0'00 0'30 both of them displaying advantages and draw- Fig. 1. Location and map of the Great Ouse estuary. Sam- backs. The first method consists of submitting pling stations are numbered from 11 (river) to 1 (sea) and quoted as TO11 to TO1 in the text (TO standing for the carbonate fraction to acid (usually HCl) Trans-Ouse). attack and drying the remaining organic mate- rial which is then combusted at 900-1000°C in depth at low water) stretches for 25 km the presence of oxygen provided from diverse before opening on the 600 km2 Wash embaye- sources (copper oxide or direct oxygen gas ment. The four rivers (Witham, Welland, Nene, flow) (Fontugne and Duplessy, 1978; Jeffrey et Great Ouse) draining to the Wash may be al., 1983; Macko, 1989). High temperatures classified as highly eutrophic as they yield high ensure that the organic carbon is completely loads of inorganic nutrients resulting in high pro- combusted to CO2. The disadvantage is that ductivity. Agricultural activities are mainly hydrolysis of some organic molecules may responsible for the high level of nitrate and phos- occur during acid attack. Moreover, acidified phate (400-600 pmol I-' and 40-80 pmol I-', sediments or suspended material often need to respectively; Fichez et al., 1992) in the river. A be rinsed prior to combustion (Rau et al., 1982; monthly averaged freshwater flow of 110 m-3 s-l Ember et al., 1987), and such a process may was measured at Denver Sluice in February (NRA, contribute to a significant loss of organic mate- unpublished data). It decreased to 38 m-3 s-l and rial (Byers et al., 1978). In the second method, 19 mb3 s-l in March and April, respectively, to the raw sample is directly combusted at a mod- reach minimum values of 3-5 m-3 s-l from June erate temperature (50O-60O0C, 0.5-2 h) in the to September and increased sharply after October presence of excess oxygen (Sofer, 1980; Hack- (9 m-3 s-') to maximum values (1 10 m-3 s-'1 in ney and Haines, 1980; Hughes and Sherr, 1983; January. Fry et al., 1984). Inorganic carbon is not oxi- '66 R. Fick t t (il. .\lwitie C¡wniistry 43 1 19V3 1 33-2-6 dized at temperatures below 500 C (Hirota and graphite standards was within f 0.1 %O and the Szyper. 1975: Kristensen and Andersen, 1987) natural sample variation was in the range of k and the organic carbon fraction alone is osi- 0.5 "r~for suspended particles and & 0.4 "tio for dized to CO2 gas. Unfortunately, such a sediment. temperature-based differentiation between inor- cganic and organic carbon seems to be only the- Biochcriiical coniposirimi oretical and there is no doubt that a significant loss of inorganic carbon occurs at 55VC (Hirota Total carbohydrate (CH). protein (PR) and s and Szyper. 1975: Byers et al., 1978) while some lipid (Ll) contents of suspended particles were refractory organic material remains unconi- anal) sed together with iS13C determination. Car- '; busted at 500'C (Friielich. 1950: Kristensen bohydrates were analysed according to Dubois and Andersen. 1987). et al. (1956) and expressed as glucose equivalents. It is certain that neither method is completely were analysed according to Lowry et al. satisfactory. However, we found that the second (1951) and expressed as bovine sero-albumine method had a significant advantage as the mod- equivalents. were extracted according to erate-temperature technique does not require Bligh and Dyer (1959). analysed according to quartz glassware. thus significantly reducing the Marsh and Weinstein ( 1966) and espressed as expense for routine determination of the tripalmitic acid equivalents. 13c . 1' C ratio. Furthermore. the method also Techniques for the analysis of organic com- significantly decreases the blank level 1% hen com- pounds have been discussed extensively (Daw- pared with the high-temperature combustion son and Liebezeit. 1981: Williams. 1985). The technique (Chesselet et al.. 1981 ). Precombusted choice for the analytical techniques used in this filters rire cleaned of organics and are not further stud) arose from previous works on suspended I affected by the moderate-temperature combus- or sediment organic matter (Fichez, 1991a. b) tion step thus giving almost undetectable blanks and proved to be suitable for the analysis of at the spectrometer detection limits. Such advan- estuarine POM (Moa1 et al., 1985: Poulet et al.. tages persuaded us to select the moderate-tem- 1986). Chemically assessing the bulk amount of peraturc combustion technique for the routine organic compounds is subject to several biases. preparation of organic CO2 samples. such as the importance of the oxidation of Filters or sediment samples mere sealed under macromolecules owing to the reaction reagents, \ íicuum in pyres glass tubes in the presence of an the accuracy of the chemical identification of excess of copper oxide. The tubes were progres- molecules or molecular bonds and the choice of 1 sively heated to 500°C in a muffle furnace. kept the standard. The relative lack of accuracy of at 500°C for 3 h and cooled down progressively. each bulk analysis is balanced by the advantage Progressive heating and cooling minimized the of simultaneously determining the main cate- occurrence of glass-tube cracking and the subse- gories of organic compounds constituting quent loss of the sample. Purification of the gas POM. Such simultaneous measurements of the sample was achieved on a vacuum cracking line three main classes of compounds along with the equipped with cold traps for the separation of bulk organic carbon content yield an estimate of H20 and CO?. Isotope ratios were measured the gross composition of organic matter (Spitzy on a V.G. Isogas SIRA 3 (V.G. Instruments. and Ittekkot. 1991 1. To assess the composition of Middlewich. UK) isotope mass spectrometer. POM. carbohydrate. protein and lipid concen- The ratios are expressed as 6I3C in %tl deviation trations were converted to carbon using conver- from the Peedee belemite (PDB) carbonate stan- sion factors of 0.45 gC ge'. 0.50 gC g-' and 0.70 dard (Craig, 1957). The analltical reproduci- gC g-I. respectively, according to the composi- bility for the 6I3C composition of internal tion of the standards used and the natural com- R. Fichez et al./Matiiie Cliemistry 43 (1993) 263-276 267 3 position of particles (Degens, 1970; Jeffrey, 1970; explained below. During most of the following Saliot et al. 1984; Fukami et al., 1985). It must be months there is a steep increase in S13C in the emphasized that average conversion factors freshwater reach, the extent of which is receding neglect the diversity in the composition of from 12 km in May to 2.5 km in August. For organic compounds (Fichez, 199 1b). However, example, SI3C values increased from -30 %O in such a conversion step makes it possible to cal- the river to -22 %O throughout the tidal fresh- culate the residual fraction of organic carbon water reach in May; this increase is not related to ? which is not accounted for by carbohydrate, the mixing of fresh and marine waters. Within protein and lipid. This fraction, described as the region of the salinity gradient there was a 'I heterogeneous, polyfunctionalized and macro- slight increase of S13C up to a value of -19 %o v molecular in nature (Cough and Mantoura, which persists from this part of the estuary as far 1990), we term complex organic matter (COM) as the middle of the Wash. In summer, S13C owing to both its biochemical complexity and values increased in the river to -28.5 %O in July poorly described molecular composition and ori- and -26 %O in August. Profiles changed mark- gin (Fichez, 1991a,b). Distinct from the unchar- edly by the September sampling with a 8 km acterized COM fraction, the pool of downstream shift of the salt wedge together carbohydrate, protein and lipid compounds is with a strong decrease of S13C in the river (-33 referred to as the CPL fraction. %O). S13C values rose rapidly (-28.4 %O) at the very beginning of the tidal freshwater reach then progressively increased to -22 %O together

c Results with the mixing of fresh and marine water. Dur- ing autumn and winter, river POC S13Cincreased from -29.5 %O in October to - 26 %O in January. Except for January, SI3Cvalues in the tidal fresh- The S13C of suspended particulate organic water reach were close to - 22 %O, slightly matter was plotted together with salinity for decreasing downstream. The January data dis- the transect along the estuary (Fig. 2). The played a rather different shape from all other need for such a presentation, instead of usual samplings, S'~Cvalues progressively increasing plots of S13C against salinity, arose from the from -26 %O in the river to -22 %O at the physical conditions in the upper estuary (tidal lower limit of the freshwater reach, decreasing to freshwater reach) where tidal freshwater -24.2 %O in the fresh-marine water mixing zone r. extended several kilometres downstream of the and increasing to -21.8 %O in the marine part of sluice during autumn and winter surveys. the estuary. c There is a general increase of 613Cvalues from An exceptional drop in SI3C values was -30 %O in the river upstream of Denver Sluice to observed in April at TO5 (25 km) which has to - 19 %O in the centre of the Wash which could be be explained. This specific sampling was made at considered as the effective marine end-member the beginning of the ebbing tide in front of the of the estuary. In March the SI3C value in the King's Lynn sewage outfall. At this time of the river end-member was -30 %O , it increased tide the gates of the sewage plant are opened to slightly to -28.5%0 in the tidal freshwater discharge the effluent into the estuary where it is reach and progressively increased to -20.4 %O then flushed to the sea by the flowing water. The at TO3 (35 km). The S13C decreased to -22.1 input of anthropic organic material depleted in %O in the more offshore part of the transect with- 13C is clearly identified by the 6 %O drop in the out any associated change in salinity (ca. 33 PSU). S13C value which is localized at TO5 due to the The April profile showed a similar trend except limited dilution of the plume at the beginning of for the drop in SI3C at TO5 (25 km) which is the release process.

*I

r. 165

3.5 -'I,1" ?Il -27 25 'II .'t, 15 1 -31 n

-34 II

...I

Fig. 1.Asisl distrihution ofh"C :ind dinitj-within the Great Ouse estuarj. b"C in "WI and salinity in PSU on x-axis. dihtance in kilometres from the riverine end-member sampling point (TO11 upstream of Denver Sluice on the .u-us¡s. R. Fichez et al./Marine Chemistry 43 (1993) 263-276 269

Biochemical composition tide period, the amount of organic material dis- charged daily at the mouth of the estuary may be Particulate organic carbon (POC), carbohy- of the same magnitude as the organic material drate, protein and lipid concentrations (Table flushed by the estuary during low freshwater- 1) increased from the river to the salt wedge flow periods (summer). Sewage could thus have where the turbidity maximum occurred then a major impact on the biogeochemical cycling decreased seaward. Particulate organic carbon and the structure of the benthic communities of concentrations were high in May and maximum the surrounding sediments if not on the biogeo- during winter. In contrast to POC, concentra- chemical budget of the estuary. tions in CPL were high during the first 7 months The biochemical composition of organic mat- of the survey and decreased during the last 4 ter expressed as a percentage of organic carbon months which covered the end of autumn and has been plotted against the distance from Den- winter. In the river end-member concentrations ver Sluice (Fig. 3). The uncharacterized fraction ranged from 96 to 1995 pg 1-' for protein, 76 to of organic carbon which is considered as com- 1725 pg 1-' for carbohydrate and 41 to 1030 pg plex organic matter (COM) represented the lar- 1-' for lipid. In end-member maximum gest pool for organic carbon. Nevertheless, from concentrations were recorded in May for carbo- March to July the CPL fraction represented hydrate, protein and lipid with especially high more than 50% of the POC for most of the values in the range 2000-3600 pg I-' for carbo- sampling. From August to January the CPL hydrate. Generally proteins were the most fraction decreased dramatically progressively important of the three identified organic species reaching values below lo%, COM originating and ranked second while lipids, carbon thus accounting for the complementary usually far below, ranked third. Maximum con- 90%. An unusual situation was observed in centrations on the estuarine transect were March, lipid accounting for less than 5% of located in the upper or middle estuary always organic carbon in the upper estuary and increas- corresponding with the POC maximum and ing to almost 50% in the lower estuary and the the turbidity maximum. The strong decrease in Wash. concentrations at the mouth of the estuary (T05), owing to dilution of estuarine waters into the Wash, is a signal for the transition Discussion and conclusions from the tidal channelled estuary to the Wash embayement. Carbon isotopic mixing models applied to An anomaly in this general decreasing trend estuarine studies are mostly based on two occurred in April as concentrations of organics sources of suspended particulate organic mat- dramatically increased at T05; this is related to ter: the riverine and the marine end-members. the sampling occasion described in the previous However, the data from the Great Ouse estuary section which involved the King's Lynn sewage are not consistent with this simple two end-mem- effluent. This outfall strongly affected the con- bers mixing model. Given the S13C values of the centration and composition of POM with an river and marine end-members of the Great Ouse especially large increase in lipid concentration. (-30 and -19 %O) and allowing for the generally The influence of the sewage was not the subject limited isotope fractionation (O to -2 %O) owing of this study and such sampling was not repeated to degradation processes (Schwinghamer et al., during the following months and will not be 1983), the strong increase in S13C that occurred discussed in detail. However, it must be empha- in the tidal freshwater reach suggests the pre- sized that even though the sewage release is a sence of at least one more source of POC (Fry process confined to the beginning of the ebb and Sherr, 1984). Tahle 1 Biochemical composition of particulate organic matter in the Great Ouse estuary folloaing a transect from the ril-er (TOI 11 to the sea (TOI, 47.6km downstream TOI 1 ) from hlarch 1990 to January 1991. POhl. particulate organic matter: POC. particulate organic carbon: CH. carhohidrates; PR. proteins: LI lipids. Concentration in micrograms per litre

Date Component Location

Toll TO10 TO9 TOX TO7 TOh TO5 TO4 TO3 TO2 TOI

Distance 1 km J

0.11 1.4 6.7 11.1 14.2 19.5 23.7 30.h 35.6 41.0 47.5

?(I O3 YO d POC 2407.3 3743.5 41115.1 4560.1 644O.5 5846.3 IfMl 1 498.5 460.9 529.4 319.8 CH 618.9 I 21 11.9 lh41.7 241 7.0 1723.h 1x33 1 501.7 134.3 125.8 130.5 67.7 PR 1020.8 2009.7 2444.2 3h30.0 405 I. 1 3179.2 8'3.8 278.1 168.P 371.x 161,s LI I?(I.X '311.4 231.3 3.54.' 448.1 34h.h YI 9 146.4 183.1 231.1 213.3 17 O4 90 POC 1418.6 3774.1 4395.5 3111.1 1738.2 1807.1 9434.0 587.1 326.11 478.7 288.4 CH 709.7 1316.1 1065.3 406.4 449 4 627.7 1774.0 214.h 19S.5 212.4 1I2.h PR 1n41.1 21n5.2 2'41.5 1559.5 923.0 11'4.8 3739.h 480.5 262.1 439.4 131.7 LI 513.4 s21 2 841.1 645.7 1677 3S1.3 5268.5 117.1 66.59 107.2 55.9 19 115 911 POC 3(lhh.h h729.2 71(19,1 41151 .s 14'0.5 32346 54855 2549.3 242l.6 1413.3 1484.l:l CH 1311íl.11 2bY4.0 2ml.i) 14n7.0 1n71.s W.5 2W).íi 1OK!.4 2590.0 2475.7 1115.7 PR w.n 3533 3 3104.2 1961.5 136O.11 1879.1 3845.8 1410.7 1363.1 985.1 934.5 LI 689.1 11s1.4 1052.5 hV!).). 421 .(I 363.8 1no4.9 3Sl.h 391.6 371.6 375.6 14nh sn POC l44cl.b S15h.5 4Ol6.0 3137.h 4h2Q.5 3111s m2.n 7311.2 701.4 535.1 735.5 CH 46 7.6 3750.1) 1 647.7 1 120.0 137O.tl VI 4 776 7 IEh.9 231.8 15h.4 254.4 PR 855.4 39Ih.7 1933.7 1141.7 1733.3 1221.4 11843 444.6 356.7 794.7 354.3 LI 1Yll.4 111 1.0 756.0 677.7 b4(l 7 391.0 365.Y 118.1 113.8 1'3.9 173.5 IS 07 YO POC 35511.2 311337.0 7373.5 41t14.5 2Yl)h.h 331.8 1055.7 22'1.9 229.5 CH 17'5.0 5366.7 2876.7 1361 7 830.8 143.5 213.4 1 18.1 106.9 PR I (178.(l 5517.8 4191.7 1710.t; 1173.6 137.5 468.9 lS(l.4 144.1 LI 754.3 1967.4 11ll5.1 ~2.t~471). 1 86.6 162.9 h3.h 43.7 18 ns 40 POC h7b3.4 .53X7.(1 3878.11 24b3.S 1523.9 1 1 17.4 1341.8 6389 3h8.2 1911.9 142.3 CH 14(l5.(l 102¡).0 614.7 371l.S 108.6 171.3 144.h 1YY.h 105.7 77 8 31.1 PR 1995.1 1504.ï 1590.5 7lKO.O 4111.3 405.7 317.1 143.4 134.3 34.3 2'J.iJ LI l(130.0 572.3 534.6 317.7 177.Il 101.5 97.3 x5.5 5.5.3 45.1 72.cr 2h 09 9n POC 1065.5 3787.3 h76h.il 6684.0 1N5.4 l631.n 1464.1 b(l8.0 173.8 3x6 215.1 CH 751.5 950.0 1419.4 1127.5 5(10.(1 1'4.4 195.5 147.3 75.5 67.4 82.0 PR 385.7 1050.8 960.4 1342.9 21'1.1 l72.tI 266.1 85.3 711.3 51.5 50.4 1- 1 151 2 4n4.7 431 .Il 453.3 1-11 7 Q5.5 I18 h 45.6 3.1 35.7 33.3 24 l(1 Yn POC 684.5 4139.0 8544.1, 12078.0 6843.0 301 7.8 5445.0 781.5 789.7 48(I.') - CH 130.4 b34.0 13711.8 lYYO.0 IO20..? Ol(1 O 79i1.0 149.1) 138.8 71.1 - PR 151.4 73 1.(I 1318.7 1257.2 1263.3 4qQ.l 2143 7 1h9.3 1h4.15 I35 - LI 95.5 110.3 387.1 976.1 2433 I844 2145 Ill5.O 87.1 (13.7 - 26 11 90 POC l(181.Y 11131.5 1 xos 1.5 I24WfJ 9547.0 3626.8 2401.3 117X.h 11155.2 lil56.h 678.5 CH 351 .S I455.11 1481.1 237 1.4 lii7il.ii 730.(1 338.4 94.(1 140.8 75.1 98.4 PR 179.h 1782.3 2278.9 963.3 1464.0 85(1.11 4Oh.3 99.b 1x2 74.6 66.5 LI 88.1 34s. 1 331.0 299.0 210.5 lh8.1 1114.1 SlJ.5 13Ïl.O 55.1 66.7 R. Ficlvz et al./Marine Chemistry 43 (1993) 263-276 27 1

Table 1 Continued

Date Component Location

TO11 TO10 TO9 TO8 TO7 TO6 TO5 TO4 TO3 TO2 TO1

Distance (km)

0.0 2.4 6.7 11.2 14.2 19.5 23.7 30.6 35.6 41.0 47.5

08/12/90 POC 628.4 21277.5 14848.5 5738.1 2645.2 - - CH 110.2 1242.9 1416.3 1518.8 561.0 - - PR 96.24 2327.4 3006.0 2994.0 671.5 - - LI 59.4 412.0 390.4 322.0 150.2 - - 13/0119 1 POC 654.4 4937.0 8078.3 16312.0 9938.5 4196.7 2677.5 770.4 1477.6 1248.8 1578.1 CH 76.3 373.5 474.7 1068.8 733.8 482.5 181.3 52.5 65.8 81.5 65.6 PR 114.4 914.3 1406.3 1165.9 1863.1 711.9 283.8 108.4 172.1 71.3 113.7 LI 41.3 125.6 112.7 233.3 173.2 87.7 61.0 53.2 122.0 78.2 109.1

A first possible source could be the detritic the upper reaches of the Great Ouse estuary non-living sediment organic material as it may from March to July has been presented in a account for a large fraction of the suspended previous work (Fichez et al., 1992). Autotrophic POM in the upper reaches of tidal shallow production, even in the most turbid part of the water estuaries, mainly because of resuspension estuary, is made possible for a large period of the processes and the long residence times of parti- year (March-July) by the turbulent motion of cles (Eisma and Cadée, 1991). The occurrence of particles in such shallow waters as those from a S13C anomaly in the tidal freshwater reach the Wash estuaries. Despite the long residence corresponded to the progressive formation of a times of particles primary production is sus- suspended particulate matter (SPM) maximum tained in the tidal freshwater reach and for (from 100 pg I-' in May to 400 pg I-' in Decem- each transect few spatial changes resulted in the ber) while its decrease (below 50 pg 1-' in March) biochemical composition of POM. Adaptation corresponded to the partial flushing of this SPM of cells to low light intensities is maximum to the sea by increasing river flow known to decrease enzymatic isotope fraction- (Fichez et al., 1992). S13C values for sediment ation subsequently increasing the SI3C value POC in the upper estuary of the Great Ouse of the (Mortain-Bertrand et al., 1988; (salinity 0-10 PSU) were in the range -21.7 to Descolas-Gros and Fontugne, 1990a, b). Auto- -20.3%0 (R.Fichez, unpublished results, 1991), cthonous primary production thus could be a thus demonstrating the benthic boundary layer source of POM enriched in 13C in the freshwater to be a significant potential source of POM reaches of shallow turbid estuaries. enriched in I3C. Values of S13C higher for the The influence of both autotrophic production sediment than for SPM indicate that marine sedi- and sediment resuspension combined with two mentary organic material may be transported end-members mixing yields a complex pattern upstream. Such a transport, related to the tidal of the spatial and temporal distribution of currents at the benthic boundary layer, has S13C. Even if the analysis of 613C is of limited already been suggested through stable isotope value in the study of systems with more than study (Mook and Tan, 1991). two organic sources (Fry and Sherr, 1984) it is Additionally, despite a high level of turbidity, of considerable interest in giving evidence of the evidence for significant primary production in multiplicity of these sources as well as in pro- 'I'

:ti hlnrrh 19W

I R. Fichez et a1,IMarine Chemistry 43 (1993) 263-276 273 ducing some clues to the identification of poten- contribute to a moderate increase in 613C at tial organic sources. From the results obtained stations TO4 and TO3 when compared with in this study we are able to show that POM TO2 and TO1 which are located in the middle composition does not depend on a simple water of the Wash. To test this hypothesis, sedi- mixing process alone but also on other environ- ments from the Wash embayement and the tidal mental conditions (light penetration, particle flats will be analysed for their stable isotope residence time, resuspension). compositions. The rise in S13C to typical marine values at the The sharp decrease in the d3C ratio of the mouth of the estuary, 30 km downstream of terrestrial organic material from -26 %O in Denver Sluice, demonstrates that, even though August to -33%0 in September may be related dilution has a major impact on estuarine POC, to the increase in the river flow with monthly little direct export of continental POC occurs for averaged values of 3m3s-' in August and most of the year, as concluded in several studies 5 m3 s-l in September (National Rivers Author- on estuaries (Salomon and Mook, 1981; Tan and ity (NRA), personal communication). After a Strain, 1983; Gearing et al., 1984; Lucotte, 1989). long period of relative dryness and prolifer- Nevertheless, the progressive increase in S13C ation of planktonic and benthic algae in the values from the river to the sea in March, Septem- ponds and agricultural drains surrounding the ber or January could indicate that some outwelling river system, strong rainfall washed this organic fluxes of POM do occur but it must be emphasized material to the river and through the estuary to that stormy events may also be responsible for a the sea. Evidence for the strong depletion of significant shift in SPM S13C at sea. 13C in the terrestrial organic material flushed to We cannot give an unambiguous explanation the estuary arises from the very negative and for the few %O drop in S13C leading to the values marked seasonal fluctuations in d3C values of around -22 %O occasionally measured in the measured in standing fresh water (Rau, 1978; middle of the wash. An input of continental Fry and Sherr, 1984; Mook and Tan, 1991). organic material through the plume of the large The downstream extent of the several %O drop Humber estuary to the Wash is very unlikely, in d3C in September as compared with August particularly as this trend was clearly observed indicates that the export of continental organic at the end of spring and during summer when material reached 35 km downstream of Denver river flows are low. This anomaly was first Sluice. However, the two sampling stations observed in March and corresponded to high located in the middle of the Wash (T01, T02) concentrations of lipid, which are known to be were not affected by this outwelling of continen- very depleted in 13C. However, from June to the tal organic material. end of the year lipids were low but COM The significant importance of COM in riverine increased markedly. We thus suggest that both and estuarine organics, is already well known lipid and COM may be partially responsible for (Laane, 1983; Berger et al., 1984; Saliot et al., the shift in S13C values in the middle of the Wash. 1984; Ittekkot, 1988). This has been confirmed Additionally, the influence from the tidal flats in this study as carbon associated with COM located at the mouth of the estuary (between 25 commonly accounted for more than 40% of and 35 km downstream from the sluice) may bulk POC. Complex organic matter is generally considered to be highly refractory to decomposi- tion and to display a conservative behaviour in Fig. 3. As for Fig. 2 for the proportional particulate organic estuaries (Courtot et al., 1985; Ittekkot, 1988; carbon content (%) of the diverse biochemical compounds: carbohydrates (CH), proteins (PR), lipids (LI) and the Spitzy and Ittekkot, 1991). We did not find any remaining fraction of POC which is assumed to be complex strong increase in the proportional importance organic matter (COM). of COM in the turbidity maximum as shown in '74 R. Ficlic: et 01. dfuriiie Cheinisir>~33 1 IV93 I 263-.?76 1 the Loire estuary (Saliot et al.. 1984). From Acknowledgements March to July the regular decrease in the impor- tance of COM as a carbon source from the river Many thanks go to the staff from UEA. to the middle of the Wash agreed with other MAFF. to the crew of R' V "Cirolana" and to studies demonstrating the uncharacterized frac- the King's Lynn Pilot Board which strong11 sup- tion of organic matter to be higher in river than ported sampling at sea. We are \cry grateful to at sea (Ittekkot and Zhang. 1989: Spitzy and NRA for providing data on river flows at Denver Ittekkot. 1991). However. from August to Janu- Sluice. Wc also thank Dr. M. Fontugne for use- ary no significant differences were observed from ful discussions of the 613C analysis and two the river to the middle of the Wash. thus demon- ononqmous referees for correcting the manu- strating the increasing importance of the humic script. One of the authors (R. Fichez) gained fraction in marine POC. financial support for this work from the MAST Carbohydrates peaked during the phytoplank- programme of the European Economic commu- ton bloom periods. especially during May in the nit). UEA Stable Isotope Laboratory Contribu- marine part of the estuary. coresponding to a tion No. 0003. classical feature in phytoplankton groiith (Anonymous. 19S5: Mayzaud et al.. 1989). Pro- teins. or niore restrictively amino acids, have References been reported to be of insignificant importance during winter periods but to represent up to 69" O Anonymous. 1985. Organic material and mineralization. of organic matter in phytoplankton blooming Rijkswsaterstaat communications No. JO. Rijksuater- staat. The Hague. pp. 100-1 19. (Laane, 1953). Nevertheless. our results showed Benner. R.. Fogel. h;l.L.. Sprrigue. E.K. and Hodson. R.E.. protein carbon to represent a significant fraction 1987. Depletion of '"C in and its implications for of POC during most of the suney as well as stahle carhon isotope studies. Nature. 32Y: 708-710. being the main characterized organic compound Berger. P.. Etcheber. H.. Ewald, M..Lavaus. G. and Belin. ahead of carbohydrate and lipid. A similar high C.. 1951. Variation of orgrinic m:itter extracted from particles along the Gironde estuary. Chem. Geol.. 45: content ofproteins or amino acids in POM have 1-1 h. been described for riverine (Wafar et al., 1989) Bligh. E.G. and Dyer. N.J.. 1959. A rapid method of rotal and estuarine systems (Saliot et al.. 1983: Poulet lipid extraction and purification, Can. J. Biochem. I'hy- et al.. 1986: Relexans et al.. 19SS) and it is pos- siol.. 37: 911-417. Brockmcin. U., Billen. G.lind Gieskes. W.K.C.. 1988. North sible to question whether or not the excess of N Sea nutrients and eutrophication. In: W.Salomons. H.L. nutrients is of importance in the contest of pro- Bayne. E.I.;. Duurrsmn and U. Fijrstner (Editors). Pollu- tein production. tion of the North Sea. an Assessment. Springer. Berlin, Turbidity maxima are generallj accepted as pp. 348- 3x9. Byers. S.C.. Mills. E.L. and Stewart. P.L.. 1978. A compar- locations for heterotrophic decomposition of ison of methods of determining organic carbon in marine POM produced upstream. as the residence time sediments, with suggestions for 'i standard method. of particles is high and light penetration low Hydrohiologia. 58: 43-47. (Kranck. 1979). However. the behaviour of Chesselet. R.. Fontugne. M.R.. Buat-hlenard. P.. Erat. U. and Lambert. C.. IYSI. The origin ofparticulate organic POM in the Great Ouse estuary may be carbon in the marine atmosphere as indicated bl its regarded as a possible example of how other stable crirbon isotopic composition. Geophjs. Res. shallow tidal estuaries function. Furthermore. Lett.. 8: 345-318. it may be inferred from this study that in shal- Courtot. P.. Ilahude. A.G.. Noureddin. S. and Chevolot. L.. lY85. Comportement conservatif des substances humi- low water and turbid estuaries the organic and ques particulaires dans la rade de Brest. 0ct:tinis. 11: inorganic reservoirs are niore tightly and intri- 4'9436. cately related than has been assumed. Craig. H.. 1957. Isotopic standards forcarhon and oxygen and R. Fichez et al./Marine Chemistry 43 (1993) 263-276 275

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