Estuaries
Vol. 26, No. 4A, p. 885-893
August 2003
Relating Spatial and Temporal Variability in Sediment Chlorophyll a and Carbohydrate Distribution with Erodibility of a Tidal Flat C A T H Y H . L U C A S ~'*, J O H N W I D D O W S 2, a n d
LAURI W A L L 2
1 School of Ocean and Earth Science, University of Southampton, Southampton Oceanography Centre, European Way, Southampton, SOt 4 3ZH, United Kingdom Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth, PL1 3191-1, United Kingdom ABSTRACT: T r e n d s in the spatial distribution o f chlorophyll a (chl a) and colloidal and total carbohydrates on the Molenplaat tidal flat in the Westerschelde estuary~ Netherlands, reflected s p a d a l d i f f e r e n c e s in physical p r o p e r t i e s o f the s e d i m e n t . Results froni a S p e a r m a n Rank Order Correlation indicated that m a n y o f the physical a n d biological m e a s u r e s covaried. Multiple regression analyses describing the relationship b e t w e e n colloidal carbohydrates a n d s e d i m e n t properties r e s u l t e d in several highly significant equations, although in all cases chl a was able to predict colloidal carbohydrate content. Relationships between sediment surface chl a and colloidal carbohydrate, a n d s e d i m e n t erodibility (i.e. critical erosion threshold, Ucat~ and mass of s e d i m e n t e r o d e d at a velocity of 30 cm s -~) determined in annular flume experiments were examined. Overall s e d i m e n t erodibility was lowest (i.e. high thresholds~ low mass eroded) for the siltiest sediments in June 1996 w h e n chl a and colloidal carbohydrates were high (56.9 Ixg gDW 1 and 320.6 Ixg gluc.equ, gDW i respectively), and greatest (i.e., low thresholds~ high m a s s eroded) at the sandier s e d i m e n t s in September 1996, when chl a a n d colloidal carbohydrates were low (1.0 I~g gDW i and 5.7 I~g gluc.equ, gDW 1, respectively). When sediments were g r o u p e d according to relative silt content~ the nrost significant relationships were f o u n d in m u d d y s a n d with a fineg r a i n e d fraction ( < 63 p,m) o f 25-50%. T h r e s h o l d s of erosion increased~ while m a s s of s e d i m e n t e r o d e d decreased, with increasing chl a and colloidal carbohydrate. A similar t r e n d was o b s e r v e d for the s a n d - m u d d y s a n d (63 I~nl 1025%). In the s a n d (63 I~nl 0-10%)~ there w e r e no relationships f o r Ucav whereas m a s s e r o d e d appeared to increase with increasing chl a and colloidal carbohydrate. T h e increased carbohydrate may stick sand grains together, altering the nature o f erosion f r o m rolling grains to c l u m p s o f r e s u s p e n s i o n .
complex matrices in the surficial layers (e.g., Yallop et al. 1994). U n d e r w o o d et al. (1995) and Und e r w o o d and Smith (1998) f o u n d that EPS accounted for 20-25% of the extracellular water-soluble (colloidal) carbohydrates produced from photoassimilated c a r b o n in benthic diatoms, but its p r o d u c t i o n d e p e n d s on n u t r i e n t availability and growth phase (Buzzelli et al. 1997; Goto et al. 1999; Staats et al. 2000b) as well as tidal and photosynthetic rhythms (Smith and U n d e r w o o d 1998). Benthic diatom chlorophyll a (chl a) and colloidal carbohydrate concentrations have been f o u n d to be closely correlated by U n d e r w o o d et al. (1995), Sutherland et al. (1998b), and U n d e r w o o d and Smith (1998). Significant correlations between erosion threshold and chl a and EPS (measured as colloidal carbohydrates) have already b e e n reported, particularly in cohesive sediments (Yallop et al. 1994; Sutherland et al. 1998a,b; Austen et al. 1999; Riethm/511er et al. 1999, 2000). T h e extent to which they alter the stability of sediments depends on a variety of physical and biological factors. Increased aerial exposure increases sediment stability. T h e sediment dewaters and the surface matrix becomes dehydrated, resulting in the sediment b e c o m i n g m o r e tightly b o u n d (Paterson et al. 1990; Yallop et
Introduction
T h e erosion characteristics of natural estuarine sediments have i m p o r t a n t implications for coastal engineering, pollutant exchange, habitat stability, and material and carbon fluxes. T h e r e c e n t develo p m e n t of in situ techniques for measuring the erodibility of natural sediments has revealed that temporal and spatial variations occur in s e d i m e n t erosion (Amos et al. 1999; U n d e r w o o d and Paterson 1993; S u t h e r l a n d et al. 1998a; Widdows et al. 1998a; Paterson et al. 9000). U n d e r s t a n d i n g the causes of these variations has b e e n the subject of i n c r e a s i n g m u l t i d i s c i p l i n a r y r e s e a r c h in r e c e n t years. Biota play an i m p o r t a n t role in modifying sedim e n t erosion, acting b o t h to stabilize and destabilize sediments (Jumars and Nowell 1984; G r a n t and D a b o r n 1994; Yallop et al. 1994; Widdows et al. 1998c). Microphytobenthos, particularly benthic diatoms inhabiting cohesive and non-cohesive sediments, increase sediment stability t h r o u g h the p r o d u c t i o n of extracellular polymeric substances (EPS) that act to bind sediment grains and form * C o r r e s p o n d i n g author; tele: + 4 4 (0)23 80596270; fax: + 4 4 (0)23 80593059; e-mail:
[email protected]. 9 2003 Estuarine Research Federation
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al. 1994). This p a p e r aims to explore the relationships between local abiotic characteristics a n d mic r o p h y t o b e n t h o s chl a and colloidal carbohydrate, as well as how spatial and t e m p o r a l variability in chl a a n d colloidal carbohydrates affect s e d i m e n t erodibility (threshold and mass e r o d e d ) on tidal fiats.
g A
0 0 0 0 0 0 0 0 0 0
Materials a n d M e t h o d s
STUDY ARFA T h e M o l e n p l a a t tidal fiat, located in the midregion of the Westerschelde estuary (SW N e t h e r lands) was chosen as the m a i n study area. Five sampiing sites were selected, each o n e characterized by different s e d i m e n t types (Table 1) and associated m a c r o f a u n a c o m m u n i t i e s . Site 2, located in the m o r e central region of the fiat, e x p e r i e n c e s s u m m e r silt accumulation. Sites 4 a n d 5 were sandier, particularly site 5, which is located in a high energy region of the Molenplaat. Sites 1 and were i n t e r m e d i a t e in nature. Siltier sites were characterized by suspension-feeding m a c r o f a u n a , in particular Cerastoderraa edute a n d Mya arenaria, while sandier sites were d o m i n a t e d by p o p u l a t i o n s of deposit feeders. In J u n e 1998, a validation c a m p a i g n took place n e a r the H o n d - P a a p tidal fiat in the Ems-Dollard estuary (Molenplaat: 51~ 3~ Hond: 53~ 6~ to test h y p o t h e s e s relating to predictions of s e d i m e n t erodibility. T h e H o n d tidal fiat has a similar r a n g e of s e d i m e n t characteristics to those of the M o l e n p l a a t (Table 2), and 5 contrasting sites (A-E) were chosen accordingly. Only data relating s e d i m e n t chl a and carbohydrates with erodibility will be p r e s e n t e d in this paper. SAMPLINC All 5 sites on the M o l e n p l a a t were s a m p l e d on m o r e than one occasion during the m a i n field c a m p a i g n s in J u n e and S e p t e m b e r 1996 and 1997. Sites A - E on the H o n d were sampled in J u n e 1998. At a p p r o x i m a t e l y midway t h r o u g h the low water e m e r s i o n period, between 4 a n d 6 s e d i m e n t cores for chl a and colloidal c a r b o h y d r a t e d e t e r m i n a tions were collected using 2.5 cm d i a m e t e r cut-off syringes. T h e cores were sectioned on-site at 0-2, 2-4, 4-6, 6-10, 10-15, a n d 15-20 m m d e p t h intervals, each of which was analyzed individually. T h e sections were freeze-dried in the dark for 24-72 h (until dry) a n d stored at - 7 0 ~ for up to g m o p r i o r to analysis. At the same time as the s e d i m e n t coring, surface scrapings (1-2 ram) of the sedim e n t were collected for water and organic c o n t e n t using a m e t a l spatula a n d placed into pre-weighed vials. Water a n d total organic (ash free dry weight [AFDW]) contents of the surficial s e d i m e n t were
g
E
~b
g V 9
o
~q
MhM~hhm@M
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Carbohydrates end Sediment Erodibility
TABLE 2.
S u m m a r y o f the m e a n sedimentological a n d b i o c h e m i c a l p r o p e r t i e s of the H o n d tidal flat in Property
% < 6 8 p,m (silt) % 63-125 pan (very fine sand) % 125-250 p,m (fine sand) % 250-500 ~rn ( m e d i u m sand) % > 500 p,m % water % AFDW Chlorophyll a (~g-gDW 1) Colloidal Caxbo. (p,g gluc.equ, gDW < )
June
1998.
Site A
Site ]~
SiVaC
Site D
SiVaE
16.65 69.15 18.15 0.80 0.25 40.40 1.60 48.74 126.54
8.05 72.15 17.85 0.30 0.65 32.80 1.30 11.44 71.68
1.20 38.20 54.40 6.05 0.15 25.90 0.45 9.50
1.05 38.25 57.95 2.65 0.10 23.70 0.50 13.24 16.79
nd nd nd nd nd 22.82 0.50 3.22 10.12
d e t e r m i n e d after oven d r y i n g at 60~ for g - 4 d, followed by loss o n i g n i t i o n at 500~ for 24 h. As p a r t of a p r o g r a m e x a m i n i n g algal p i g m e n t diversity, m i c r o p h y t o b e n t h o s chl a w a s m e a s u r e d in freeze-dried s e d i m e n t s by reverse-phase H P L C . S a m p l e s were extracted in 90% a c e t o n e , ultra-sonicated for 80 s, a n d c e n t r i f u g e d at 8,000 r p m (2,000 X g) for 15 rain to r e m o v e the particles f r o m suspension. T h e H P L C m e t h o d , d e s c r i b e d fully in L u c a s a n d H o l l i g a n (1999), is a modification f r o m M a n t o u r a a n d Llewellyn (1983). T h e resuits are expressed as b~g chl a g D W -1 o f s e d i m e n t . Colloidal c a r b o h y d r a t e , used as an i n d e x o f EPS, was m e a s u r e d in freeze-dried s e d i m e n t s using a m o d i f i c a t i o n o f the p h e n o l - s u l p h u r i c acid m e t h o d of D u b o i s et al. (1956) d e s c r i b e d by U n d e r w o o d et al. (1995). g ml of 25%o saline was a d d e d to 0.5 g (+ 0.1 g) o f freeze-dried s e d i m e n t . T h e resulting s o l u t i o n was m i x e d o n a r o t a m i x e r for 10 s, t h e n c e n t r i f u g e d at 3,500 r p m (2,700 X g) f o r 15 min. 0.4 ml o f 5% w / v p h e n o l a n d 2 ml A n a l a r G r a d e c o n c e n t r a t e d H~SO4 were a d d e d to 1 ml o f the sup e r n a t a n t . O n c e c o o l e d , a b s o r b e n c y was m e a s u r e d at 490 n m a n d D-Glucose was u s e d as the s t a n d a r d . Results are e x p r e s s e d as b~g g l u c o s e equivalents (gluc.equ.) g D W -1 of s e d i m e n t . S e d i m e n t erodibility was m e a s u r e d o n n a t u r a l s e d i m e n t s using a p o r t a b l e a n n u l a r flume, described by W i d d o w s et al. (1998b). S u s p e n d e d particulate m a t t e r (SPM) c o n c e n t r a t i o n (rag 1-1), mass of s e d i m e n t e r o d e d (g m e), a n d s e d i m e n t e r o s i o n rate (rag m e s 1) were m e a s u r e d in res p o n s e to a stepwise increase in c u r r e n t velocity (GV) f r o m 1 0 - 5 0 cm s -1, in 5 cm s -1 i n c r e m e n t s , each lasting 15 rain. D u p l i c a t e r u n s were c a r r i e d o u t for e a c h site, with g o o d r e p l i c a t i o n of results (Widdows et al. 9000). T h e free-stream c u r r e n t v e locities were relatively c o n s t a n t d o w n to within 1 2 c m of the bed, a n d were s t a n d a r d i z e d in the f l u m e a n d field to 10 cm (Widdows et al. 9000). Critical e r o s i o n velocity (Um~) was d e t e r m i n e d as the c u r r e n t velocity r e q u i r e d to increase SPM a b o v e a t h r e s h o l d of 100 m g 1 1 T h e r e l a t i o n s h i p b e t w e e n m e a s u r e d fiee-stream c u r r e n t velocity
25.86
(i.e., 10 cm a b o v e the bed) a n d calculated shear stress (based o n log vertical profile 1 c m a b o v e the b e d w h e n s m o o t h t u r b u l e n t flow over f i n e - g r a i n e d cohesive m u d ) in the f l u m e is d e s c r i b e d by the following e q u a t i o n (see W i d d o w s et al. 9000 for details): S h e a r Stress (Pa) - 0 . 0 0 0 8 U ~
0 . 0 0 0 6 U + 0.0052
w h e r e U - m e a n c u r r e n t velocity ( c m s -1) a n d r 2 - 0.99. Results SEDIMENT BIOGEOCHEMISTRY
T r e n d s in the spatial d i s t r i b u t i o n o f chl a a n d colloidal c a r b o h y d r a t e s o n the M o l e n p l a a t reflected d i f f e r e n c e s in the physical p r o p e r t i e s of the sedi m e n t at e a c h site (Fig. 1). In J u n e a n d S e p t e m b e r h i g h e s t m e a n values o f chl a a n d colloidal carboh y d r a t e s were m e a s u r e d at the siltiest site (2), while the lowest values o b s e r v e d at the h i g h l y dyn a m i c s a n d y site 5. I n J u n e , each site was significantly different f r o m the o t h e r in t e r m s of colloidal c a r b o h y d r a t e (Kruskal-Wallis 1-way analysis of v a r i a n c e (ANOVA): H 4 - 10.4, p < 0.05; p o s t h o c D u n n ' s Test: p < 0.05) a n d chl a ( H 4 - 14.2, p < 0.01) contents. By S e p t e m b e r , chl a a n d colloidal c a r b o h y d r a t e c o n c e n t r a t i o n s h a d d e c l i n e d across the M o l e n p l a a t . This t e m p o r a l variability was especially m a r k e d at the siltier sites, especially site 2. I n S e p t e m b e r , t h e r e were n o significant site differences in colloidal c a r b o h y d r a t e s , a l t h o u g h chl a still displayed significant site d i f f e r e n c e s (p
0 . 1 0 ) ; l o g ( m a s s e r o d e d + 1) = 4 . 1 8 9 - 0 . 8 2 9 l o g ( c o l l , c a r b o . + 1), r 2 = 5 9 . 1 % , p < 0.02. D a t a p o i n t s a r e m e a n s + SD.
was c h o s e n , as in the majority of cases, Umt has a l r e a d y b e e n attained. T h e data s h o w n h i g h l i g h t the spatial variability in m i c r o p h y t o b e n t h o s biomass. S e d i m e n t erodibility was lowest (i.e., h i g h thresholds, low mass e r o d e d ) at the siltier sites in J u n e w h e n chl a a n d colloidal c a r b o h y d r a t e s were high, a n d greatest (i.e., low thresholds, h i g h mass e r o d e d ) at the s a n d i e r sites in S e p t e m b e r w h e n chl a a n d colloidal c a r b o h y d r a t e s were low (p < 0.001).
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10000 0.10; colloidal carbohydrate: r e 8 9 . 1 % , p < 0.02).
Because of the relatively wide range of s e d i m e n t types e x a m i n e d during the f l u m e experiments, U ~ t and mass of s e d i m e n t e r o d e d versus chl a and colloidal carbohydrate relationships were explored on a sediment-type basis. T h e data were divided into 3 subgroups according to Riethmflller et al. (1998), based on the relative a m o u n t of finegrained (6S > m ) fraction: 0-10% (sand), 1 0 - 2 5 % (sand-muddy sand), and 25-50% ( m u d d y s a n d ) . Linear regressions are presented for Um~ (Fig. 5) and mass of s e d i m e n t eroded (Fig. 6). T h e m o s t significant relationships b e t w e e n sed-
"~-- 9
ID0
bohydrates, multiple regression analyses relating Umt with variables associated with m i c r o p h y t o b e n t h o s - i n f l u e n c e d stabilization ( c o l l o i d a l carbohydrate, chl a, and average low water e m e r s i o n period, w h i c h i n f l u e n c e s s e d i m e n t s t a b i l i z a t i o n t h r o u g h dehydration of EPS), resulted in the following equations: U~,t - 20.6 + 0.025(colloidal carbohydrate,
pog g l u c . e q u . g D W - 0 . 4 0 S ( e m e r s i o n h)
2)
Carbohydrates and Sediment Erodibility
where F - 8.44,
p < 0.0045,
r ~ - 50%;
and Umt-
22.6
0.801(chl a, p,g.gDW -1)
+ 0.084(colloidal carbohydrate, p,g gluc. equ. gDDr"1) - 0.902(emersion h) where F - 6.87,
p < 0.0079,
r ~ - 52%
Although values of Umt were highly correlated with colloidal carbohydrate, chl a, and emersion period when all the data were combined, the significance of these relationships broke down when J u n e and S e p t e m b e r data were analyzed separately. In June, U~,~t was positively but weakly correlated with colloidal carbohydrate (p < 0.05), chl a (p < 0.10), and a combination of chl a, colloidal carbohydrate, and emersion period (p < 0.10). In September, there were no significant relationships between these parameters.
Discussion SEDIMENT ]~IOGEOCHEMISTRYAND ERODIBILITY
T h e strong correlation between colloidal carbohydrates and chl a observed on the Molenplaat has b e e n f o u n d in several studies (Grant et al. 1986; Paterson et al. 1990; Madsen et al. 1998; Underwood and Paterson 1998). T h e linear regression p r e s e n t e d in this study falls within the 95% prediction limits of the model derived by U n d e r w o o d and Smith (1998), primarily for cohesive sediments. Strong correlations are usually f o u n d in siltier sediments associated with epipelic diatom dominated communities that p r o d u c e large quantities of water-soluble colloidal exopolymers during loc o m o t i o n (Decho 1990). T h e silty sites on the Molenplaat are d o m i n a t e d by benthic diatoms, primarily species of Navicuta and Nit~schia (Barranguet et al. 1997; Lucas and Holligan 1999). Weaker correlations observed in the sandier sediments on the Molenplaat are likely to be the result of changes in c o m m u n i t y composition. Sandy sediments are d o m i n a t e d by epipsammic diatoms and cyanobacteria (Stal 1994; Yallop et al. 1994), n e i t h e r of which p r o d u c e s large quantities of colloidal carbohydrates (Madsen et al. 1998; de W i n d e r et al. 1999). Although significant, the relationship between chl a and colloidal carbohydrates on the H o n d did not fall within the 95% c o n f i d e n c e limits of either the Molenplaat data or the m o d e l of Und e r w o o d and Smith (1998). Adverse weather conditions with high rainfall coupled with shorter low water emersion periods (2.5-5 h c o m p a r e d with
89'1
4.5-7 h) may have p r o m o t e d solubilization of the mucopolysaccharides (Smith and Underwood 1998) and p r e v e n t e d stabilization and accumulation of the mucilage matrix. T h e r e are no taxon o m i c data on c o m m u n i t y composition for the H o n d , although H P L C analysis of sediments indicated that diatoms d o m i n a t e d during the sampling period. Factors relating to photosynthesis (e.g., d e p t h of photic zone, p h o t o i n h i b i t i o n , growth phase, and n u t r i e n t availability) are beyond the scope of this p a p e r and will not be discussed (but see Goto et al. 1999; Staats et al. 2000a,b; Perkins et al. 2001). T h e relationships between s e d i m e n t properties and m i c r o p h y t o b e n t h o s biomass and colloidal carbohydrate observed in the present study are well d o c u m e n t e d in the literature (Colijn and Dijkema 1981; Sundb~ck 1984; Santos et al. 1996), with silty sediments typically containing m o r e chl a and carbohydrates than sandy sediments. Cross-shore gradients of m i c r o p h y t o b e n t h o s d i s t r i b u t i o n have b e e n r e p o r t e d , with u p p e r and middle shore regions supporting greater biomass ( U n d e r w o o d and Paterson 1993; Paterson et al. 1994), primarily a result of increased p h o t o p e r i o d for photosynthesis (Pinckney and Zingmark 1991) and r e d u c e d physical disturbance by wave and c u r r e n t action ( U n d e r w o o d and Paterson 1993). T h e r e were no simple linear gradients between height on shore (emersion period) and sediment properties such as grain size, chl a, carbohydrates, silt, and water contents on either the Molenplaat or H o n d tidal flats, and this is reflected in measures of s e d i m e n t erodibility. Taking all the data combined, measures of sedi m e n t erodibility (threshold and mass eroded) were related to sediment chl a and colloidal carbohydrate content, in that threshold increased and mass e r o d e d decreased with increasing chl a and colloidal carbohydrate content. Although colloidal carbohydrates and chl a are closely coupled, it is the colloidal carbohydrates representing EPS that are involved in sediment stabilization and not chl a per se. T h e significance of the correlations tend to be greater with chl a, possibly because mucopolysaccharide p r o d u c t i o n is m o r e variable, and resistance to degradation to solubilization is low (Smith and U n d e r w o o d 1998; Staats et al. 2001). In spite of these overall correlations, several anomalies in the relationships were apparent, highlighted by the data points omitted from the regressions. Given the wide range of s e d i m e n t types, the datasets were split into 8 groups based on the relative a m o u n t of fine-grained ( < 63 p~m) fraction: sand (0-10%), sand-muddy sand (1025%), and m u d d y sand (25-50%) according to Riethm/511er et al. (1998). Although statistically sig-
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c . H . Lucas et al.
nificant correlations were n o t obtained because of low sample n u m b e r s and a high degree of scatter--particularly in the coarser sediments where diatom mats are m o r e patchily d i s t r i b u t e d - - t h e r e was a trend of increasing influence of microphyt o b e n t h o s on sediment erodibility as the finegrained fi-action increased. This has also b e e n observed by Riethmfiller et al. (1998). In sandy sediments there was no a p p a r e n t relationship between Um~ and chl a and colloidal carbohydrate. In contrast to the other sediment types, mass of sediment e r o d e d a p p e a r e d to increase, a l t h o u g h it must be n o t e d that the relationship was not statistically significant. T h e r e appears to be a change in the erosion characteristics of sandy sediments containing larger a m o u n t s of m i c r o p h y t o b e n t h o s . O n c e the threshold of erosion had b e e n reached, the sand lifted off in thin sheets held together by mucilage rather than as individual sand grains, increasing the mass of s e d i m e n t e r o d e d soon after Um~. This was observed during the flume experiments. In the m u d d y sand, chl a and colloidal carbohydrates increased sediment stabilization by increasing the threshold of erosion and r e d u c i n g the mass of sedi m e n t eroded. In addition to the c o m b i n e d effect of sediment type and chl a and colloidal carbohydrates content, consideration of o t h e r factors that influence sedim e n t erodibility still needs to be considered. Alt h o u g h an overall r e d u c t i o n in chl a and colloidal carbohydrate c o n t e n t in S e p t e m b e r was reflected in lower values for critical erosion velocity, trends between erosion threshold and the m e a s u r e d mic r o p h y t o b e n t h o s variables were m o r e a m b i g u o u s c o m p a r e d with J u n e . Clearly, o t h e r factors such as grazing and b i o t u r b a t i o n and probably microphyt o b e n t h o s c o m m u n i t y composition are affecting the threshold of erosion. Riethm/511er et al. (2000) observed that when c o m p a r i n g the influence of m i c r o p h y t o b e n t h o s on erosion shear stress at two tidal flats, there were sizable differences between the two datasets, and two-thirds of the variation at one location (Sylt-Remo Bight) was due to site-specific features n o t m e a s u r e d in the study. A majority of studies have focused on the role of m i c r o p h y t o b e n t h o s in stabilizing s e d i m e n t , whereas the role of bioturbators in destabilizing the s e d i m e n t has received less attention. Net sedim e n t stability is d e p e n d e n t on the relative balance between the two processes, which will vary temporally and spatially. T h e relative influence of biological stabilizers and destabilizers on sediment erodibility has been examined for the Molenplaat, with the results from 1996 published in Widdows et al. (2000). In addition to the influence of mic r o p h y t o b e n t h o s on sediment stabilization, significant relationships were observed between sedi-
m e n t erodibility (i. e., mass of sediment e r o d e d and erosion rate) and density of the clam Macoma batthica. This deposit feeder is a major b i o t u r b a t o r of surface sediments, and on the Molenplaat, density increased significantly between J u n e and September due to r e c r u i t m e n t and growth during the SUIIIII1ei, ACKNOWLEDGMENTS T h e authors would like m thank Peter H e r m a n for providing the bathymet~ic a n d grain size m a p s of the Molenplaat based on 1995 sampling campaigns carried o u t by N I O O , and Jack Middelburg for providing grain size data for 1997. Patrick Holligan provided helpful c o m m e n t s during the writing of this paper. This work is a contribution to the ELOISE P r o g r a m m e (ELOISE No. 136) in the framework of the ECOFLAT (EcoMetabolism of a Tidal Flat) project carried out u n d e r contract ENV4-ct96-0216, jointly s p o n s o r e d by the ENVIRONMENT a n d MAST P r o g r a m m e s of the EU. LITERATURE CITED AMos, C. L., G. R. DABOt~N, H. A. CHm~TIAN, A. ATKINSON,AND A. ROBERTSON. 1992. In situ erosion m e a s u r e m e n t s of finegrained sediments f r o m the Bay of Fundy. Marine Geology 10: 175--196. A u s T ~ , I., 11 J. ANDF~,SFa~, AND K. EDVJVANO. 1999. T h e influence of b e n t h i c diatoms and invertebrates on the erodibility of an intertidal m u d f l a t in the Danish Wadden Sea. Est~arine, Cbastal and Shdf Science 49:99-111. B~OUET, C., R M.J. H ~ , AND J. J. s ~ . 1997. Microp h y t o b e n t h o s biomass a n d c o m m u n i t y composition studied by p i g m e n t Momarkers: I m p o r t a n c e and fate in the cm-bon cycle of a tidal flat. Journal of Sea Research 38:59-70. B u z ~ l J l , E., R. CIANNA, E. MAI~CNOra, AND M. BRUNO. 1997. Influence of n u t r i e n t factors on production of mucilage by A~phora coffaeafc,rmis vat. perp~silla. Cc,ntiner~talShelf Research 17:1171-1180. COlNN, E AND K. S. DgI~UA. 1981. Species composition of benthic diatoms and distribution of chlorophyll a on an intertidal flat in the Dutch Wadden Sea. Marine Ecdogy Progress Series 4: 9-21. DECHO, A. W. 1990. MicroMal exopolymer secretions in ocean environments: Their role(s) in food webs and marine processes. Oceanography and Marine Bidogy: Art Ar~n~al Revue 28: 73-153. I)E W~,rDER, B., N. STAATS, L.J. STaZ, AND D. M. PATERSON. 1999. Carbohydrate secretion by p h o m t r o p h i c communities in tidal sediments. Journal of Sea Research 42:131-146. Dugols, M., K. A. Gns.vs, J. K. HAMILTON,}~ A. RF~FX, AND E SMITH. 1956. Colo~imet~ic m e t h o d for determination of sugars and related substances. AnalTtical Chemistry 28:350-356. COTO, N., 12 KAWANU~, O. M r r a N u ~ , AND H. TFa~I. 1999. I m p o r t a n c e of extracellular organic carbon p r o d u c t i o n in the total p i i m a r y p r o d u c t i o n by tidal-flat diatoms in comparison m phytoplankton. Marine Ecology Progress Series 190:289-295. GRANT, J., U. V. BATItMANN, AND E. L. MILLS. 1986. The interaction between benthic diatom films and s e d i m e n t transport. E~tuarine, Coastal ar~d Shelf Science 23:225-238. G~NT, J. AND G. R. DAsom< 1994. The effects of biomrbation on sediment t r a n s p o r t on an intertidal mudflat. Netherlands Journal of Sea Research 32:63-72. JuMAI~S, P. A. ANDA. R. M. NOWF,LL. 1984. Effects of b e n t h o s on sediment transport: Difficulties with functional grouping. Cou tinental Shelf Research 3:115-130. Lucas, C. H. AND R M. HOLLIGAN. 1999. Nature a n d ecological implications of algal p i g m e n t diversity on the Molenplaat tidal
Carbohydrates and Sediment Erodibility flat (Westerschelde estuary). Marine Ecology Progress Series 180: 51-64. MADSEN, K. N., R NK.SSON, AND K. SUNDt~CK. 1993. T h e influe n c e of b e n t h i c microalgae on the stability of a subtidal sedi m e n t . Journal of Experimental Marine Biolog7 and Ecology 170: 159-177. MAbrroul~, R. F. C. AND C. A. LI~WF~L'ZN. 1983. T h e rapid det e r m i n a t i o n of algal chlorophyll a n d c a r o t e n o i d p i g m e n t s a n d their b r e a k d o w n p r o d u c t s in n a t u r a l waters by reverse-phase h i g h - p e r f o r m a n c e liquid ctn-omatography. Anal/tica Chiv~ica Acta 151:297-313. PATERSON, D. M., R. M. CRAWFORD, AND C. LITTLE. 1990. Subaerial exposure a n d c h a n g e s in the stability of intertidal estuarine sediments. E~tuarine, Coastal and Shdf Science 30:541556. PATERSON, D. M., rE J. TOLItI_rRST,J. A. KELLY, C. HONEYWILL, E. M. G. T. DE DECKERE, V. HUET, S. A. S n A u ~ x , K. S. BLACa, J. DE BROUWER, AND I. DAVIDON. 2000. Variations in s e d i m e n t properties, Skeffling m u d f l a t , H u m b e r estuary, U.K. Continen tal Shelf Research 20:1378-1396. PATERSON, D. M., M. L. YALLOP, AND C. GEORGE. 1994. Spatial variability in s e c ~ n e n t erodibility on the island of Texel, p. 107-120. In W. E. Krurnbein, D. M. Paterson, a n d L. Stal (eds.), Biostabilization of Sediments. Verlag, O l d e n b u r g , Ge> man}~ PEPaClNS, R. G., G. J. C. U>mE1~wooI), V. BROTA& G. C. SNOW, B. J e s u s , AND L. RIBemo. 2001. Responses o f m i c r o p h y t o b e n t h o s to light: P r i m a r y p r o d u c t i o n a n d carbohydrate allocation over an e m e r s i o n period. Marine Ecology Progress Series 223:101-112. PINCKNEY,J. AND R. G. ZINOSgXV,K. 1991. Effects of tidal stage a n d s u n angles on intertidal b e n t h i c microalgal productivity. Ma fine Ecology Progress Series 76:81-89. RIETHMi2nJ.w, R., J. H. M. HAKVOORT,M. H E I N e , , K. HEU~ANN, H. KuIm, AND G. W n r e . 1998. Relating erosion s h e a r stress to tidal flat colour, p. 283-293. In K. S. Black, D. M. Paterson, a n d A. C r a m p (ads.), S e d i m e n t a r y Processes in the Intertidal Zone, Volume 139. Geological Society, L o n d o n , U.K. RIETHIVII7x . ~ , R., M. HEINEKE, H. KUIIL, AND R. K E ~ - R L r D I GF~. 2000. Chlorophyll a c o n c e n t r a t i o n as an i n d e x o f sedim e n t stabilization by m i c r o p h y t o b e n t h o s ? Continental Shelf Research 20:1351-1372. SAYros, R J . R,J. C~aw~, ANI) L. R SOUZA-SANTOS. 1996. Spatial distribution a n d d y n a m i c s of m i c r o p h y t o b e n t h o s biomass in the G i r o n d e estuary (France). Oceanolog~caActa 20:549-556. SMITH, D. J. AND G. J. c. UNOERWOOD. 1998. E x o p o l y m e r prod u c t i o n by intertidal epipehc diatoms. Liv~nology and Oceanograph7 43:1578--1591. STA&TS, N., E. M. G. 22 I)E DECImV,E, B. I)E WINDER, AND L. J. STALL.2001. Spatial p a t t e r n s of b e n t h i c diatoms, carbohydrates a n d m u d on a tidal flat in the Ems-Dollm-d esmm-y. H~drobiologia 448:107-115. STA&TS, N., L. J. STAL, B. DE WINDER, AND L. R. Mu~. 2000a. O x y g e n i c p h o t o s y n t h e s i s as driving process in exopolysaccharide p r o d u c t i o n of b e n t h i c diatoms. Marine Ecology progress Series 193:261-269. STARTS, N., L.J. STAL, AND L. R. MU~. 2000b. Exopolysaccharide p r o d u c t i o n by the epipelic d i a t o m CTlindrotheca dosterium: Ef-
893
facts of n u t r i e n t conditions. Jonrnal of Experimental Marine Bi ology and Ecdogy 249:15-27. STAL, L. 1994. Microbial mats: Ecophysiological interactions related to biogenic s e d i m e n t stabilization, p. 41-53. h~ W. E. Krurnbein, D. M. Paterson, a n d L. Stal (eds.), Biostabilization o f Sediments. Verlag, O l d e n b u r g , Germany. SUNOBAC~ K. 1984. Distribution o f m i c r o b e n t h i c chlorophyll a a n d diatom species related to s e d i m e n t characteristics. Ophelia supplement 3:229-246. StrI~Fa~aND, 22 F., C. L. A ~ o s , AND J. G ~ r r . 1998a. T h e effect o f b u o y a n t biofilms on the erodibility o f sublitroral s e d i m e n t s o f a t e m p e r a t e microtidal estuary. Liv~ndogy and Oceanography 43:225-235. StrI~Fa~aND, 22 E,J. G ~ N T , AND C. L. AMos. 1998b. T h e effect o f carbohydrate p r o d u c t i o n by the d i a t o m Nilzschia c~r.oilinea ta on the e r o d i N h t y o f sediment. Liwnology and Oceanography 43:65-72. UNDF~WOOD, G. J. c . AND D. M. PATF2,SON. 1993. Seasonal c h a n g e s in d i a t o m biomass, s e d i m e n t stability a n d b i o g e n i c stabihzation in the Severn estuary. Jc~urnal of the Marine Biolog~ ical Association of the United Kingdom 73:871-887. UNDERWOOD, G . J . c . , D. M. PATERSON, AND R. J. PARKES. 1995. T h e m e a s u r e m e n t o f microbial carbohydrate e x o p o l y m e r s f r o m intertidal sediments. Limndogy and Oceanograph/ 40: 1243-1253. UNDFa~WOOD, G . J . C . AND D.J. SMrrI~. 1998. Predicting epipehc e x o p o l y m e r c o n c e n t r a t i o n s in intertidal s e d i m e n t s f r o m sedinaent ctflorophyn a. MicrobiaIEcology 35:116-125. WIDDOWS, J., M. D. gRINSLEY, N. BOWLEY, AND C. BAX~Tr. 1998b. A b e n t h i c a n n u l a r f l u m e for in sire m e a s u r e m e n t of s u s p e n s i o n f e e d i n g / b i o d e p o s i t i o n rates a n d erosion potential o f intertidal cohesive s e d i m e n t . E~tuarine, Coastal and Shelf Science 46:27-38. WIDDOWS, J., M. D. BRINSLEu AND M. ELLIOTT. 1998a. Use o f an in sire f l u m e to quantify particle flux (biodeposition r a m s a n d s e d i m e n t erosion) for an intertidal m u d f l a t in relation to c h a n g e s in c u r r e n t velocity a n d b e n t h i c m a c r o f a u n a , p. 8 5 97. ,rr~ K. S. Black, D. M. Paterson, a n d A . C r a m p (ads.), Sedi m e n t a r y Processes in the I n m r t i d a l Zone. Geological Society, Spatial P u b h c a t i o n 139. L o n d o n , U.K. WIDDOWS,J., M. D. BmNSLEu P. N. SALKFJm, AND M. ELLIOTr. 1998C. Use o f annulai- fltm~es to d e t e r m i n e the i n f l u e n c e of c u r r e n t velocity a n d bivalves on material flux at the sedimentwater interface. E~t~aries 21:552-559. WIDDOWS,J., M. D. BRINSLEY,1~ N. SALKELD,AND C. H. Lucas. 2000. I n f l u e n c e of biota on spatial a n d t e m p o r a l variation in s e d i m e n t erodiNlity a n d material flux on a tidal flat (Wesmrschelde, N e t h e r l a n d s ) . Marine Ecology Progress Series 194:23-37. YALLOP, M. L., g. DE WINDER, D. M. PATERSON,AND L.J. STAL. 1994. Comparative structure, p r i m a r y p r o d u c t i o n a n d biogenic stabilization o f cohesive a n d non-cohesive m a r i n e sedim e n t s i n h a b i t e d by m i c r o p h y t o b e n t h o s . Estuarine, Coastal and She,'Science 39:565-582.
Received for consideration, December 19, 2001 Revised, August 1, 2002 Accepted for publication, September 17, 2002
