Tidal variations of particulate carbohydrates in the Mersey estuary

Estuarine, Coastal and Shelf Science (1992) 34, 37-48

Tidal Variations o f P a r t i c u l a t e C a r b o h y d r a t e s in the Mersey Estuary

Martin R. Preston a and Prabhakar

Prodduturu*

Oceanography Laboratories, Earth Sciences Department, Liverpool University, P.O. Box t47, Liverpool L69 3 B X , U.K. Received 7January 1991 and in revised form 17 May 1991

K©ywords: estuaries; carbohydrates; suspended particles; sediments The composition of monosaccharides and their variations in concentration in suspended particles and sediments have been examined for samples from two sites in the River Mersey estuary. Samples were collected over a tidal cycle and the compositional changes as a function of tidal state are reported. Higher concentrations of carbohydrates were found in the lower-salinity region with glucose being the dominant monosaccharide at both sample sites. Carbohydrates contributed between 14 and 17°o of the organic carbon present. The detailed monosaccharide speciation indicates that the primary sources of monosaccharides are angiosperm leaves and grasses rather than phytoplankton. Carbohydrate concentrations were somewhat higher in the suspended particles than in the underlying sediments and various hypotheses are advanced to explain this phenomenon.

Introduction Particulate organic matter (POM) in estuaries derives from allochthonous sources such as terrestrial plants and soils, anthropogenic sources such as industrial and wastewater discharges, and autochthonous sources which are dominated by primary production. T h e complexity of estuarine systems has limited the extent to which it is possible to make definitive statements about the nature and magnitude of inputs, cycling and sinks of organic matter. O f the many organic compounds that make up the estuarine and marine P O M the carbohydrates form an important part (Cowie & Hedges, 1984a; Hedges et al., 1986, 1988; Mopper, 1980; Ochiai et al., 1987, 1988). However comparatively little is known of their occurrence and behaviour in the estuarine environment and, in particular, how the abundance and composition of the carbohydrate fraction differs with time and position. T h e work described in this paper addresses these issues and forms part of a continuing programme of investigations of natural and anthropogenic estuarine organic matter conducted in these laboratories (e.g. A1-Omran & Preston, 1987; Readman et al., 1986; Preston & AI-Omran, 1986, 1989; Reeves & Preston, 1989, 1991). °To whom correspondenceshould be addressed. *Present address: Geochemistry Division, KDMIPE, O.N.G.C., Kaulagarh Road, Dehradun, India 248195. 0272-7714/92/010037 + 12 $03.00/0

© 1992AcademicPress Limited

M.R. Preston&P. Prodduturu

38

53"30"N

\ 53"tO'N 2"37'W

3"08"W

Figure 1. The River Mersey estuary showing the positions of the sample sites, (•).

The study area

T h e Mersey estuary (Figure 1) flows into Liverpool Bay at the south-east corner of the eastern basin of the Irish Sea. T h e catchment area of the River Mersey covers approximately 4560 km 2 of north-western England, much of which is heavily industrialized particularly in the lower reaches of the river. T h e River Mersey estuary extends for about 50 km from the upper tidal limit to its mouth into the Irish Sea. Along the estuary there is extensive industrial activity with concomitant waste discharges together with a considerable number of treated and untreated sewage outfalls. There are extensive m u d flats within the estuary which are exposed at low tide and which are colonized in part by such plant species as common reeds (Phragmites australis) and cord grass (Spartina angelica). T h e high tidal range (8.4 m mean springs) induces a very high turbidity (frequently 0 . 5 - > 3 - 0 g l -~ in the lower, wider part of the estuary) which considerably restricts planktonic growth, though growth of algae such as Enteromorpha is evident on dock walls. T h e estuary may be considered to be divided into two regions (see Figure 1). A narrow upper part dominated by riverine flow and a broad, shallow, lower region where tidal mixing and sediment resuspension are more important. T h e Widnes site is representative of the former conditions and the Speke site of the latter. T h e sites are also accessible at all stages of the tide.

Methods

Samples of water for suspended particulate matter (SPM) analysis were collected at hourly intervals at two sites in the Mersey estuary: Widnes West Bank in the upper estuary and Speke in the lower estuary. T h e sites are shown in Figure 1. Samples were collected at approximately 1 m depth using pre-cleaned 10-1 glass jars. HgC12 (20 mg 1-1) was added to each sample to inhibit biological changes in the carbohydrate fraction. Simultaneous

Tidal variations of particulate carbohydrates

39

samples were taken for salinity measurement. At each site, surface sediment samples were collected from about 5 m from the water's edge at low tide. T h e s e samples were collected in aluminium foil-capped, pre-cleaned glass jars using a stainless steel spatula. T h e y were returned to the laboratory at the first opportunity and frozen until required for analysis. In the laboratory, water samples were filtered as soon as practicable through ashed (450 °C) and pre-weighed W h a t m a n glass fibre filters. Samples were then frozen until required for analysis. Prior to analysis, samples of both S P M and surface sediment were oven-dried ( < 40 °C), filters were weighed to determine S P M loadings and both types of sample were ground in an agate ball mill to pass through a 0-351-mm mesh sieve. Subsamples were extracted for elemental analysis using a Carlo Erba Model 1106 Elemental Analyser after treatment with dilute acid (1 M HC1) to remove inorganic carbonates.

Carbohydrates analysis T h e analytical technique utilized in this work was that of Cowie and Hedges (1984b) which is a refinement of a combination of the preparation and extraction steps of M o p p e r (1977) and the equilibration step described by Bethge et al. (1966). Depending on the sample type and the organic carbon levels, 25-500-mg samples (equivalent to an organic carbon loading of 10-30 mg) were weighed into 100-ml Pyrex tubes with P T F E - l i n e d screw caps. A volume of 1-2 ml o f 7 2 % sulphuric acid sufficient to fully cover the sample was added by pipette followed by a small, P T F E - c o v e r e d magnetic stirring bar. T h e samples were then stirred for 2 h at laboratory temperature, after which sufficient distilled water was added to produce a 1.2 M solution of acid. T h e tightly capped tubes were then stirred at 100 °C in a boiling water bath for 3 h after which the hydrolysis was halted by placing the tubes in an ice bath. At this stage an aliquot (100-200 ~tl of a 1-mg m l - x solution in p'yridine) of a solution of adonitol, a five-C alditol, was added to act as an internal standard for the later G C quantitation. Immediately after hydrolysis and internal standard addition, finely ground barium hydroxide (approximately 90% of the stoichiometrically required amount) was added. T h e tightly capped tubes were placed on a shaker to aid the precipitation of barium sulphate and to avoid incomplete reaction due to the formation of protective films which may develop on the Ba(OH) z crystals. After this step the mixture was brought to a p H of 6.5 (+_0.2) by further addition of Ba(OH) 2, quantitatively transferred to a 75-ml glass centrifuge tube and centrifuged for about 15 rain at 2500 r p m (750 g). T h e supernatant was subsequently removed by decantation. T h e neutralized solution was passed through a 15-20-ml column of 1:1 mixed cation (Dowex 50W-X8, 20-50 mesh, H + form) and anion (Dowex 1X8-200, 100-200 mesh, formate form) exchange resins at 1-5-2-0 ml rain- ~ and the column was rinsed with two bed volumes of distilled water. T h e neutralized and deionized hydrolysate eluant was transferred to a 100 ml flask and rotary evaporated to dryness at 60 °C. T h e large volume ( ~ 75 ml) of water to be removed meant that this procedure was slow and required high rotation speeds and a good vacuum to speed it up. Each sample was redissolved in pyridine and an aliquot was transferred to a 3.5-ml glass vial (because of the relative instability of the derivatized sugars, some sample was generally retained in a dried, frozen form for confirmatory analysis if required). An equal volume of pyridine containing 0.4% w/v LiCIO 4 was then added, the vial was sealed with a P T F E - l i n e d cap and the solution was left to equilibrate in an aluminium heating block for 48 h at 60 °C.

40

M . R. Preston & P. Prodduturu

After equilibration, 100-200 ~1 of a solution of sorbitol, a six-C alditol, was added as a reference standard. T h e mixture was then blown down to dryness under a gentle stream of nitrogen gas. Trimethylsilyl derivatives were formed by adding 200 ~tl of Regisil R C - 2 [bis(trimethylsilyl) trifluroacetamide+ 1% trimethylchlorosilane; Regis Chemical Co.] and incubating the sample for 10 min at 60 °C. After cooling, the samples were again reduced to dryness under nitrogen gas, redissolved in 1 ml of pyridine and 1 ~tl aliquots were injected into the GC. Gas chromatographic analysis was carried out using a Perkin Elmer Sigma 2B gas chromatograph fitted with a F I D detector and a Sigma 15 data station. A split-injection technique onto a 25 m x 0.22 m m id. BP10 fused silica capillary column (SGE) was used. T h e chromatographic conditions used were: injection port 250 °C, detector 275 °C, oven p r o g r a m m e d for a 4-min delay followed by a rise of 4 ° m i n - : to 240 °C. Helium was used as a carrier gas with a column head pressure of 2 bar. U n d e r these conditions all sugars elute in under 25 min and at least one major peak of each sugar can be quantified. Eight neutral sugars were identified by reference to authentic standards and quantified using an internal standard technique. Confirmatory analysis was performed using a Finnigan Ion T r a p model 700 mass spectrometer coupled to a Hewlett Packard model 5890A G C fitted with the same capillary column and using identical chormatographic conditions. Although procedural blanks were routinely run no significant corrections were required. T h e analytical reproducibility of the method was estimated as __7°:~ (Prodduturu, 1990).

Results T h e full data set collected for the two sites over the sampling period is given in T a b l e 1. At both stations there was considerable temporal variation in the suspended load. T h e mean S P M concentration at the Speke station was about four times that found at Widnes ( 6 9 0 m g l - : , Speke; 175mg1-1, Widnes). T h e r e was no clear relationship between suspended load and salinity at either station although there is some indication that higher salinities and higher S P M burdens go together in the lower estuary. Plots of the P O C content against S P M (Figure 2) reveal that at each station there is an inverse relationship between these two parameters. T h i s phenomenon has been reported before by, for example, Meybeck (1982) and Ittekot and Arain (1986) and arises from the dilution of organic matter by resuspended sediments which are dominated, at high flows, by inorganic matter. Mean organic carbon to nitrogen ratios were very similar at the two sites at around 14:1 but the Widnes samples showed considerably greater variability (Speke mean O C / O N = 13.8, SD = 3.42, n = 9; Widnes mean O C / O N = 14"2, SD = 6-60, n = 9). T h e s e atomic mass ratios are fairly similar to that reported by Hedges et al. (1986) for material from the Amazon River and, indeed, an average of about 10 has been observed in most rivers. Soil organic matter commonly has a C / N ratio of about 10 (Meybeck, 1982) whereas marine plankton have a typical ratio of around 5 (Hamilton & Hedges, 1988). T h e ratio for vascular plant debris has been reported to fall in the range of 20-300 (Hedges et al., 1985, 1986). In a contaminated estuary like the Mersey, domestic and/or industrial inputs may also be significant. Sewage organic matter from the region has a C / N ratio of about 3-5 ( D r P. C. Head, Northwest Water plc, pers. comm.) but no data are available for industrial waste. T h e present observations for the Mersey are therefore consistent with the hypothesis that the suspended material is predominantly derived from land with only a minor contribution from marine components. T h e influence of land-derived

Tidal variations of particulate carbohydrates

41

TABLE 1. Analysis of s u s p e n d e d particles from the two sites

Speke samples Sample T i m e (h) Salinity (%0) S P M ( m g I -i) Organic C (%) Organic N (%) C/N L y x (lag g ~) Ara (lag g ~) R h a (lag g ~) F u c (lag g ~) Xyl (lag g ~) M a n (lag g ~) Gal (lag g i) G l u (lag g i) T . P C H O ( l a g g ~) L y x (mg g O C i) Ara (mg g O C i) Rha (mg g O C ]) F u c (mg g O C i) Xyl (mg g O C ~) M a n (mg g O C J) Gal (rag g O C ~) G l u (rag g O C ~) T . P C H O ( m g g O C ') T o t . H e x o s e s (lag g ~) T o t . Pentoses (lag g- ~) T o t . D e o x y - H e x (lag g ~) L y x (mole % ) Ara (mole % ) R h a (mole %) F u c (mole %) Xyl (mole °.0) M a n (mole %)) Gal (mole %) Glu (mole %) Hexoses (%) Pentoses (%) D e o x y - H e x (%) Hex/Pent Man/Xyl (Ara+Gal)

S1

$2

$3

$4

$5

$6

$7

$8

$9

0941 18-75 206"00 2-96 0"27 12-50 122-00 92-00 269"00 153"00 270"00 432"00 243"00 1029"00 2610-00 4"12 3.11 9.09 5.17 9.12 14.59 8-21 34-76 88-18 1704-00 484"00 422.00 5.40 4.00 9,80 6.20 12.00 16.00 9.00 38-00 62.68 21.37 15.97 2.93 1.33 20.90

1041 19-05 295-00 3"06 0"27 13"00 171-00 32-00 154-00 263-00 100-00 224"00 72"00 570"00 1586"00 5"59 1.05 5.03 8.59 3.27 7.32 2.35 18-63 51-83 866"00 303-00 417.00 12.40 2.30 9,10 17.30 7-20 13.40 4-30 34-00 51.82 21.77 26.41 2.38 1.87 10.08

1141 21"54 677-00 2-75 0-23 11-50 177-00 53-00 172-00 340"00 117-00 413"00 122"00 651'00 2045-00 6"44 1-93 6.25 12.36 4-25 15.02 4.44 23.67 74"36 1186-00 347"00 512-00 9-90 2.90 7-90 17-40 6-50 19.20 5.70 30-30 55.25 19.44 25.31 2.84 2.94 12.39

1241 22-96 250"00 2-68 0-26 11-00 322-00 51"00 240"00 412"00 298"00 1086"00 322"00 1371"00 4102"00 12-01 1-90 8'96 15"37 11.12 40.52 12.01 51-16 153-06 2779-00 671-00 652-00 9.00 1.40 5.60 10-60 8-30 25.40 7.50 32-00 65.03 18.82 16.15 3.46 3.04 13.19

1341 23-61 1788-00 2-01 0-18 17"00 474-00 352"00 271"00 226"00 467"00 406"00 713"00 1562"00 4471"00 23.58 17.51 13"48 11-24 23,23 20"20 35.47 77"71 222.44 2681-00 1293-00 497.00 11-90 8-90 5-60 5.20 11.80 8.50 15.00 32.90 56.43 32.72 10.77 1.72 0.72 35.61

1541 23'28 735-00 2"26 0"20 19-00 374"00 514'00 280"00 232"00 464'00 312'00 712'00 1662'00 4550'00 16'55 22'74 12"39 10"27 20'53 13'81 31"50 73"54 201.33 2686.00 1352'00 512-00 9-30 12-70 5.70 5.30 11.50 6.40 14.70 34.30 55"50 33.52 10.98 1.66 0"56 41.82

1641 22"10 1186"00 2-17 0-21 18"00 502"00 337'00 313-00 219-00 461-00 614'00 715'00 1290'00 4451"00 23"13 15.53 14"42 10"09 21'24 28"29 32.95 59'45 205-12 2619-00 1300-00 532"00 12'70 8'50 6"50 5"00 11 '70 12"90 15"10 27.30 55'38 32'99 11-64 1'67 1'11 32.57

1741 21-83 938-00 2-20 0-24 9-00 570-00 421-00 274'00 232'00 460-00 511"00 779"00 1310-00 4587-00 25.91 19.14 12.45 10.55 20"91 23"23 35.41 60"91 208-50 2630-00 1451-00 506-00 13-90 10-30 5.50 5.20 11-30 10-40 15-90 27.30 53.70 35"58 10-72 1-51 0"93 36"10

1741 21-57 138-00 3-26 0-32 13-50 313"00 508-00 161-00 180-00 170-00 241"00 584"00 773-00 3054"00 13"40 15.58 4.94 5.52 5-21 7-39 17-91 23"71 93-68 1598-00 1115-00 341-00 15"90 18"50 4-80 6-00 6-20 7-30 17"80 23"50 48.55 40.64 10.82 1.19 1"18 47"36

W1

W2

W3

W4

W5

W6

W7

W8

W9

Widnessamples Sample T i m e (h) Salinity (%0) S P M (mg 1 ~) Organic C (%) Organic N (°o) C/N L y x (lag g ')

0941 1041 1141 1241 1341 10-33 3.07 14-61 18-89 14.52 60.00 378.00 302-00 144-90 155.00 3-33 2.59 3-49 3-11 3.59 0.20 0-20 0-36 0.37 0.42 28-00 22-00 9.70 8.70 10.00 1329.00 360.00 489-00 411.00 658.00

1441 1541 1641 1741 14-06 13.20 11.19 10-03 266.00 64,00 149.00 58.00 2'87 3"38 3.29 3-52 0'30 0"48 0-29 0-31 12.00 9.30 13.50 14-50 574,00 717.00 596.00 1082-00

continued

M. R. Preston & P. Prodduturu

42

TABLE 1. Continued.

Widnes samples Sample

Wl

Ara(~tgg -~) Rha(llgg -j) Fuc(lagg -~) Xyl(lagg -~) Man(lagg -~) Gal(~tgg J) Glu (lag g 1) T . P C H O (lag g-~) Lyx (rag g OC ~) Ara (rag g OC i) Rha (rag g OC -~) Fuc (rag g OC ~) Xyl (mg g O C ~) Man (mg g OC ~) Gal (mg g OC '~ ) Glu (mg g OC -~) T . P C H O ( m g g O C -j) Tot. Hexoses (lag g z) Tot. Pentoses (lag g-') Tot. Deoxy-Hex(lagg-') Lyx (mole °,b) Ara (mole Oro) Rha (mole %) Fuc (mole %) Xyl (mole %) Man (mole %) Gal (mole %) Glu (mole %) Hexoses (°b) Pentoses (%) Deoxy-Hex (°o) Hex/Pent Man/Xyl (Ara + Gal)

625.00 262.00 233.00 117.00 201.00 704.00 1536.00 5007.00 39.91 18-77 7-87 7-00 3-51 6.04 21.14 46.13 150-36 2441-00 2071.00 495.00 29"30 13"80 4-70 4-70 2-60 3-70 13-00 28.20 44"90 45"70 9.40 0-98 1.72 38.29

W2

W3

W4

251.00 497,00 373-00 212.00 305.00 285-00 218.00 267.00 259.00 207.00 337.00 357.00 229.00 311.00 491.00 423.00 623.00 1156.00 1528.00 1768.00 2560-00 3428.00 4597.00 5892-00 13-90 14-01 13.22 9.69 14.24 I 1-99 8-19 8.74 9-16 8-42 7-65 8-33 7.99 9.66 11.48 8.84 8.91 15.79 16-33 17.85 37.17 59.00 50'66 82-32 132.36 131.72 189.45 2180.00 2702,00 4207.00 818.00 1323.00 1141.00 430.00 572.00 544-00 11 "90 12'00 8"00 8"30 12"20 7"30 5-80 6.20 4.60 6-60 6.00 4-60 6-90 8.30 7-00 6.30 6-40 8-00 11.70 12.70 18-80 42.30 36-00 41-60 60"30 55"10 68"40 27"10 32"50 22'30 12.40 12.20 9.20 2.23 1.70 3.07 1.11 0.92 1.37 35.47 39.59 45-89

W5

W6

414.00 273.00 232.00 336-00 402.00 262-00 237.00 413-00 166.00 376-00 309.00 584.00 1135.00 1610.00 3553-00 4428.00 18.33 20-00 11-53 9-51 6.46 11-71 11-20 9-13 6,60 14.39 4.62 13.10 8.61 20-35 31.62 56.10 98.97 154.29 1610.00 2570.00 1309.00 1260.00 634.00 598.00 20'50 14"60 12'90 7"00 6.00 7.00 11,40 6.10 7-40 10-50 4-30 8-00 8-00 12-40 29.40 34-20 41'70 54"60 40'80 32"10 17.40 13-10 1.02 1.70 0.70 0-91 29.90 30.41

W7

W8

1108.00 525.00 490.00 501-00 449.00 492.00 464"00 734-00 545,00 1234-00 1220.00 1329.00 2109'00 2572.00 7102"00 7983.00 21.21 18-12 32.78 15-96 14.50 15-23 13-28 14-95 13.73 22.31 16.12 37.51 36'09 40.40 62.40 78.18 210,12 242.64 3874,00 5135.00 2289,00 1855-00 939.00 993.00 11 "30 8"50 17~50 7'50 6.40 5.90 6.50 6.40 7.30 10-50 7-20 14.70 16.00 15-80 27.70 30.60 50"90 61"10 36'10 26"50 12'90 12.30 1-41 2.31 1.18 1.68 46.63 34.26

W9 1480.00 450.00 747.00 367-00 593.00 1326.00 1350.00 7395.00 30.74 42-05 12-78 21-22 10.43 16.85 37.67 38.35 210-09 3269.00 2929.00 1197.00 16" 10 22"00 5.50 10-20 5-50 7-40 16-50 16.80 40'70 43'60 15.70 0.93 1-62 46.42

SPM, suspended particulate matter; Lyx, lyxose; Ara, arabinose; Rha~ rhamnose; Fuc, fucose; Xyl, xylose; Man, mannose; Gal, galactose; Glu, glucose; T.PCHO, total particulate carbohydrates; Deoxy-Hex, deoxy-hexoses; Hex/Pent~ total hexoses/total pentoses; Man/Xyl, weight °0 mannose/weight % xylose.

organic matter in the Mersey estuary has previously been noted in observations of lignin d i s t r i b u t i o n s ( R e e v e s & P r e s t o n , 1989). Total carbohydrate concentrations (T.PCHO)

r a n g e d f r o m a b o u t 52 to 2 2 3 m g g - ~ O C

i n t h e S p e k e s a m p l e s ( m e a n = 144-3, S D = 6 7 . 5 ) w h i l s t a t t h e o t h e r site t h e r a n g e w a s f r o m 9 9 to 2 4 3 m g g - ] O C ( m e a n = 1 6 8 . 9 , S D = 4 6 . 6 ) . T h e r e l a t i o n s h i p b e t w e e n T . P C H O and the organic carbon content was however markedly different between the two sample s e t s ( F i g u r e 3). I n t h e l o w e r e s t u a r y t h e r e w a s a f a i r l y s t r o n g n e g a t i v e r e l a t i o n s h i p b e t w e e n t h e s e t w o p a r a m e t e r s ( r = - 0 . 8 0 , n = 9) w h e r e a s i n t h e u p p e r e s t u a r y t h e r e w a s a w e a k p o s i t i v e t r e n d ( r = 0 - 3 6 , n = 9 ) . I t is e v i d e n t f r o m t h i s t h a t t h e r e is a d i l u t i o n o f carbohydrate in the lower estuary by organic material depleted in this fraction. T h e t i d a l v a r i a b i l i t y o f T . P C H O c o n c e n t r a t i o n s is s h o w n i n F i g u r e 4. T h e g e n e r a l t r e n d a t b o t h s i t e s is t o w a r d s a d e c r e a s e i n c o n c e n t r a t i o n t o w a r d s t h e h i g h t i d e ( 1 1 4 1 h ) w i t h a

Tidal variations of particulate carbohydrates

43

43

+ ,+'

+ 2 o o..

I

0

500

1000 SPM (rag

1,500

2000

I -I )

Figure 2. The relationship between POC and SPM (both sites). *, Speke; +, Widnes.

(o)

4.

2

I

~°z o t0 L~

i ......

i ...........

2'2

2"4

2"6

t

2'8

~ ...........

3

3"2

3:4

3"6

J

3~'8

4

(b) +

8

+ 6

+

+ 4

+

2

O2 POC (%)

Figure 3. The relationship between T.PCHO and POC, l I August 1988. (a) Speke, (b) Widnes.

subsequent increase on the ebb. T h i s trend is somewhat clearer for the Speke samples. At both sites there is a decrease in concentration at high slack water (around 1200 h) presumably due to some settling out o f relatively' carbohydrate-rich' material. I f the tidal variability o f the different monosaccharide classes (i.e. hexoses, pentoses and deoxy-hexoses) is examined (Figure 5), it can be seen that at both sites it is the hexoses

44

M. R. Preston & P. Prodduturu

250

5

(o)

200

~,

4

/

150

3

+"

D

I

////

O0 a ~ ~

o

50

o~

0

2 I T O3

I::

F:

iJi

I0p

it2

i 13

J 14

16-

i 15

200 ~

~ 17

180

o, :m

-

./

//'/~

10 0

"/

+I

50~ 0

2 - L _ _ _ _

..............

9

L ....................

I0

tl

~.

I2.

I

............ E

13 14 Time ( h )

I5

1

I

I6

t7

0

18

Figure 4. Tidal variation of T.PCHO in the Mersey estuary, 11 August 1988. (a) Speke, (b) Widnes. [3, T.PCHO (rag g OC '); +, T.PCHO (rag g SPM-~).

Z

(a)

b)

2.5 2

/

T ~. i-5 E

/

I

0"5 0

•

9

I

" i

i

~

I

i

I0

II

12

13

14

15

i

16

i

17

.F

18

I0

II

13

14

15

16

17

18

Time ( h )

Figure 5. Tidal variation of total hexoses (•), pentoses ( + ) and deoxy-hexoses (*). (a) Speke, (b) Widnes.

w h i c h follow t h e d i s t r i b u t i o n o f T . P C H O m o s t closely, reflecting t h e d o m i n a n c e o f this class. P e n t o s e s too follow the T . P C H O d i s t r i b u t i o n fairly closely b u t the d e o x y - h e x o s e s t e n d to h a v e a fairly c o n s t a n t a b u n d a n c e t h r o u g h o u t t h e tidal cycle, t h o u g h t h e r e is

Tidal variations of particulate carbohydrates

45

70 6O

5O ~o

40

~D

+o

3o

~ 20

o o

I0

o

o

o i

i

I0

80

3'0

4'0

5'o

do

80

Man (%1

Figure 6. Plot of weight% (Ara and Gal) against weight% Man. The rectangle represents the range reported by Cowie and Hedges (1984a) for angiosperm leaves and grasses, x , Widnes; O, Speke.

evidence of a weak trend of increasing concentration over the study period in the Widnes samples. O f the eight individual monosaccharides quantified, glucose was dominant in both sample sets representing some 31% of the total in the Speke samples and 32% in the Widnes samples. On average the m o l e % decreased in the order G l u > M a n > G a l > L y x > Xyl > Fuc > Ara > Rha for the Speke samples and Glu > Lyx > Gal > Ara > Xyl = M a n > Fuc > Rha for the Widnes samples. Over the tidal cycle the absolute concentrations of monosaccharides within the S P M ranged by factors of between 1-5 and 7. T h e average contributions of the individual sugars to the total organic carbon fraction were 1.45, 1-09, 0-96, 0.99, 1-32, 1-89, 2-0 and 4"7% (Speke) and 2-1, 1-85, 1.05, 1-12, 1-11, 1-42, 2-62 and 5.61% (Widnes) for Lyx, Ara, Rha, Fuc, Xyl, Man, Gal and Glu, respectively. In total, the monosaccharides contributed 14.3 (Speke) and 16.9 % (Widnes) of the organic carbon present which translates into between 5 and 6°,0 of the organic matter present (assuming a carbon content of organic matter of 36% as derived from the Redfield formula). T h e r e do not appear to be any consistent relationships between the individual monosaccharide concentrations in the particulate phase and the salinity at which they were collected other than a tendency for occasional high concentrations to be found at the upstream site. Samples from the lower estuary are generally more homogeneous. Statistical analysis showed that the concentration of the T . P C H O in the Speke samples was most strongly correlated with just five of the monosaccharides measured, namely Lyx, Rha, Xyl, Gal and Glu, which all exhibited correlation coefficients of > 0.8 (significant at the >99°/0 level). T h e upstream samples showed strongest correlations with Gal, Rha and M a n (r = > 0"8 significant at the > 99% level) and slightly lower correlations with r = 0.6-0.8 (significant at the > 95% level). Biogeochemical source indicators

T h e use of monosaccharides as indicators of source materials has been described by Cowie and Hedges (1984a). Marmose to xylose ratios can, for example, be used to provide some general indications. T h e ratios found in the present work (0-56-3.04, Speke; 0-77-1.42 Widnes) are similar to those found in such sources as phytoplankton, leaves, grasses and angiosperm woods which all have ratios less than about 4.

46

M . R . Preston ~ P. Prodduturu

TABLE2. Analysis of sediments from the two sampling sites (a) Concentration data

Sugar

Concentration (~tgg- t dry sediment)

Concentration (mg g- ~OC)

Speke sediment Lyx Ara Rha Fuc Xyl Man Gal Glu TCHO Widnes sediment Lyx Ara Rha Fuc Xyl Man Gal Glu TCHO

62.6 35.1 35.6 29.7 42.6 65.8 78.2 197-1 546-6

4-9 2.7 2.8 2.3 3.3 5.1 6-1 15-4 42-6

150-7 57-3 165.7

7-0 2-7 7.7

242.8

11.2

214-3 505"1 213.7 768-2 2317-7

9-9 23.4 9.9 35-6 107-3

Mole %

7-46 2.83 6.76 10.99 10.61 20-84 8-82 31.69

13-01 7.29 6.09 5.64 8.86 11.40 13.55 34.15

(b) Other data

Parameter Organic C (%) Organic N (%) C/N Hexoses (% ) Pentoses ( % ) Deoxy-Hex (%) Hex/Pent Man/Xyl (Ara + Gal)

Speke sediment

Widnes sediment

2-16 0-247 10.56 61.35 20.95 17.75 2-94 1-96 17.06

1-28 0-111 11-53 59-1 29.16 11-74 2.03 1.29 31.65

TCHO, total carbohydrate. All other abbreviations as in Table 1.

Cowie a n d Hedges (op. cit.) have suggested that it is possible to differentiate b e t w e e n woody a n d n o n - w o o d y tissues b y the e x a m i n a t i o n of the s u m m a t i o n of the weight per cent of arabinose a n d galactose (calculated o n a glucose-free basis). T h e p r e s e n t values lie w i t h i n the range 10.07 to 47.36% (Speke) a n d 29.67 a n d 46.45% ( W i d n e s ) w h i c h are c o n s i s t e n t with the c o n c l u s i o n s o u t l i n e d in the p r e v i o u s p a r a g r a p h b u t r a n g e too widely to b e o f m u c h f u r t h e r assistance to i n t e r p r e t a t i o n ; however, it m a y be possible to d i s c o u n t a n g i o s p e r m woods as a m a j o r source to the W i d n e s samples o n the basis o f their smaller r a n g e o f values. F u r t h e r source identification can be a t t e m p t e d by e x a m i n i n g the r e l a t i o n s h i p b e t w e e n the c o m b i n e d p e r c e n t a g e of arabinose a n d galactose (calculated o n a glucose-free basis)

Tidal variations of particulate carbohydrates

47

and mannose. Most of the values for the two data sets plot within the rectangle attributable to angiosperm leaves and grasses (Figure 6).

Comparison with surface sediments Data on the chemical composition of surface sediments collected from the two sites are given in T a b l e 2. At the Speke site the organic carbon content and the C / N ratio are similar to those for the suspended particles. Individual monosaccharide concentrations decreased in the order Glu > M a n > Fuc > Xyl > Gal > Rha > Lyx > Ara with glucose accounting for 31.7°0 of the total. In general the monosaccharide concentrations were somewhat lower in the sediment than in the suspended particles with the exception of that of fucose which was similar to the S P M average concentration. T h e T . P C H O content of the sediment, 1 0 7 - 3 m g g -~ OC, m a y be compared to that of 1 4 4 - 3 m g g -~ O C for the suspended particles. In the Widnes samples glucose was again dominant with the individual concentrations decreasing in the order Gal > M a n > Lyx > Xyl > Ara > Rha > Fuc. T h e total carbohydrate content of the sediments was about four times lower than the average for the suspended particles and constituted only 4.3 % of the sedimentary organic carbon. At both sites there were distinct chemical differences between the suspended particles and the underlying sediment. These differences may be a result of the contrasting ' resuspendability' of different sedimentary fractions, perhaps due to such factors as buoyancy or surface area (Reeves & Preston, 1991). T h e y may also be due to the fact that resuspended particles m a y be transported considerable distances with the result that the composition of a particle is more a function of its original source than its immediate position. A further possibility is that there are post-depositional changes in the carbohydrate composition. T h i s might be anticipated in view of the generally high biological usefulness of such compounds. Clearly, further studies are required to resolve these alternatives.

Conclusions T h e monosaccharide contents of suspended particles and sedimentary material from two sites within the River Mersey estuary have been examined. In general, concentrations of carbohydrates were higher in the lower-salinity region with glucose being the dominant component at both sample sites. Carbohydrates contributed between 14 and 17% of the organic carbon present or about 5 - 6 % of the organic matter present. N o clear relationship between particle composition and salinity was observed though concentrations were generally significantly higher on the ebb tide, perhaps reflecting the greater influence of freshwater-derived material. At both sites the hexoses, and glucose in particular, were the dominant components and examination of the detailed monosaccharide composition suggests that the primary source of these compounds as angiosperm leaves and grasses. Carbohydrate concentrations were somewhat higher in the suspended particles than in the underlying sediments perhaps due to variations in source material and/or its physical properties or post-depositional diagenetic changes. T h e r e is insufficient evidence to distinguish between these hypotheses and this will form the subject of further research.

Acknowledgements P. Prodduturu was supported by a National Overseas Scholarship provided by the Ministry of Welfare, G o v e r n m e n t of India during the period of this study.

48

M . R . Preston & P. Prodduturu

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