variations in sediment properties, skeffling mudflat, humber estuary, uk

*Corresponding author. Tel.: 0044-1334-463-467; fax: 0044-1334-463-443.E-mail address: [email protected] (D.M. Paterson).

Continental Shelf Research 20 (2000) 1373}1396

Variations in sediment properties, Ske%ingmud#at, Humber Estuary, UK

D.M. Paterson!,*, T.J. Tolhurst!, J.A. Kelly!, C. Honeywill!,E.M.G.T. de Deckere", V. Huet#, S.A. Shayler$, K.S. Black!,

J. de Brouwer", I. Davidson!

!Sediment Ecology Research Group, Gatty Marine Laboratory, University of St Andrews,St Andrews, KY16 8LB, UK

"Netherlands Institute of Ecology, Centre of Estuarine and Coastal Ecology, 4400 AC Yerseke, The Netherlands#CNRS-IFREMER, CREMA L'Houmeau, BP5, 17137 L'Houmeau, France

$Department of Earth Sciences, University of Cardiw, CF1 3YE, UK

Received 14 July 1999; received in revised form 22 December 1999; accepted 15 January 2000

Abstract

The generic importance of biogenic mediation of sediment erosion and transport is a matterof debate and a multidisciplinary approach is required to investigate biologically mediatedmechanisms of sediment stability. Biogenic in#uence on sediment behaviour can be inferredfrom a variety of correlative parameters that act as proxies for biological e!ects. These includepigment content, organic content and biomass. These biological `indicatorsa are routinelymeasured by biologists on a number of di!ering scales and depth resolutions. Few attemptshave been made to examine the importance of an appropriate `matcha between the erosionprocess, the measured physical response and the scale/resolution of the measured biologicalparameter. This scale dependency was examined along an extensive shore normal transect onthe Ske%ing mud#at (Humber Estuary, UK). Measurements of physical sediment properties,macrobenthos and selected biogeochemical properties (extracellular polymeric substances)were made. Biogeochemical properties were measured on a `traditionala cm scale and ata depth resolution of 5 mm but also on a microspatial scale, at a 0.2 mm depth resolution.Sediment stability was measured using a cohesive strength meter (CSM). Correlation analysiswas used to determine the interactions between variables. A complementary investigation of thesediment micro-fabric (low-temperature scanning electron microscopy) was also conducted.Results demonstrate that the depth resolution of biogeochemical measurements is an in#uentialfactor in the interpretation of the biogenic stabilisation of intertidal cohesive sediments.Sediment stability varied with time and with bed feature. Stability increased with time except

0278-4343/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved.PII: S 0 2 7 8 - 4 3 4 3 ( 0 0 ) 0 0 0 2 8 - 5

where in#uenced by other factors such as rain which markedly reduce surface stability. Criticalerosion threshold increased towards the shore whilst suspension index (erosion rate) decreased,and crests were generally more stable than troughs. The study emphasises the temporal andspatial variability of mud#at stability and the importance of biological processes on theerosional behaviour of cohesive sediments. ( 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Mud#ats; EPS; Extracellular polymeric substances; Erosion; Biogenic stabilisation; Diatoms;

Bedforms; Chl a; In situ measurements

1. Introduction

The stability of intertidal cohesive sediments is in#uenced by a large number ofinteracting physical and biological parameters (Paterson, 1997; Burt et al., 1997; Blacket al., 1998b; Black and Paterson, 1998a). In many natural intertidal systems, biolo-gical activity can be the primary control on sediment stability with variation instability directly linked to spatial and temporal variation in benthic biota (Kornmanand de Deckere, 1998; Widdows et al., 1998; Paterson, 1989; Sutherland et al., 1998;Tolhurst et al., 1999). The natural variation or `patchinessa of benthic systems canvary at several spatial scales (Reise, 1985; Thrush, 1991) and recognition of thepotential importance of small-scale variation led to the development of techniquesand devices capable of measuring the variation of properties (Black & Paterson, 1997;Harper et al., 1997; Wiltshire et al., 1997; Taylor and Paterson, 1998; Black et al.,1999). Cone penetrometry and other surrogate methods (Hansbo, 1957; Kravitz, 1970)have been used to determine sediment stability while some in situ erosion devices,such as the Cohesive Strength Meter (CSM) system (Paterson, 1989; Tolhurst et al.,1999) are capable of measuring small-scale spatial and temporal variation in sedimentstability. These techniques can now be combined to relate physical and biochemicalparameters at similar scales for the "rst time. It has been shown that micro-scaleanalysis of biochemical and sediment properties reveals information missed by coarsesampling methods (Davison et al., 1997; Wheatcroft and Butman, 1997) and thatrelated correlations can be improved (Paterson et al., 1998). This paper examines therelationship between biological and physical parameters and sediment stability overan intertidal transect on the Humber Estuary and includes examination of the bedforms, crest/trough formation characteristic of some stations (Black and Paterson,1998a; Dyer, 1998). Measurements of biochemical sediment properties were made ontwo spatial scales (lm and mm) to determine which was most relevant to sedimentstability.

2. Materials and methods

The Ske%ing mud#at on the Humber estuary is about 5 km wide with a 1 : 1000slope with a tidal range of about 6 m. Details of the site and the stations used can be

1374 D.M. Paterson et al. / Continental Shelf Research 20 (2000) 1373}1396

found in Black and Paterson (1998a) and Dyer (1998). Measurements of sedimentproperties were made at four stations A, B, C and D located 200, 580, 1550 and 2180 mfrom the shore, respectively. A crest/trough system of bedforms develops seawards ofstation A, covering a large proportion of the mud#ats, only the extreme upper shore(station A) was free of these bedforms. Stability measurements were made simulta-neously at stations B, C and D with three CSM devices. Measurements were made onboth crests and troughs, where present. The experiments were carried out in April1997 as part of the EU INTRMUD campaign. Grain size pro"les can be found inChristie et al. (2000). The major clay mineral was illite ('50% of (2 lm fraction atall sites) and further sedimentological description of the sites can be found in Blackand Paterson (1998a).

2.1. Macrobenthos

Sediment samples were taken to characterise the fauna at each sampling station.Three cores (i.d. 10 cm) were taken at each sampling location, with both crests andtroughs being sampled. The upper 30 cm of the sediment was sieved through a 0.5 mmmesh sieve and the remnant "xed with formaldehyde. These samples were analysed forspecies density and biomass.

2.2. Sediment sampling

Chl a, colloidal carbohydrate and water content analysis were conducted for eachstation by two methods * coarse core (syringe) analysis and by the "ne-scaleCryolander technique of Wiltshire et al. (1997). The coarse cores were taken usingmodi"ed Perspex syringes (36 mm i.d.), the upper cm of the coarse cores was split intotwo sections, 0}5 and 5}10 mm, then sectioned in centimetre increments from 1 to5 cm depth. The Cryolander discs were obtained by freezing the upper region ofsediment with liquid-nitrogen vapour (Wiltshire et al., 1997) to obtain frozen discs ofsediment (5 cm diam., by 1 cm depth). The discs were kept frozen, under liquidnitrogen and transported to the laboratory. The frozen discs were cut into blocks(1 cm2) while still frozen, and these were sectioned at 0.2 mm intervals, to a depth of2 mm, using a freezing microtome. The sections were re-frozen and lyophilised(freeze-dried), ready for further analysis.

Where the stations contained crest and trough systems, pairs of sediment sampleswere taken from adjacent crest and trough bedforms at regular intervals during theexposure period using both methods. Paired measurements of colloidal carbohydrateand Chl a were made from these samples.

2.3. High-performance liquid chromatography (HPLC)

Chl a was quanti"ed by reverse-phase HPLC. 1 ml of 100% acetone was added tothe lyophilised sediment sample (0.04}0.1 g) and pigments were extracted (24 h,!703C, in the dark). Acetone and sediment were separated by "ltration througha 0.2 lm coarse "lter (WhatmanTM). Chl a standards (derived from Anacystis nidulans,

D.M. Paterson et al. / Continental Shelf Research 20 (2000) 1373}1396 1375

SigmaTM) were analysed with every sample batch as an internal standard. The HPLCsystem consisted of a quaternary high-pressure pump (Perkin Elmer 410), anautosampler (Waters WISP 417) and a diode-array detector (Waters 910). The columnwas a nucleosil C18 (Capital HPLC Ltd) kept in a column oven at 253C. Extractionswere injected onto a binary gradient using the same methods as Wiltshire andSchroeder (1994) and Wiltshire et al. (1997). The #ow rate was 0.8 ml min~1 and thetwo solvents used were eluant A: 80% methanol, 10% water, 10% bu!er (1.5 g acetate,7.7 g ammonium acetate in 100 ml of distilled water), eluant B: 90% methanol, 10%acetone. Chl a eluted at 25 min, and analysis of peak area was used to quantify thepigment.

2.4. Colloidal carbohydrate

Colloidal carbohydrate was extracted using the method of Underwood et al. (1995)and quanti"ed using the methods of Dubois et al. (1956). As used here, colloidalcarbohydrate was de"ned as the carbohydrate content extracted in saline at 203C.Between 4 and 150 mg of lyophilised sediment was extracted in 1.2}5 ml of saline(2.5%) for 15 min at 203C, then centrifuged for 15 min. 1 ml of the supernatant wasremoved and colloidal carbohydrate analysed by the phenol}sulphuric acid assay:0.5 ml of 5% aqueous phenol (wt/vol) was added to 1 ml of supernatant, followedimmediately by 2.5 ml of concentrated sulphuric acid and the solution vortexed. Theabsorbency was measured spectrophotometrically at 485 nm after equilibration of thesolutions (1 h). A standard curve was produced using glucose and the carbohydratecontent expressed as glucose equivalents per dry weight of sediment (lg gl. equ.g~1 dry wt sed.) (Underwood et al., 1995).

2.5. Grain size analysis

The core samples were freeze-dried and analysed using a Malvern particle sizer(Malvern 3600 E). No further pre-treatment was applied to the samples.

2.6. Low-temperature scanning electron microscopy (LTSEM)

Low-temperature scanning electron microscopy (LTSEM) was used to visualise thesurface and vertical faces of the cryolander samples collected during the "eld study.Frozen discs of sediment were stored in liquid nitrogen and then fractured, undernitrogen, to provide samples for LTSEM (Paterson, 1995). The samples were vis-ualised using an SEM (Jeol JSM 35CF), modi"ed for cryogenic examination (OxfordInstruments CT1500). LTSEM of samples prevents water loss and preserves thestructure of hydrated material (Paterson, 1995; DeH farge, 1997).

2.7. Sediment stability measured with the CSM device

The CSM (Paterson, 1989; Tolhurst et al., 1999) is an in situ device for measuringthe erosion threshold of exposed intertidal sediments. The CSM creates a vertical jet


of water to erode the sediment surface. The device consists of a water-"lled chamber30 mm in diameter that is pushed into the sediment. The jet of water is directed at thesediment surface within the test chamber. The velocity of the jet is increased systemati-cally through each experiment. Bed erosion is manifested by the decrease in thetransmission of light across the chamber caused by the suspension of sediment. Theerosion is over a small area of the bed (0.0007 m2), which enables the device to detectsmall-scale spatial variations in stability. The device can be deployed rapidly and eachmeasurement takes 5}10 min, making the CSM suitable for measuring temporalvariations in sediment stability over a single tidal exposure. The CSM jet wascalibrated using the literature values for the suspension of sorted sand fraction toconvert velocity data to N m~2 (Tolhurst et al., 1999). The sensitivity and errormargins of the CSM system are discussed by Tolhurst et al. (1999) and value incomparison with other devices (Tolhurst et al., 2000).

Three CSM MKIII devices were used to determine sediment stability. Identicaldeployment protocols and jet programming was followed at each site. Data werecollected in up to sets of six replicates. Replicate measurements were taken on crestsand troughs. Stability was expressed as a threshold for sediment erosion, the point atwhich the transmission above the bed was decreased against background level by a set% (usually 10%). This jet pressure has been calibrated to an equivalent bed shearstress in N m~2; and the suspension index (S

i), a relative measure of the erosion rate,

was calculated from the decrease in transmission (increase in suspended sedimentconcentration) with increasing pulse velocity (Tolhurst et al., 1999).

3. Results

3.1. Macrobenthos

Two species, Macoma balthica and Streblospio shrubsolii were found at all stations(Table 1). High densities of Oligochaeta were found at stations A and B. Nereisdiversicolor and Retusa alba were the dominant predators at these stations, whileNephtys hombergii was dominant at stations C and D. The total density and number ofspecies decreased along the transect from station A to D. The biomass also decreasedtowards the sea. Biomass was dominated by three species: Macoma balthica, Nereisdiversicolor and Nephtys hombergii. At stations B, C, and D, the biomass in the troughswas lower than on the crests. However, the species density in the troughs was bothhigher (station C) and lower (stations B and D) than the crests.

3.2. Distribution of colloidal carbohydrate and Chl a

The coarse coring methods produced far lower levels of carbohydrate or Chla content in the sediments than the Cryolander method (Fig. 1). In addition, theformer produced rather consistent values with little variability within or betweenstations although the content fell below reliable limits of detection for the coarsecoring method by station D. Discrimination between sedimentological features was


Table 1Distribution of species biomass (g m~2) over the Ske%ing intertidal #at, Humber estuary

Station A B B C C D DCrest/trough ri! ru! ri ru ri ru

Deposit feedersCossura 0.002Oligochaeta 0.576 0.204 0.192 0.026

Interface feedersCorophium volutator 0.016Macoma balthica 4.902 11.083 8.096 2.788 2.04 0.87 0.461Spio martinensis 0.006 0.002Streblospio shrubsolii 0.039 0.011 0.01 0.012 0.033 0.005

Omnivores/predatorsCrangon crangon 0.291 0.097Eteone 0.006Nephtys hombergii 0.149 1.465 1.294 1.323 0.345Nereis diversicolor 4.782 1.006 1.347 0.212 0.111Retusa alba 0.012 0.145 0.069 0.038 0.071

Surface deposit feedersAbra tenuis 0.69Hydrobia ulvae 0.043Tharyx marioni 0.006Nemertinae 0.041

!ri"crest, ru"trough.

not apparent in the coarse core results while consistent di!erences were noted betweenthe troughs and crests of the crest/trough features at station B and C using theCryolander technique (Fig. 1). The Cryolander results revealed a general pattern ofdecreasing biomass and organic content in a seaward direction from the upper station(Station A). There was no strong temporal signal in Chl a or colloidal carbohydratecontent throughout the exposure period at any of the stations as determined by thecoarse coring method (Fig. 2). In keeping with the cross-shore pro"le results, stationA showed the highest surface content and also greatest intra-station variability(Fig. 2). Station D showed the least biological activity as determined by thesurface elevation in level of Chl a or colloidal carbohydrate. There was alsorelatively little variation between crest and trough features and again the relativelevel of content was shown to decline in a seaward direction along the transect(Fig. 2).

Analysis of pooled data showed highly signi"cant di!erences between stations for2 mm Cryolander slices (ANOVA F"30.37, DF"6, P(0.001) and 0.2 mm slices(ANOVA F"19.10, DF"6, P(0.001). Notably, station A had more colloidalcarbohydrate than all other stations except for the crest at station B. Station D (bothtrough and crest) had signi"cantly less colloidal carbohydrate than all other stations.There were no statistically signi"cant di!erences between crests and troughs at each


Fig. 1. Variation in colloidal carbohydrate measured over the intertidal transect. Station A had no crest ortrough features on 4th July 1997. Open bars"colloidal content in the upper 200 lm. Dark bars representcoarse core colloidal content.

station although all crests had higher colloidal carbohydrate content than the corre-sponding station troughs. The same relationships between stations and features werepresent in both the 2 and 0.2 mm slices, except that there were no signi"cantdi!erences between the crests at stations C and D for the 0.2 mm samples. There werevery highly signi"cant di!erences between stations for pooled analysis of Chl a in2 mm Cryolander slices (ANOVA F"52.79, DF"6, P(0.001) and of surface0.2 mm slices (ANOVA F"4.054, DF"6, P(0.001). More speci"cally (at theP(0.05 level using Tukey's pairwise comparisons), pooled (2 mm) Cryolander slicesshowed signi"cant di!erences at the P(0.05 level between 17 of 21 possible pairs.Most notably station A (higher) contained higher levels of Chl a than all other stationfeatures and station D trough had lower than all other stations. There were statist-ically signi"cant di!erences between crests and troughs at stations C and D. Similarrelationships between stations and features were found for the top 0.2 mm slice of theCryolander.

3.3. Depth proxles of Chl a and carbohydrate

Comparison of the depth pro"les by the two techniques highlighted the variation indiscrimination provided by the two methods. Depth pro"les of Chl a and colloidalcarbohydrate from the coarse resolution coarse core samples (Fig. 3) indicate that forstation A and the crests of stations B and C, Chl a concentration was highest at thesurface and decreased with depth. The greatest decrease being at station A, which had


Fig. 2. Time series of colloidal carbohydrate and Chl a content in exposed sediments from the Humberestuary taken by coarse coring at low tide on 4th July 1997.


Fig. 3. Depth pro"les of colloidal carbohydrate and Chl a content within sediment analysed by cores takenfrom four stations, during tidal exposure, in the Humber estuary on 4th July 1997. Resolution ofmeasurement on mm scale.

a higher initial Chl a content than the other station (Fig. 2) and showed an overalldecrease of 85%. At station D, Chl a concentrations were low (Fig. 1) and changedlittle with depth, in keeping with the low biomass found at this station.

Analysis of Cryolander samples gives superior depth resolution to the coarse corefor material within the surface 2 mm of sediment (Figs. 4 and 5 ). For stations A and B,Chl a and colloidal carbohydrate content were elevated in the upper 100 lmand declined with depth. At station C, there was a di!erence between the Chla content of the crest and troughs systems with the troughs showing a lowercontent and no increase near the surface. Station D showed a small, statist-ically insigni"cant, enhanced content at the surface and a small but consistentdi!erence between the troughs and the crests, the latter having a slightly higherChl a content (Fig. 5). The content of carbohydrate showed generally similarpatterns although the discrimination of crests and troughs was less marked(Fig. 5). Station D again showed little evidence of biological activity in the form ofelevated levels of colloidal carbohydrate while station A had by far the highest levels(Fig. 5).


Fig. 4. Depth pro"les of Chl a content within sediment analysed by Cryolander samples taken from fourstations, during tidal immersion, in the Humber estuary on 4th July 1997. Resolution of measurement onlm scale.

3.4. Comparison of coring techniques

Both the Cryolander and the coarse core results generally show a decrease incolloidal carbohydrate and Chl a content with sediment depth. There was a signi"cant


Fig. 5. Depth pro"les of colloidal carbohydrate content within sediment analysed by Cryolander samplestaken from four stations, during tidal immersion, in the Humber estuary, 4th July 1997. Resolution ofmeasurement on lm scale.

correlation (Spearman's Rank) between means of the 2 mm Cryolander slices andthe 5 mm coarse core surface samples for colloidal carbohydrate content(rs"0.786, n"7, P"0.036) and Chl a content (r"0.793, n"7, P"0.033). There


was also a signi"cant correlation between colloidal carbohydrate and Chl a contentusing both coring methods (r"0.637, n"21, P"0.002) and a stronger correlationbetween them when mean Cryolander results were used (r"0.959, n"14,P(0.0001). There was also a signi"cant correlation (Pearsons between each pairedrepetition) of coarse cored carbohydrate and Chl a contents (r"0.679,n"16, P(0.004). When paired replicates Cryolander samples of Chl a and carbohy-drate were analysed, a highly signi"cant correlation was obtained (r"0.892,n"21, P(0.001 for 2 mm slices and r"0.931, n"21, P(0.001 for 0.2 mm slices).

3.5. Water content

At all stations (except A which lacked bedforms) there was a signi"cant di!erencebetween the average water content of the crests and the troughs. The average watercontent of the crests was lower than the troughs for station B (61 and 89%, respective-ly: P(0.01) but higher for station C (57 and 51%, respectively: P(0.001). At stationD, the average water content of the crests (50%) was higher than the troughs (44%)(P"0.023).

3.6. Low-temperature scanning electron microscopy

The surface microfabric of the stations varied. The upper stations had moreevidence of surface colonisation by Bacillariophyceae (diatoms) (Figs. 6(A)}(D) and7(A)}(D)). However, the coverage was sparse and areas without apparent colonisationcould be found at most stations (Fig. 6(B)). Freeze fracture techniques revealed thatthe sediments of stations A and B were dominated by the mud fraction but includingoccasional silt/sand grains. In general, more silt/sand material and less microbialcolonisation was evident along the transect in a seawards direction (compareFigs. 6(E)}(G) and 7(E)}(G)). The surface of station D was devoid of live diatoms ineither the crest or trough feature (Fig. 7(A)}(D)). Fracture face analysis revealedsome di!erences in microstructure between the crests and the troughs. The crestsshowed micro-lamination in the sediment fabric with clear areas of "ner sedimentbetween layers of coarser grains (Fig. 7(E)). This was not found in the trough station(Fig. 7(F)).

3.7. CSM stability measurements

CSM devices were deployed at stations B, C and D, Ske%ing mud#ats. Fifty fourstability measurements were taken at station B over a single exposure period. Thecrests were signi"cantly more stable than the troughs (two sample t-test assumingunequal variances, P'0.001). There was a trend towards a decrease in the stability ofthe crests over the measurement period (Fig. 8), however spatial variation was highand this apparent trend was not signi"cant (linear regression r2"!0.50). There wasno signi"cant change in the stability of the troughs with time (linear regressionr2"0.23). Rain fell from 10 : 50 am (120 min after exposure) for about half an hour atstation B which coincided with low stabilities on both crests and troughs (see


Fig. 6. Low-temperature scanning electron micrographs of Humber sediments taken during the middle ofthe tidal exposure period. (A) Station A. Diatoms present in low density (bar marker 10 lm). (B) Crest ofstation B. No diatoms or other microphytobenthos visible at the surface (bar marker 100 lm). (C) Station A.Patchy areas of bio"lm coverage (bar marker 100 lm). (D) Station B. Fracture face form crest featureshowing diatom (Gyrosigma, sp.) within the sediment structure (bar marker 10 lm). (E}G) Fracture faces ofthe sur"cial sediments from stations A and B. (E) Station A. Open card-house structure, indicative of highwater content and porosity. (F) Station B, crest feature. Sediment fairly dense, less open structure thanstation A. (G) Station B, trough feature. Sediment structure more open than crest feature, more similar tostation A. (All bar markers 100 lm).


b

Fig. 7. Low-temperature scanning electron micrographs of Humber sediments taken during the middle ofthe tidal exposure period. (A) Station D, surface of a crest feature. No obvious biological colonisation (barmarker 100 lm). (B) Station D: Detail of crest. Evidence of some biological e!ects. Web structure of organicmaterial between grains (bar marker 10 lm). (C) Station D, trough features. Outline of sediment grainsvisible among organic debris (bar marker 100 lm). (D) Detail of surface of trough at station D. Fibrousorganic residue obvious on sediment surface (bar marker 10 lm). (E}G) Fracture faces of the sur"cialsediments from station D. (E) Low magni"cation of crest feature. Note obvious layering of sediment withalternate "ne and coarse laminations (bar marker 1 mm). (F) Station B, trough feature. Sediment fairlycoarse and no evidence of any layering (bar marker 600 lm). Detail of the surface region of a trough. Noevidence of biological colonisation and also no obvious organic content (bar marker 100 lm).

Section 4). There are also di!erences in the erosion pro"les of the crests and troughs(Fig. 9). The crests tended to have slightly shallower erosion pro"les, and the S

ivalues

(relative erosion rate) of the crests were signi"cantly lower than the troughs (twosample t-test assuming unequal variances, P"0.046). However, the erosion pro"les ofthe crests during the period of rain are not signi"cantly di!erent from those of thetroughs.

Forty six CSM runs were taken at station C over the exposure period (Fig. 8). Thecrests were signi"cantly more stable than the troughs (two sample t-test assumingunequal variances, P(0.002). There was no signi"cant change in the stability of thecrests with time (linear regression r2"0.18). There was a general decrease in thestability of the troughs over the measurement period (linear regression r2"!0.92).It began to rain heavily around 11 : 30 am (150 min after exposure) for about half anhour, which coincided with the lowest recorded stabilities on the crests. There are alsodi!erences in the erosion pro"les of the crests and troughs (Fig. 10). The crests tend tohave shallower erosion pro"les, and the S

ivalues of the crests were signi"cantly lower

than the troughs (two sample t-test assuming unequal variances, P"0.039). However,the erosion pro"les of the crests during the period of rain were not signi"cantlydi!erent from those of the troughs.

Seventeen CSM runs were taken at station D at the beginning of the exposureperiod (Fig. 8). The crests were signi"cantly more stable than the troughs (two samplet-test assuming unequal variances, P(0.001). There were only slight di!erences in theerosion pro"les of the crests and troughs (Fig. 11). The crests had shallower erosionpro"les, and lower S

ivalues than the troughs.

In summary, critical erosion threshold increased towards the shore whilst suspensionindex (erosion rate) decreased, and crests were generally more stable than troughs.

3.8. Correlation of the measured sediment parameters with sediment stability

The data were pooled for the entire exposure period and correlated (Pearsonproduct moment) to determine which sediment properties and organisms had thestrongest co-variation with sediment stability (Table 2). A signi"cant correlation wasaccepted when P"0.05. A correlation matrix comprising 15 variables was produced,including six prominent species or groups of macrofauna: Oligochatea, Macomabalthica, Streblospio sp., Nephtys sp., Nereis diversicolor, Retusa sp. and a standard suite


Fig. 8. Cohesive strength meter (CSM) derived erosion thresholds over the immersion period of theintertidal #ats on the Humber estuary. Data represent average erosion thresholds for measurement taken ateach time given as minutes after tidal exposure (n"6).

of sediment properties (Table 2). Cryolander Chl a was signi"cantly correlated withthe erosion threshold. While the next strongest, but not signi"cant relationship toerosion threshold, was a negative relationship (!0.77) with erosion rate (S

i). This

suggests that as the surface of the sediment becomes more resistant to erosion, theerosion rate was also in#uenced. Coarse core measurements and Cryolander colloidalcarbohydrate, cryolander Chl a, and Macoma balthica biomass were signi"cantlycorrelated with S

ivalues. None of the sedimentological features (water content, grain

size, or sediment fractions below 16 or 63 lm) were correlated with either erosionthreshold or erosion rate.


Fig. 9. Average erosion pro"les derived by cohesive strength meter analysis of crests and troughs at stationB on the Humber estuary. The erosion pro"les of the crests are shallower than those from the troughs,except during and immediately after a period of rain (top right) when they were very similar. The pro"lesdiverged in structure as the sediments dried (top right to bottom left) (n"6).

4. Discussion

4.1. Correlation with erosion threshold

Only cryolander Chl a was signi"cantly correlated with the erosion threshold(Table 2). This suggests that the photosynthetic biomass (diatoms) was signi"cant incontrolling sediment stability. From the literature (Holland et al., 1974; Yallop et al.,1994; Paterson, 1997), the secretion of EPS would be the expected mechanism for thise!ect. The total EPS content of the sediments was not measured but only the colloidalform that has been shown to represent diatom biomass most strongly (Underwoodand Smith, 1998; Taylor et al., 1999). However, the cryolander colloidal carbohydratewas not signi"cantly correlated with stability (r"0.7, n"6). This was unexpected.The Chl a concentration re#ects the photosynthetic biomass, and hence the potentialof the system to produce EPS. However, Chl a is relatively immutable compared tocolloidal carbohydrate and therefore likely to be less variable temporally. The troughsoften contained surface water, whilst the crests did not. Removal of colloidal carbohy-drate by dissolution into this surface water may explain why the troughs had lowercolloidal carbohydrate concentrations than the crests. The Chl a concentration wouldnot be a!ected by surface water as it is intra-cellular. Colloidal EPS is also the major


Fig. 10. Average erosion pro"les derived by cohesive strength meter analysis of crests and troughs atstation C on the Humber estuary. The erosion pro"les of the crests are shallower than those from thetroughs, except during and immediately after a period of rain (data set 2, top right) when they were verysimilar. The pro"les diverged in structure as the sediments dried (bottom, data sets 3 and 4) (n"6).

Fig. 11. Average erosion pro"les derived by cohesive strength meter analysis of crests and troughs atstation D on the Humber estuary. Fewer data were gathered for station D but little difference was discernedbetween crests and troughs (n"6).


stabilising mechanism reported for diatoms, but not for cyanobacterial and other"lamentous forms (Paterson, 1995; Yallop et al., 1993). It seems in the Humber thatthe presence of Chl a covaries with stabilising activity, but that this is not alwaysdirectly related to colloidal carbohydrate content.

The erosion threshold is determined by all sediment properties. Field observationssuggested that there was more loose surface #occulent material in the troughs, whichwould have a low erosion threshold. Chl a concentration (diatom biomass) anderosion threshold were higher on the crests than troughs. Both colloidal carbohydrateand Chl a calculated from the coarse cores were not correlated with erosion threshold.This is not surprising, as diatoms are rarely found deeper than 2 mm so measuringtheir biochemical indicators on a 5 mm scale results in dilution. If the upper 2 mm ofsediment is very stable then the sediment will not erode, regardless of the sedimentbelow. The coarse core method does not provide the necessary resolution to deter-mine the in#uence of the surface sediment properties on stability. However, the fabricof the sediment below the surface is important in determining the erosion rate.

4.2. Correlation with erosion rate (Sivalues)

The Si

values give a relative measure of the erosion rate for the CSM device(Tolhurst et al., 1999). The S

ivalues were signi"cantly negatively correlated with

Cryolander colloidal carbohydrate and Chl a, indicating that colloidal carbohydrateis the likely controlling in#uence on erosion rate (Tolhurst, 1999). Coarse coremeasurements of colloidal carbohydrate were signi"cantly correlated with the S

ivalues

suggesting that the properties of the sediment with depth were important in determiningthe erosion rate. Coarse core measurements of Chl a were not correlated with theSivalues. M. balthica biomass was signi"cantly negatively correlated with the S

ivalues

suggesting that M. balthica may stabilise the sediment. The role of M. balthica may becomplex since it feeds at the surface but deposits organic-rich faeces and pseudo-faecesand has been noted to reduce the erosion threshold (Widdows et al., 1998).

4.3. Correlation with water content

Water content was signi"cantly negatively correlated with median grain size andsigni"cantly positively correlated with the grain size fraction below 16 and 63 lm.Thus, as the grain size decreases the water content increases. Cryolander Chl a andcolloidal carbohydrate were signi"cantly positively correlated with water content.Diatoms distribution has been shown to in#uence sediment grain size (Underwoodand Paterson 1993; Kornman and de Deckere, 1998), so diatoms may have an indirectin#uence on water content via altering grain size distributions. Coarse core, Chl a andcolloidal carbohydrate were not correlated with water content. M. balthica and N.diversicolor biomass were signi"cantly correlated with water content. Porosity in-creased shoreward, probably due to decreasing grain size and increasing density ofmacrobenthos. It is possible that either these organisms prefer "ne sediment (andhence are correlated positively with water content) or that bioturbation increases theporosity and thus the water content of the sediment.


Table 2Correlation of sediment properties measured during the INTRMUD Humber Estuary "eld campaign

!Bold "gures indicate signi"cant correlation at P"0.05.

4.4. Correlation with grain size

The grain size data are discussed as a whole as the proportion of one grain sizerange is coupled to another. Correlations indicated below are all statistically signi"-cant. The median grain size was negatively correlated with Cryolander colloidalcarbohydrate. The (16 and (63 lm fractions were positively correlated withCryolander colloidal carbohydrate and the (16 fraction was positively correlatedwith Chl a concentration. This is partly because diatoms tend to be found in"ner-grained sediments, but the possibility that EPS helps to stabilise the sediment,trapping "ne-grained particles cannot be overlooked (Kornman and de Deckere, 1998).

The biomass of M. balthica and N. diversicolor macrobenthos were negativelycorrelated with the median grain size. The biomass of M. balthica and N. diversicolormacrobenthos were positively correlated with the (16 lm and the (63 lmgrain size fraction. These species tend to be found in areas with a high propor-tion of "ne sediment. Correlation with density (numbers per m2) of the di!erentspecies showed a similar pattern as the biomass, but the correlation was lesssigni"cant.

4.5. Correlation with colloidal carbohydrate and Chl a concentration

As Chl a concentration is a measure of diatom biomass, and diatom biomass largelydetermines colloidal carbohydrate concentration, these two sediment properties areconsidered together. It should be noted that Chl a at depths below 2 mm can be


classed as photosynthetically inactive biomass. The coarse cores were analysed toa depth of 5 cm, however only the "rst section (0}5 mm) was included in thecorrelation. Coarse core colloidal carbohydrate concentration was positively corre-lated with coarse core Chl a concentration. There was no correlation with othersediment properties including Cryolander colloidal carbohydrate and Chl a concen-tration, showing that the scale on which sediment properties are measured signi"-cantly a!ects the data obtained.

Chl a content was highest in the surface 200 lm and decreased rapidly with depth.Meso-scale analysis of Chl a reveals a similar trend. Micro-scale analysis of colloidalcarbohydrate shows that it remains relatively constant with depth with the exceptionof station A. In the upper 2 mm, Chl a and colloidal carbohydrate are closely related,however, below this depth the relationship becomes decoupled. Cryolander Chl a hasa stronger relationship to colloidal carbohydrate than coarse core Chl a.

4.6. The inyuence of rain

The high water content in the troughs was probably responsible for the lowerconcentration of colloidal carbohydrate. The period of rain was accompanied bylower erosion thresholds on the crests, but not in the troughs. There was no apparentchange in the colloidal carbohydrate concentration on the crests, so this reduction instability was not due to removal of colloidal carbohydrate. It is unclear why thestability decreased; one possibility is that the surface water increases the water contentof the sediment and may also change the hydration state and binding capacity of thecolloidal carbohydrate. After the rain, the stability of the crests recovered rapidly,presumably due to draining and drying. The S

ivalues also show a similar change, the


erosion pro"les of the crests at the time of the rain were very similar to those of thetroughs, and after the rain return to their previous shape. This suggests that the rainhas a short-lived e!ect on sediment stability and that the erosion rate was signi"cantlycontrolled by the erosion threshold. The e!ect of fresh water may re#ect the ionicbalance of the medium and it is unclear at present if a similar reduction in stabilityoccurs during tidal immersion with salt water.

4.7. Sediment stability

The diatom biomass in the top 2 mm seems to be a major control on sedimentstability. Diatom bio"lms are found on the sediment surface and their in#uence onsediment properties will decrease with depth. A bio"lm acts as a `skina on thesediment surface, no erosion can occur until this skin is broken. Visual observation oferosion in both the "eld and laboratory shows that erosion often starts around areasof weakness in the bio"lm. Once the bio"lm has been eroded the sediment below isrevealed and its erosion will be increasingly determined by physical factors. Thisexplains why meso-scale analysis of colloidal carbohydrate and Chl a is a poorpredictor of erosion threshold, but a better predictor of the S

ivalue (erosion rate). The

coarse core is measuring the sediment properties in the top 5 mm that will partlydetermine the erosion rate but not the erosion threshold. The surface is still importantin determining the erosion rate, as the rate at which the surface erodes determines therate at which the rest of the sediment can erode. This explains why the micro-scaleanalysis also correlates with the erosion rate, as the stability of the sediment surface isimportant in determining the erosion rate. It is suggested that measurements ofsediment properties for comparison to sediment stability should be made on thesurface 2 mm of the sediment. Measurements below 2 mm provide information ofrelevance to the erosion rate.

5. Conclusion

This study has shown that the temporal and spatial variability of sediment proper-ties (stability, erosion rate) on a macro- and micro-scale are clearly signi"cant. Thisvariation is related to sediment bedform and is in#uenced by the organisms inhabitingthe bed. In addition, there is a clear temporal signal that can be a!ected by localclimatic conditions (e.g. rain). Rain reduces sediment stability and therefore thevariation between bedform features. However, this e!ect was short-lived and recoverywas rapid. The scale of measurement was shown to be important. Biological variablesmust be measured on a relevant scale otherwise false inference can be made in terms ofthe importance of biological products (e.g. EPS). This understanding may lead toa better interpretation and modelling of the interactions of the biological andsedimentological variables controlling sediment dynamics. The consequent variabilityin the material supplied to the water column may have a signi"cant ecologicalin#uence in terms of food supply and benthic productivity.


Acknowledgements

This work was funded by the European Commission Mast 3 programme under the`INTRMUDa award (MAS3-CT95-0022). The participation of T.J. Tolhurst wasunder the auspices of a studentship award from the University of St Andrews. Thework of Prof. K. Dyer in co-ordination of the INTRMUD project was muchappreciated and the valuable comments of Dr. C. Amos and two anonymous re-viewers on the manuscript.

References

Black, K.S., Paterson, D.M., 1997. Measurement of the erosion potential of cohesive marine sediments:a review of current in situ technology. Journal of Marine Environmental Engineering 26, 43}83.

Black, K.S., Paterson, D.M., 1998a. LISP-UK Littoral Investigation of sediment properties. An introduc-tion. In: Black, K.S., Paterson, D.M., Cramp, A. (Eds.), Sedimentary Processes in the Intertidal Zone,Special publication, Vol. 139. Geological Society, London, pp. 1}10.

Black, K.S., Paterson, D.M., Cramp, A., 1998b. Sedimentary Processes in the Intertidal Zone. GeologicalSociety, London, p. 409.

Black, D.M., Paterson, D.M., Davidson, I.D., 1999. Sediment micro-fabric of oil rig drill spoil heaps:preliminary observations using low-temperature scanning electron microscopy. Environmental Scienceand Technology 33, 1938}1990.

Burt, N., Parker, R., Watts, J. (Eds.), 1997. Cohesive Sediments. Wiley, UK, p. 458.Davison, W., Fones, G.R., Grime, G.W., 1997. Dissolved metals in surface sediment and a microbial mat at

100 lm resolution. Nature 387 (6636), 885}888.DeH farge, C., 1997. Cryoscanning electron microscopy and high resolution scanning electron microscopy of

organic matter and organomineral associations in modern microbial sediments. Geomaterials, Petrol-ogy and Sedimentology 324 (2a), 553}561.

Dubois, M., Gilles, K.A., Hamilton, J.K., Rebers, P.A., Smith, F., 1956. Colorimetric method for determina-tion of sugars and related substances. Analytical Chemistry 28, 350}356.

Dyer, K.R., 1998. The typology of intertidal mud#ats. In: Black, K.S., Paterson, D.M., Cramp, A. (Eds.),Sedimentary Processes in the Intertidal Zone, Special publication, Vol. 139. Geological Society,London, pp. 11}24.

Hansbo, S., 1957. A new approach to the determination of the shear strength of clay by the fall-cone test.Proceedings of the Royal Swedish. Geotechnology Institute, Vol. 14, 1}49.

Harper, M.P., Davison, W., Tych, W., 1997. Temporal, spatial and resolution constraints for porewatersampling devices using di!usional equilibration: dialysis and DET. Environmental Science and Techno-logy 31, 3110}3119.

Holland, A.F., Zingmark, R.G., Dean, J.M., 1974. Quantitative evidence concerning the stabilisation ofsediments by marine benthic diatoms. Marine Biology 27, 191}196.

Kornman, B.A., De Deckere, E.M.G.T., 1998. Temporal variation in sediment erodibility and suspendedsediment dynamics in the Dollard Estuary. In: Black, K.S., Paterson, D.M., Cramp, A. (Eds.), Sedimen-tary Processes in the Intertidal Zone, Special publication, Vol. 139. Geological Society, London,pp. 231}241.

Kravitz, J.M., 1970. Repeatability of three instruments used to examine the undrained shear strength ofextremely weak saturated cohesive sediments. Journal of Sedimentary Petrology 40, 1026}1037.

Paterson, D.M., 1989. Short-term changes in the erodibility of intertidal cohesive sediments related to themigratory behaviour of epipelic diatoms. Limnology and Oceanography 34 (1), 223}234.

Paterson, D.M., 1995. Biogenic structure of early sediment fabric visualized by low-temperature scanningelectron microscopy. Journal of Geological Society, London 152, 131}140.

Paterson, D.M., 1997. Biological mediation of sediment erodibility, ecology and physical dynamics. In:Burt, N., Parker, R., Watts, J. (Eds.), Cohesive Sediments. Wiley, New York, pp. 215}229.


Paterson, D.M., Yates, M.G., Wiltshire, K.H., McGrorty, S., Miles, A., Eastwood, J.E.A., Blackburn, J.,Davidson, I., 1998. Microbiological mediation of spectral re#ectance from intertidal cohesive sediments.Limnology and Oceanography 43 (6), 1207}1221.

Reise, K., 1985. Tidal #at ecology: an experimental approach to species interactions. Springer, Berlin.Sutherland, T.F., Grant, J., Amos, C.L., 1998. The e!ect of carbohydrate production by the diatom

Nitzschia curvilineata on the erodibility of sediment. Limnology and Oceanography 43 (1), 65}72.Taylor, I., Paterson, D.M., 1998. Microspatial variation in carbohydrate concentrations with depth

in the upper millimetres of intertidal cohesive sediments. Estuarine Coastal and Shelf Science 46,359}370.

Taylor, I., Paterson, D.M., Mehlert, A., 1999. The quantitative variability and monosaccharide compositionof sediment carbohydrates associated with intertidal diatom assemblages. Biogeochemistry 45 (3),303}327.

Thrush, S.F., 1991. Spatial patterns in soft-bottom communities. Trends in Ecology and Evolution 6, 75}79.Tolhurst, T.J., 1999. Microbial mediation of intertidal sediment stability. Ph.D. Thesis, University of

St Andrews, UK.Tolhurst, T.J., Black, K.S., Shayler, S.A., Mather, S., Black, I., Baker, K., Paterson, D.M., 1999. Measuring

the in situ erosion shear stress of intertidal sediments with the Cohesive Strength Meter (CSM).Estuarine Coastal and Shelf Science 49, 281}294.

Tolhurst, T.J., Black, K.S., Paterson, D.M., Mitchener, H.J., Termaat, G.R., Shayler, S.A., 2000. A compari-son and measurement standardisation of four in situ devices for determining the erosion shear stress ofintertidal sediments. Continental Shelf Research 20 (10/11), 1397}1418.

Underwood, G.J.C., Paterson, D.M., Parkes, R.J., 1995. The measurement of microbial carbohydrateexoploymers from intertidal sediments. Limnology and Oceanography 40 (7), 1243}1253.

Underwood, G.J.C., Smith, D.J., 1998. Predicting epipelic diatom exopolymer concentrations in intertidalsediments from sediment Chl a. Microbial Ecology 35, 116}125.

Wheatcroft, R.A., Butman, C.A., 1997. Spatial and temporal variability in aggregated grain-size distribu-tions, with implications for sediment dynamics. Continental Shelf Research 17 (4), 367}390.

Widdows, J., Brinsley, M., Elliott, M., 1998. Use of an in situ #ume to quantify particle #ux (biodepositionrates and sediment erosion) for an intertidal mud#at in relation to changes in current velocity andbenthic macrofauna. In: Black, K.S., Paterson, D.M., Cramp, A. (Eds.), Sedimentary Processes in theIntertidal Zone, Special Publication, Vol. 139. Geological Society, London, pp. 85}97.

Wiltshire, K.H., Blackburn, J., Paterson, D.M., 1997. The Cryo-lander: a new method for in situ samplingof unconsolidated sediments minimising the distortion of sediment fabric. Journal of SedimentaryResearch 67 (5), 980}984.

Wiltshire, K.H., Schroeder, F., 1994. Pigment patterns in suspended matter from the Elbe Estuary.Northern Germany. Netherlands Journal of Aquatic Ecology 28 (3-4), 255}265.

Yallop, M.L., de Winder, B., Paterson, D.M., Stal, L.J., 1994. Comparative structure, primary productionand biogenic stabilization of cohesive and non-cohesive marine sediments inhabited by microphyto-benthos. Estuarine, Coastal Shelf Science 39, 565}582.


variations in sediment properties, skeffling mudflat, humber estuary, uk

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