Schubel & Carter, 1984; Leonard et al., 1994; French et .... 9 Automatic water sam,. *** Salt marsh erosion/. Rome i q. Kongsmark 9 dam ...... Postma, H., 1967.
HELGOL,~-NDER MEERESUNTERSUCHUNGEN Helgol~inder Meeresunters. 51,253-268 (1997)
A fine-grained sediment budget for the Sylt-Romo tidal basin M. Pejrup, M. Larsen & K.
Edelvang
Institute of Geography, University of Copenhagen; •ster Voldgade i0, DK-1350 Copenhagen K, Denmark
ABSTRACT: A b u d g e t for net accumulation of fine-grained sediment (< 63 pro) has b e e n set up for the Sylt-Roma tidal basin. Net accumulation within the basin was computed from '-'l~ core dating and m a p p i n g of the intertidal and supratidal surface sediments. It was found that a yearly m e a n value of 58 - 10:3 tons of sediment was deposited in the tidal basin. The largest s e d i m e n t source for the net input of fine-grained sediment is the North Sea contributing about 64 % to the net budget; the fluvial input a n d primary production contribute 14 % and 15 %, respectively. Local salt marsh erosion accounts for about 5 % of the b u d g e t and atmospheric deposition for only 2 %. The total amount of sediment deposited in the investigated area was low compared with earlier investigations in the Wadden Sea. This is explained partly by the intensive diking of the natural salt marshes fringing the area in the past, and partly by the exposed conditions of most of the intertidal flats. An index describing the trapping efficiency of the water e x c h a n g e d b e t w e e n the North Sea a n d the SyltRomo tidal area is defined as the ratio b e t w e e n yearly net sediment input from the North Sea and yearly e x c h a n g e d water volume b e t w e e n the tidal basin and the sea. This index shows that in the Sylt-Romo tidal basin, fine-grained s u s p e n d e d sediment "filters" out of the e x c h a n g e d sea water at a rate that is 12 times lower than in the Gr~dyb tidal basin. It is concluded that the net deposition of fine-grained sediment in a tidal basin is mainly a function of physiographical a n d hydrodynamical parameters a n d to a lesser degree of sediment availability.
INTRODUCTION S h a l l o w e s t u a r i n e a r e a s a r e k n o w n to i m p o r t l a r g e q u a n t i t i e s of f i n e - g r a i n e d s e d i m e n t f r o m t h e a d j a c e n t s e a s (cf. S c h u b e l & C a r t e r , 1984; L e o n a r d et al., 1994; F r e n c h e t al., 1994). T h i s is also t r u e for t i d a l a r e a s i n t h e W a d d e n S e a (cf. B a r t h o l d y & P h e i f f e r M a d s e n , 1985; P e j r u p , 1988a; B r e u n i n g - M a d s e n , 1995; a n d L a r s e n et al., 1996). T h e a c c u m u l a t i o n of f i n e - g r a i n e d s e d i m e n t i n t h e W a d d e n S e a is m a i n l y c a u s e d b y t h e sett l i n g - a n d s c o u r l a g m e c h a n i s m s as d e s c r i b e d b y V a n S t r a a t e n & K u e n e n (1957, 1958) a n d P o s t m a (1967). H o w e v e r , o t h e r p r o c e s s e s s u c h as a g g r e g a t i o n of f i n e - g r a i n e d s e d i m e n t p a r t i c l e s m a y a l s o b e i m p o r t a n t . T h e role of s a l t f l o c c u l a t i o n h a s b e e n d e s c r i b e d as p o t e n t i a l l y i m p o r t a n t b y P e j r u p (1988b, 1991) a n d b i o a g g r e g a t i o n i n t h e f o r m of f a e c a l p e l l e t s h a s b e e n s h o w n to b e i m p o r t a n t (cf. H a v e n & M o r a l e s - A l a m o , 1972; E d e l v a n g & A u s t e n , 1997). F i n e - g r a i n e d s e d i m e n t b u d g e t s h a v e o n l y b e e n s e t u p for a s m a l l n u m b e r of t i d a l a r e a s i n t h e W a d d e n Sea. F o r G r i d y b , t h e n o r t h e r n m o s t t i d a l a r e a , B a r t h o l d y & P h e i f f e r M a d s e n (1985) c o m p u t e d a n a n n u a l n e t a c c u m u l a t i o n of 142 - 103 t o n s of f i n e - g r a i n e d s e d i m e n t ; a n d L a r s e n et al. (1996) c o m p u t e d a n e t a c c u m u l a t i o n of 2 t o n s for t h e s m a l l 9 Biologische Anstalt Helgoland, H a m b u r g
254
M. Pejrup, M. Larsen & K. E d e l v a n g
tidal area K6 n i g s h a f e n w h ic h is a part of the Sylt-R~mo tidal basin. O n the basis of literature studies, Eisma & Irion (1988) e s t i m a t e d the n et a c c u m u l a t i o n of f i n e - g r a i n e d s e d i m e n t (< 125 l~m) in the w h o l e W a d d e n Sea, i ncl u d i n g the Wash on the East A n g l i an coast, to be 5 9 106 t/year. Apart from the a b o v e - m e n t i o n e d studies, only few attempts h a v e b e e n m a d e to establish f i n e - g r a i n e d s e d i m e n t b u d g e t s for other tidal areas in the W a d d e n Sea. Extrapolating from the figures g i v e n by Bartholdy & P h e i f f e r - M a d s e n (1985), the n et a c c u m u l a t i o n in the W a d d e n Sea w o u l d a m o u n t to a p p r o x i m a t e l y 8 9 106 t / y e a r w h i ch is 1.6 times that found by Eisma & Irion (1988). F l e m m i n g & N y a n d w i (1994) s u g g e s t e d that some Wadd e n Sea tidal areas m a y be d e p l e t e d of f i n e - g r a i n e d s e d i m e n t s b e c a u s e of h u m a n activities such as land r e c la m a t io n in the area. On the basis of a d e t a i l e d study of settling velocities of bottom s e d im e n t s b e h i n d the barrier islands of S p i e k e r o o g a n d Baltrum they found that grain sizes less than 3.5 O (88 t~m) w e r e u n d e r r e p r e s e n t e d in t h e b o t t o m sedim e n t s of the tidal flats c o m p a r e d to other less m a n i p u l a t e d tidal basins. Similar results w e r e p r e s e n t e d by Dronkers (1986), w h o s h o w e d that i n t en se diking of the O o s t e r s c h e l d e estuary r e v e r s e d the net import of f i n e - g r a i n e d s e d i m e n t s to a net export. T h e s e findings e m p h a s i z e the n e e d for further studies of f i n e - g r a i n e d s e d i m e n t budgets. T h e objectives of this study are to set up a b u d g e t for the n e t a c c u m u l a t i o n of f i n e - g r a i n e d s e d i m e n t for the Sylt-Romo tidal basin, and to c o m p u t e the relative i m p o r t a n c e of the different sedim e n t sources. STUDY AREA Th e Sylt-R~ma tidal basin covers an area of a p p r o x i m a t e l y 400 km 2 of w h i c h the intertidal area comprises about 45 % (Fig 1). T h e tide is s e m i - d i u r n a l with an a v e r a g e tidal r a n g e of ab o u t 1.8 m, classifying it as a micro-tidal estuary acco r d i n g to Davis (1964) or low er m e s o - t i d a l after H a y e s (1979). T h e tidal prism is ab o u t 530 - 1 0 6 m 3 a n d m a x i m u m tidal cu rr en t velocities of about 2 m / s e c are m e a s u r e d in the tidal inlet, d e c r e a s i n g to about 0.2 m / s e c on the tidal flats in the i n n e r parts of the area (Edelvang, 1995). Th e tidal basin is restricted both to the north and to the south by artificial dams, the R o m a d a m and H i n d e n b u r g dam, respectively. T h e bottom s ed i m e n ts in the intertidal zone r a n g e from sand flats w i t h m o r e than 95 % sand, m i x e d m u d flats containing 10-50 % sand, to m u d flats c o n t a i n i n g m o r e than 50 % f i n e - g r a i n e d sediments. Two small rivers d is c h a r g e t h r o u g h sluice gates into the tidal area. T h e rivers Brede ~. and Vid~ h a v e c a t c h m e n t areas of 464 k m 2 and 1364 km 2, respectively. T h e m e a n freshw a t e r d i s c h a r g e from these two rivers a m o u n t s to about 1 .106 m 3 or 0.2 % of the tidal prism over one tidal cycle. Most of the natural salt m a r s h e s f r i n g i n g the area h a v e b e e n diked, a n d only about 10 k m 2 of u n d i k e d salt m a r s h e s are currently e x p o s e d to i n u n d a t i o n d u r i n g st o r m surges. METHODS Total net accumulation T h e total net a c c u m u l a t i o n has b e e n e s t i m a t e d from 21~ core dating of t h e s e d i m e n t on the intertidal flats a n d on the salt marsh. 21~ is a natural radioactive P b - i s o t o p e existing in the a t m o s p h e r e . It is the product of the d e c a y of 238U in the earth crust d e c a y i n g
Sediment budget for Sylt-Romo tidal basin
I 21~
255
core (dated)k~.
I 21~ b core (not dated
Rome
9 Automatic water sam, * * * Salt marsh erosion/
dam
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i
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Jordsand List i
-
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tP
Emmerlev
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It
Keitum I
sy,t ~
Hindenburg darn= tO
5 km i
Fig. 1. Map of the investigated tidal basin with indication of sites for sediment core sampling, water sampling stations, and sites for measuring salt marsh erosion
256
M. Pejrup, M. Larsen & K. E d e l v a n g
to 22~Ra a n d further to the gas 222Rn w h i c h is emitted from the earth crust to the atmosphere, w h e r e it further decays to 21~ that is r e t u r n e d to the earth surface by precipitation. W h e n 21~ is introduced in the estuarine e n v i r o n m e n t it adheres to the cohesive s e d i m e n t particles s u s p e n d e d in the w a t e r a n d is transported a n d deposited t o g e t h e r with these particles. The s e d i m e n t cores were collected as box cores in the s u p r a t i d a l areas a n d as n o r m a l s e d i m e n t cores using a l u m i n i u m tubes with 8 cm diameter in the intertidal areas. After sampling, each core was split vertically into two halves a n d s u b s e q u e n t l y cut horizontally into 1 cm slices. The m e t h o d is described in greater detail by Larsen et al. (1996). The m e a s u r e d 21~ activity in the s e d i m e n t can be divided into (a) s u p p o r t e d a n d (b) u n s u p p o r t e d activity. The supported part originates from the decay of 222Rn within the s e d i m e n t a n d the u n s u p p o r t e d 21~ activity originates from 2u)pb precipitated to the surface. The half-life of this isotope is 22.3 years a n d it is a s s u m e d that a c o n s t a n t a m o u n t of 21~ is deposited from the atmosphere every year. The age of the different s e d i m e n t layers can be c o m p u t e d by m e a s u r i n g the u n s u p p o r t e d 21~ activity from each layer, t h e r e b y establishing a vertical profile of the 2~~ activity. The 21~ activity was measured as 21~ activity by c~-spectrometry a n d 2mPb activity was a s s u m e d to b e e q u a l to the m e a s u r e d 2mpo activity. To calculate the age of the different s e d i m e n t layers, the Constant Rate of Supply method (C.R.S.) is used, where the flux of 2~~ is a s s u m e d to be constant in time for the investigated area. This m e t h o d is r e c o m m e n d e d by several authors for f i n e - g r a i n e d s e d i m e n t a c c u m u l a t i o n areas with variable deposition rates (e.g. A p p l e b y & Oldfield, 1978; Pheiffer-Madsen & Sorensen, 1979; E1-Daoushy, 1988). Subsequently, the total a c c u m u l a t i o n rate of s e d i m e n t can be estimated from the slope of the activity profile b e n e a t h the mixing layer at the top of the s e d i m e n t core. The vertical activity profile is described by a m a t h e m a t h i c a l model b a s e d on the following advectiondiffusion equation: dS - -
dt Where S D z w 1 t
is is is is is is
the the the the the the
dS
d2S =
D . - -
dz 2
- w -
- -
- 1 . S
dz
activity of 21~ (dpm/g) mixing coefficient (cm2/year) depth b e n e a t h surface (cm) linear a c c u m u l a t i o n rate (cm/year) d e c a y i n g constant for 21~ (year q) a n d time (years)
S u b s a m p l e s from the other core sections were used for grain size analysis. Each sample was divided by wet sieving into a fraction coarser t h a n 63 l~m a n d one finer t h a n 63 llm after it h a d b e e n s u s p e n d e d in 0.002 M Na4P207 in a n ultrasonic bath. G r a i n size analyses of the f i n e - g r a i n e d particles were s u b s e q u e n t l y carried out in a S e d i g r a p h 5100, which m e a s u r e s the d a m p e n i n g of a soft x-ray b e a m t h r o u g h a s e d i m e n t a t i o n cell cont a i n i n g the f i n e - g r a i n e d suspension.
S e d i m e n t b u d g e t for Sylt-Romo tidal basin
257
Fluvial input The input of f i n e - g r a i n e d s u s p e n d e d s e d i m e n t from the rivers was e s t i m a t e d on the basis of daily w a t e r samples collected with an automatic ISCO 2700 w at er sampler. Sample v o l u m e s w e r e about 0.7 litres. T h e s a m p le s w e r e filtered through W h a t m a n n G F / F glass fibre filters to d e t e r m i n e the mass c o n c e n tr at i o n of s u s p e n d e d sediment. T h e filters w e r e c o m b u s t e d at 500 ~ to obtain a m e a s u r e of organic content. Th e in-situ settling velocities of the fluvial s e d i m e n t are d e t e r m i n e d with a Braystoke SK 110 w a t e r s a m p l e r / analyser (Pejrup, 1988b).
Salt marsh erosion Salt m ar s h erosion was e s ti m a t e d from r e p e a t e d levellings of the e r o d i n g m ar sh cliffs at s e l e c t e d sites. T h e levelling was carried out with a theodolite e q u i p p e d with an electronic distance m e t e r of the type Topcon. Data w e r e stored in a solid state m e m o r y as x a nd y coordinates w h i c h w e r e later transferred to a c o m p u t e r for automatic drafting of the cliff shape. T h e retreat of the salt m a r s h cliff could t h e n be d e t e r m i n e d as the area b e t w e e n s u c c e s s i v e levellings multiplied with the a v e r a g e h e i g h t of the cliff.
RESULTS Fine-grained sediment budget Areal distribution of accumulation
Based on a p p r o x i m a t e l y 370 surface s e d i m e n t sam p l es a m ap s h o w i n g the distribution of the surface s e d i m e n t s in the intertidal a n d supratidal zones was established. Th e m a p (Fig. 2) is a modification of the s e d i m e n t m a p for the tidal basin d r a w n by K6ster et al. (1995). T h e surface sediments w e r e d i v i d e d into the following four facies: Salt marsh: c o n t a i n i n g varying a m o u n t s of s e d i m e n t > 63 l~ (mostly m o r e than 90 % < 63 Dm). M u d fiats : c o n t a i n i n g more than 50 % of s e d i m e n t < 63 lain. M i x e d m u d flats: containing 10-50 % of s e d i m e n t < 63 lam. Sand flats: c o n t a i n i n g less than 10 % of s e d i m e n t < 63 l~m. Each facies was further s u b d i v i d e d into an e x p o s e d an d a sh el t er ed unit with r e g a r d to the p r e v a i l i n g w e s t e r l y winds. A total of f o u r t e e n s e d i m e n t cores r e p r e s e n t i n g the a b o v e eight g e o m o r p h o l o g i c a l units w e r e d a t e d by application of the 21~ method. Four of these sam p l es w e r e not datable d u e to low contents of f i n e - g r a i n e d sediment. T h e b u d g e t is therefore b a s e d on ten d a t e d s e d i m e n t cores and a t h o r o u g h classification of the surface sediments. An e x a m p l e of a d a t e d s e d i m e n t core from the K a m p e n tidal fiats is s h o w n in Figure 3. Th e c o m p u t e d a c c u m u l a t i o n rates for all cores are listed in Table 1. A c c u m u l a t i o n rates r e p r e s e n t i n g e a c h of the g e o m o r p h o l o g i c a l units are c o m p u t e d either as a v e r a g e v a l u e s of the samples t a k e n within this unit or as single v a l u e s from one core t a k e n within the unit. The e x p o s e d m u d flat unit is not r e p r e s e n t e d by any d a t e d cores b e c a u s e it occupies a small area only. T h e a c c u m u l a t i o n v a l u e for this unit was
:!'I
258
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M. P e j r u p , M. L a r s e n & K. E d e l v a n g
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Fig. 2. Map of surface sediment distribution within the Sylt-Romo tidal basin. This map is b a s e d on a sediment map by K6ster et al. (1995)
S e d i m e n t b u d g e t for S y l t - R o m o t i d a l b a s i n
259
Kampen tidal flat 0
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~
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a
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1995
1975
1955
1935
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Fig. 3. Example of a s e d i m e n t core dated by use of the 21~
1875
1855
method
d e t e r m i n e d a s t h e a r i t h m e t i c m e a n of t h e e x p o s e d m i x e d m u d flat u n i t a n d t h e s h e l t e r e d m u d flat u n i t . F u r t h e r m o r e , a c c u m u l a t i o n r a t e s for t h e s a n d f l a t s w e r e c o m p u t e d o n t h e b a s i s of a c c u m u l a t i o n r a t e s d e t e r m i n e d for e x p o s e d m i x e d m u d flats a f t e r c o r r e c t i o n of t h e c o n t e n t of f i n e - g r a i n e d s e d i m e n t s in t h e s a n d flat. T h i s is a r a t h e r artificial w a y of e s t i m a t i n g t h e f i n e - g r a i n e d s e d i m e n t a t i o n o n t h e s a n d flats, b u t it is c o n s i d e r e d m o r e r e a l i s t i c t h a n j u s t d i s r e g a r d i n g t h e s e i n t e r t i d a l f l a t s in w h i c h t h e c o n t e n t of f i n e - g r a i n e d s e d i m e n t is so l o w t h a t t h e ~ ~
dating method cannot be used.
Table 1. A c c u m u l a t i o n rates from 10 different sites within the Sylt-Remo tidal basin. Values in the table are not corrected for the s e d i m e n t ' s s a n d content. A: Pejrup et al. (1995); B: Larsen et al. (1994); C: E d e l v a n g et al. (1993); D: Pejrup & Bartholdy (1991); E: B r e u n i n g - M a d s e n (1995) No
Site
Geomorphological unit
Exposure
1 2 3 4 5 6 7 8 9 10
K o n g s m a r kA K o n g s m a r kB R~mo d a m A Ballum E Ballum c KampenE Tipkenh~igelc K 6 n i g s h a f e nD K ~ n i g s h a f e nD K 6 n i g s h a f e nD
Salt m a r s h M u d flat M u d flat Salt m a r s h Mixed m u d flat Mixed m u d flat Salt m a r s h Salt m a r s h Salt m a r s h M u d flat
Sheltered Sheltered Sheltered Exposed Exposed Sheltered Sheltered Sheltered Sheltered Sheltered
A c c u m u l a t i o n rate (total kg/m2/year) 1.53 3.38 1.50 0.61 2.19 1.93 2.45 1.16 0.45 5.10
• • • • • • • • • •
0.07 0.29 0.09 0.13 0.14 0.17 0.37 0.10 0.04 0.30
260
M. P e j r u p , M. L a r s e n & K. E d e l v a n g
T h e a r e a of e a c h g e o m o r p h o l o g i c a l u n i t w a s m e a s u r e d f r o m t h e s e d i m e n t m a p . T h e r e s u l t s , t o g e t h e r w i t h t h e t o t a l s e d i m e n t a c c u m u l a t i o n r a t e s , a r e l i s t e d i n T a b l e 2. A n e t b u d g e t for t h e f i n e - g r a i n e d s e d i m e n t w a s t h e n d e t e r m i n e d b y m u l t i p l y i n g t h e a c c u m u l a t i o n r a t e s w i t h t h e i r r e s p e c t i v e a r e a s . T h e r e s u l t i n g a c c u m u l a t i o n , c o r r e c t e d for m e a n c o n t e n t of f i n e - g r a i n e d s e d i m e n t , a m o u n t s to a b o u t 58 9 103 t / y e a r ( T a b l e 2). It a p p e a r s t h a t t h e m a j o r p a r t of t h e f i n e - g r a i n e d s e d i m e n t a c c u m u l a t i n g i n t h e S y l t - R ~ m o t i d a l b a s i n is f o u n d o n t h e m u d fiats a n d m i x e d m u d flats. T h e salt m a r s h a r e a s a c c u m u l a t e a s i g n i f i c a n t a m o u n t of t h e f i n e - g r a i n e d s e d i m e n t d e p o s i t e d d u r i n g e x t r e m e h i g h w a t e r l e v e l s , e.g. d u r i n g s t o r m s u r g e s . Finally, it is s h o w n t h a t t h e a c c u m u l a t i o n of f i n e - g r a i n e d s e d i m e n t o n t h e s a n d flats a c c o u n t s for a b o u t 13 - 10 "~ t / y e a r d u e to t h e l a r g e a r e a this g e o m o r p h o l o g i c a l u n i t o c c u p i e s in t h e t i d a l b a s i n . Table 2. Classification and total accumulation of fine-grained sediment in Sylt-Romo tidal basin. Classification of each unit is based on the contents of fine-grained sediment : s a n d flat < 10 %; mixed mud flat : 10-50 %; a n d mud flat > 50 % Depositional e n v i r o n m e n t
Area (km 2)
Accumulation rate (kg/m2/year)
Average (% < 63 gm)
Accumulated sediment (t/year)
Salt marsh sheltered Salt marsh exposed Mud flat lee Mud flat exposed Mixed m u d flat sheltered Mixed m u d flat exposed Sand flats I Sand flats II Total supra-tidal Total intertidal Total area of accumulation
4.9 5.6 4.1 1.3 20.4 18.6 25.0 110.0 10.5 179.3 189.8
1.6 0.6 3.3 2.7 2.0 2.2 2.2 2.2 -
94 50 73 88 29 25 2 5 -
7 400 1 700 9 900 3 100 12 000 10 400 1 100 12 100 9 100 48 600 57 700
Sediment
sources
Fluvial input S u s p e n d e d s e d i m e n t s a m p l e s w e r e c o l l e c t e d i n t h e r i v e r s B r e d e / k a n d Vid& o v e r t h e 3 y e a r s f r o m 1992 to 1994. T h e s u s p e n d e d s e d i m e n t c o n c e n t r a t i o n in t h e s e s a m p l e s w a s d e t e r m i n e d as d e s c r i b e d e a r l i e r a n d s e d i m e n t d i s c h a r g e w a s c o m p u t e d b y m u l t i p l y i n g s u s p e n d e d c o n c e n t r a t i o n w i t h d a i l y m e a n v a l u e s of t h e w a t e r d i s c h a r g e d e t e r m i n e d f r o m the relationship between water level and measured discharge (Sonderjyllands Amtsr~d, 1993, 1994, 1995). T h e f l u v i a l i n p u t a v e r a g e d o v e r t h e t h r e e y e a r s is s h o w n i n T a b l e 3. T h e s e m e a n v a l u e s a r e b a s e d o n 3 y e a r s of m e a s u r e d , s u s p e n d e d s e d i m e n t t r a n s p o r t r a n g i n g f r o m 11 - 103 to 22 9 103 t / y e a r t r a n s p o r t e d i n t o t h e S y l t - R o m e t i d a l b a s i n . T h e m e a n v a l u e o v e r t h e i n v e s t i g a t i o n p e r i o d w a s 17.3 - 103 t / y e a r . A p p r o x i m a t e l y 3 / 4 of t h i s a m o u n t o r i g i n a t e s f r o m Vid~ a n d t h e r e m a i n i n g 1/4 f r o m B r e d e ~ . A p p r o x i m a t e l y 1/3 of t h e s u p p l i e d s u s p e n d e d m a t t e r w a s o r g a n i c , c o r r e s p o n d i n g to 6 - 103 t / y e a r . A c c o r d i n g to f i n d i n g s b y v a n Es (1977), it is a s s u m e d t h a t 90 % of t h e s u p p l i e d o r g a n i c m a t t e r is r a i n -
S e d i m e n t b u d g e t for S y l t - R e m e t i d a l b a s i n
261
Table 3. Average values for the investigation period 1992-94 of fluvially supplied fresh water Brede A Discharge (m3/sec) S u s p e n d e d sediment concentration (mg/1) Organic matter (%) Total s u s p e n d e d transport (t/year) Total fresh water discharge (106 m:~/year)
6.7 19.2 39.0 4600 210
Vid~
Total
17.0 22.4 34.2 12 700 524
23.7 21.5 35.5 17 300 734
e r a l i z e d , t h u s l e a v i n g o n l y a b o u t 10 % or 600 t of o r g a n i c m a t t e r for p e r m a n e n t d e p o s i tion. Furthermore, although the water samples in the rivers were collected at the surface, s o m e of t h e s u s p e n d e d p a r t i c l e s h a v e e q u i v a l e n t s e t t l i n g d i a m e t e r s > 63 ]~m as s h o w n b y t h e i n - s i t u s e t t l i n g t u b e a n a l y s e s . A n e x a m p l e of s u c h a n a n a l y s i s is g i v e n i n F i g u r e 4. F o r t h e B r e d e A it h a s b e e n e s t i m a t e d t h a t , as a y e a r l y a v e r a g e , 35 % of t h e t o t a l s u s p e n d e d s e d i m e n t w a s c o a r s e r t h a n 63 p m . A b o u t 11 9 103 t of t h e s u p p l i e d s e d i m e n t is i n o r g a n i c a n d a b o u t 35 % of t h i s is c o a r s e r t h a n 63 ~ m , t h u s l e a v i n g a b o u t 7.5 9 103 t of i n o r g a n i c m a t t e r for d e p o s i t i o n . T h e t o t a l r e t a i n e d f i n e - g r a i n e d f l u v i a l i n p u t m u s t t h e r e f o r e b e red u c e d f r o m 17 - 103 t / y e a r to 8 9 10 "~t / y e a r .
99,95
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Fig. 4. Sediment grading curve for s u s p e n d e d sediment collected in Brede A. The horizontal axis indicates the equivalent settling diameter for the s u s p e n d e d s e d i m e n t
262
M. P e j r u p , M. L a r s e n & K. E d e l v a n g
Salt marsh erosion T h e r e s u l t s of t h e l e v e l l i n g m e a s u r e m e n t s of t h e e r o d i n g salt m a r s h cliffs a r e s h o w n in T a b l e s 4 a n d 5. Six t e s t s t r e t c h e s w e r e s e l e c t e d a n d l e v e l l e d in t h e y e a r s 1992, 1993 a n d 1994, a l l o w i n g t w o y e a r l y e r o s i o n r a t e s to b e e s t i m a t e d . T h e a m o u n t of s e d i m e n t e r o d e d w a s c a l c u l a t e d as: E = Ce/Me W h e r e E is t h e e r o d e d a m o u n t of s e d i m e n t m 3 / m Ce is t h e c a l c u l a t e d e r o s i o n i n m 3 a n d M e is t h e m e a s u r e d l e n g t h in m. T h e t o t a l e r o s i o n w a s c a l c u l a t e d b y m u l t i p l y i n g t h e e s t i m a t e d E v a l u e w i t h t h e total l e n g t h of t h e e r o s i o n a l s a l t m a r s h of t h e d i f f e r e n t t e s t sites. Finally, this f i g u r e w a s m u l t i p l i e d w i t h t h e c o m p u t e d b u l k d e n s i t i e s of t h e i n v e s t i g a t e d salt m a r s h e s . T h i s a m o u n t w a s f u r t h e r c o r r e c t e d for t h e c o n t e n t of s e d i m e n t c o a r s e r t h a n 63 ~ m a n d t h e n e x t r a p o l a t e d to t h e w h o l e l e n g t h of e r o d i b l e salt m a r s h e s in t h e a r e a (Table 6). F r o m T a b l e 6 it c a n b e s e e n t h a t in 1992-93 a b o u t 2.8 9 103 t / y e a r w e r e e r o d e d f r o m t h e salt m a r s h e s f r i n g i n g t h e t i d a l b a s i n , w h e r e a s in 1993-94 a b o u t 2.1 9 103 t / y e a r w e r e e r o d e d . A s e v i d e n t f r o m t h e v a l u e s l i s t e d in T a b l e s 4 a n d 5, this f i g u r e is fairly c o n s t a n t a l o n g t h e i n v e s t i g a t e d s t r e t c h e s f r o m y e a r to year. O n t h i s basis, a m e a n v a l u e of 2 . 5 . 1 0 ~ t / y e a r of s e d i m e n t e r o d e d a l o n g t h e salt m a r s h w a s e s t i m a t e d .
Table 4. Salt marsh erosion measurements 1992-1993 Site
Gross (east) Gross (west) Keitum K6nigshafen Kongsmark Havneby
Test length (m)
Measured erosion (m3)
Total length (m)
Relative erosion (m:~/m)
Bulk density (kg/m 3)
Total erosion (tonnes)
410 300 230 80 250 250
74.6 57.9 128.0 22.6 -
1000 1300 1125 575 1000 1500
0.182 0.193 0.557 0.283 -
1040 980 1120 1010 1040 1280
189.2 245.9 701.2 164.1 -
Table 5. Salt marsh erosion measurements 1993-1994 Site
Gross (east) Gross (west) Keitum Kbnigshafen Kongsmark Havneby
Test length
Measured erosion
Total length
Relative erosion
Bulk density
Total erosion
(m)
(m 3)
(m)
(m3/m)
(kg/m 3)
(tonnes)
410 300 230 80 250 250
73.7 54.1 120.4 17.9 60.2 21.8
1000 1300 1125 575 1000 1500
0.180 0.180 0.523 0.224 0.241 0.087
i040 980 1120 1010 1040 1280
187.2 229.3 659.0 130.1 250.6 167.0
S e d i m e n t b u d g e t for Sylt-Ramo tidal basin
263
Table 6. Total and mean erosion of erodible salt marsh measured by the survey method Year
1992-93 1993-94 mean
Total erosion (t)
Length of test areas (m)
Total length of erodible salt marsh (m)
Average (% < 63 gm)
Supply (t/year)
1300 1600 -
4000 6500 -
10000 10000 10000
85 85 85
2800 210o 2500
Atmospheric input Th e a t m o s p h e r i c contribution to the f i n e - g r a i n e d s e d i m e n t b u d g e t was c a l c u l a t e d from p u b l i s h e d data. An a v e r a g e deposition rate of 0.055 g/m2/day was m e a s u r e d by Lufthygienische O b e r w a c h u n g (1992: pers. comm.) b a s e d on three m e a s u r i n g stations n e a r Brunsbtittel. This gives a yearly input of 20 t/km2/year to the Sylt-Ramo tidal basin. Studies in the North Sea area carried out by M c C a v e (1973) y i el d ed a v e r a g e a t m o s p h e r i c deposition rates of 2.8 t/km2/year. This rather low v al u e is supported by B r o e c k e r et al. (1958) wh o found deposition rates of 2.0 t/m2/year for the Mid-Atlantic ridge. Based on data p r e s e n t e d by Schutz (1993), Larsen et al. (1996) e s t i m a t e d a total w et and dry deposition rate of undissolvable m a t t e r to 2.2 t/km2/year at W e s t e r h e v e r Lighthouse on the p e n i n s u l a of Eiderstedt about 60 k m south of the Sylt-Romo tidal basin. It is likely that high contents of dissolvable salts, such as ammonia, nitrate and s o d i u m chloride, explain the high values of a t m o s p h e r i c deposition at Brunsbtittel w h i ch incidentally is an industrial site. Such c o m p o u n d s could easily contribute 3-10 t/km2/year, d e p e n d i n g on the distance b e t w e e n the m e a s u r i n g station a n d the sea. It was thus d e c i d e d to use the values of Schulz (1993) to estimate the a t m o s p h e r i c deposition in the Sylt-Romo tidal basin. On this basis the atmospheric input to the f i n e - g r a i n e d s e d i m e n t b u d g e t a m o u n t s to 0.9 9 10 :] t/year.
Primary production The contribution from both b e n th ic a n d p e l a g i c primary production has also b e e n e s t i m a t e d on the basis of p u b l i s h e d data. C a d 6 e & H e g e m a n (1974) found v a l u e s of 100 (+ 40) g C / m 2 / y e a r for the benthic p r i m a r y production - a value that is s u p p o r t e d by results from the Dollard estuary p r e s e n t e d by v a n Es (1977), who e s t i m a t e d a b e n t h i c primary p r o d u c t i o n of 116 g C/m2/year. A s m u s & A sm u s (1985) found v a l u e s of 150 g C / m 2 / y e a r for the benthic primary p r o d u c t i o n in K6nigshafen, and Bruns et al. (1995) found a m e a n v al u e of 111 (+ 35) g C / m 2 / y e a r c o m p u t e d for the w h o l e of Sylt-Romo tidal basin. Furthermore, v a n Es (1977) found a p e l a g i c primary production of 7.5 g C/m2/year. On the basis of the a b o v e data it was d e c i d e d to use a primary p r o d u c t i o n of 118 g C/m2/year, for the c o m p u t a t i o n of the o r g a n ic input to the s e d i m e n t budget. This is e q u i v alent to ab o u t twice the a m o u n t of o r g a n ic matter, co r r esp o n d i n g to 236 g/m2/year. At least 90 % of this is m i n e r a l i z e d before a c c u m u l a t i o n (van Es, 1977) w h i c h l e a v e s 10 % as input to the net b u d g e t of f i n e - g r a i n e d s e d i m e n t. For a tidally i n f l u e n c e d a r e a of 390 k m 2 this co rr es p o n d s to about 9 - 103 t / y e a r from p r im ar y production.
264
M. Pejrup, M. Larsen & K. E d e l v a n g R e l a t i v e i m p o r t a n c e of d i f f e r e n t s e d i m e n t s o u r c e s
T h e net b u d g e t of f i n e - g r a i n e d s e d i m e n t in the Sylt-Romo tidal basin can n o w be d i v i d e d a c c o r d i n g to the different s e d i m e n t sources. This is s h o w n in Table 7. T h e total a m o u n t of f i n e - g r a i n e d s e d i m e n t a c c u m u l a t e d in the tidal basin amounts to a yearly avera g e of 58 9 1 0 3 t. Table 7. Fine-grained sediment budget of the Sylt-Rom~ tidal basin. The contribution from the different sediment sources is rounded to thousands Sediment source
Net accumulation (t/year)
Relative contribution (%)
Rivers Primary production Salt marsh erosion Atmospheric deposition North Sea (residue)
8 000 9 000 3 000 1 000 37 000
14 15 5 2 64
Total
58 000
100
It w o u l d a p p e a r that fluvial input and primary p r o d u c t i o n are of e q u a l amounts, contributing 14 % and 15 % respectively. This is b a s e d on the assumption that all fluvially s u p p l i e d s e d i m e n t and the n o n - m i n e r a l i z e d fraction of the primary p r o d u c t i o n are deposited within the tidal basin. C o n s i d e r i n g this assumption, the calculated figures must be r e g a r d e d as r e p r e s e n t i n g m a x i m u m values. The s a m e a r g u m e n t applies to salt marsh erosion (5 %) and atmospheric deposition (2 %), the latter b e i n g of only m i n o r i m p o r t a n c e for the total net a c c u m u l a t i o n of f i n e - g r a i n e d sediments. The contribution from the adjacent North Sea is e s t i m a t e d as the residue b e t w e e n the total a c c u m u l a t e d a m o u n t of sedi m e n t and the s e d i m e n t from other sources. In this w a y it is found that the North Sea contributes 64 % of the net input of f i n e - g r a i n e d sediment. Following the a r g u m e n t a t i o n above, this figure must be c o n s i d e r e d a m i n i m u m value.
DISCUSSION Total accumulated
sediment
To e v a l u a t e the f i n e - g r a i n e d s e d i m e n t b u d g e t of the Sylt-Romo tidal basin it is comp a r e d with the net b u d g e t for the Gr~dyb tidal area e s t a b l i s h e d by Bartholdy & PheifferM a d s e n (1985). T h e s e authors found a total a c c u m u l a t i o n of f i n e - g r a i n e d s e d i m e n t of 142 9 103 t/year, half of which was d e p o s i t e d on the salt marsh and half on the m u d fiats. In the p r e s e n t study, an a c c u m u l a t i o n of 9 - 1 0 3 t / y e a r was e s t i m a t e d for the salt marsh and about 49 9 103 t / y e a r for the intertidal flats. C o r r e c t i n g for the different areas c o v e r e d by salt m a r s h and intertidal flats in the two tidal areas, the a v e r a g e a c c u m u l a t i o n rate on the salt m a r s h e s of the Sylt-R~m~ tidal basin corresponds to only half the a v e r a g e v a l u e of the Gr~dyb tidal area. Furthermore, the a v e r a g e a c c u m u l a t i o n rates on t h e intertidal flats are about 9 times h i g h e r in the Gr~dyb tidal basin c o m p a r e d to those m e a s u r e d for the Sylt-Rome tidal basin. T h e s e differences can partly be e x p l a i n e d by the h i g h e r per-
S e d i m e n t b u d g e t for Sylt-R~mo tidal basin
265
c e n t a g e of sheltered salt marshes a n d intertidal flats in the Gr~dyb tidal basin. Thus, the morphology of the tidal area is shown to be a major d e t e r m i n a n t for a c c u m u l a t i o n of fineg r a i n e d sediments. This is consistent with suggestions m a d e by Bartholdy & PheifferM a d s e n (1985) who a r g u e d that a c c u m u l a t i o n rates lower than those found in the Gr~dyb tidal b a s i n can be expected in more exposed tidal basins. Based on this comparison the results o b t a i n e d by F l e m m i n g & N y a n d w i (1994) are likely to represent a g e n e r a l t r e n d in the depositional p a t t e r n of f i n e - g r a i n e d s e d i m e n t in the W a d d e n Sea. Trapping efficiency To facilitate the comparison of f i n e - g r a i n e d s e d i m e n t a c c u m u l a t i o n in different tidal basins, Bartholdy & Pheiffer-Madsen (1985) defined the value C as a g e n e r a l m e a s u r e of t r a p p i n g efficiency, where: C=
yearly a c c u m u l a t e d s e d i m e n t yearly e x c h a n g e d w a t e r volume
In this m a n n e r different tidal basins of very different size can be compared. This concept was a d a p t e d by Larsen et al. (1996) who estimated C to be 0.7 g/m 3 for the K6nigshafen tidal basin, a small s u b c a t c h m e n t of the Sylt-Romo tidal basin. This compares with values for Jade Bay of 1.8 g/m 3, Dollard 3.1 g / m 3 a n d Gr~dyb 1.5 g / m 3 (cf. Bartholdy & Pheiffer-Madsen, 1985). The C-value c o m p u t e d for the Sylt-Rgmo tidal b a s i n is 0.2 g / m 3, s u g g e s t i n g that net deposition for the a v e r a g e v o l u m e of water e x c h a n g e d with the North Sea is n i n e times smaller than that for the Gr~dyb tidal basin. However, w h e n calculating in this way one c a n n o t speak of t r a p p i n g efficiency with regard to water e x c h a n g e with the sea b e c a u s e the supply from all s e d i m e n t sources is included. A n o t h e r i n d e x for comparison of t r a p p i n g efficiency with regard to tidal e x c h a n g e of sea water could be defined as: E=
Yearly a m o u n t of s e d i m e n t deriving from the sea Yearly e x c h a n g e d w a t e r volume
In this way, t r a p p i n g efficiency b e t w e e n the Gr~dyb a n d the Sylt-Rom~ tidal basins can be compared, the E values b e i n g s h o w n in Table 8. The t r a p p i n g efficiency computed in the m a n n e r s u g g e s t e d above can be considered a m i n i m u m value, the input from the sea b e i n g c o m p u t e d as the residue b e t w e e n total net a c c u m u l a t i o n a n d supply from other s e d i m e n t sources. It would a p p e a r that the differences in t r a p p i n g efficiencies b e t w e e n the Gr~dyb a n d the Sylt-R~mo tidal b a s i n Table 8. Relative accumulation (C) and trapping efficiency (E) for Gr~dyb and Sylt-R~m~ tidal basins Tidal area
C value (g/m ~)
E value (g/m 3)
Gr~dyb Sylt-Romo
1.46 0.16
1.23 0.10
266
M. P e j r u p , M. L a r s e n & K. E d e l v a n g
c h a n g e f r o m a f a c t o r of n i n e to a f a c t o r of t w e l v e w h e n c o m p a r i n g C a n d E - v a l u e s res p e c t i v e l y . S i n c e s e d i m e n t c o n c e n t r a t i o n s i n t h e G r ~ d y b a n d t h e Lister D y b t i d a l i n l e t s a r e fairly similar, t h e E i n d e x s e e m s to b e b e t t e r s u i t e d to e x p r e s s t h e t r a p p i n g e f f i c i e n c y of a t i d a l b a s i n . CONCLUSION T h e S y l t - R o m o t i d a l b a s i n is a n e t s i n k for f i n e - g r a i n e d s e d i m e n t . As a y e a r l y a v e r a g e , 58 9 103 t of f i n e - g r a i n e d s e d i m e n t s a r e d e p o s i t e d in t h e t i d a l b a s i n itself a n d o n salt m a r s h e s f r i n g i n g it. A b o u t 9 9 103 t / y e a r d e p o s i t o n t h e salt m a r s h a n d a b o u t 4 9 9 103 t / y e a r o n t h e i n t e r t i d a l flats. T h e m a i n s i n g l e s e d i m e n t s o u r c e is t h e N o r t h Sea, c o n t r i b u t i n g 64 % to t h e n e t s e d i m e n t b u d g e t . O t h e r i m p o r t a n t s o u r c e s a r e f l u v i a l i n p u t (14 %) a n d prim a r y p r o d u c t i o n (15 %). Of m i n o r i m p o r t a n c e to t h e a c c u m u l a t i o n of f i n e - g r a i n e d s e d i m e n t is t h e e r o s i o n of salt m a r s h d e p o s i t s (5 %) a n d a t m o s p h e r i c d e p o s i t i o n (2 %). T h e t r a p p i n g e f f i c i e n c y w i t h r e g a r d to w a t e r e x c h a n g e b e t w e e n t h e N o r t h S e a a n d t h e SyltR o m o t i d a l b a s i n is v e r y l o w c o m p a r e d to t h a t of, for e x a m p l e , t h e G r ~ d y b t i d a l b a s i n , t h e D o l l a r d e s t u a r y a n d J a d e Bay. T h i s r e s u l t s u p p o r t s s u g g e s t i o n s m a d e b y o t h e r a u t h o r s t h a t i n t e n s i v e d i k i n g is o n e m a j o r c a u s e of d i m i n i s h e d d e p o s i t i o n of f i n e - g r a i n e d s e d i ment within a tidal basin. However, another important factor influencing deposition r a t e s is t h e e x p o s u r e of t h e i n t e r t i d a l flats to t h e p r e v a i l i n g w e s t e r l y w i n d s . It m a y t h e r e fore b e c o n c l u d e d t h a t t h e a m o u n t of f i n e - g r a i n e d s e d i m e n t d e p o s i t e d i n a t i d a l b a s i n is m a i n l y a f u n c t i o n of p h y s i o g r a p h i c a l a n d h y d r o d y n a m i c a l p a r a m e t e r s a n d to a l e s s e r d e g r e e of s e d i m e n t availability.
Acknowledgements. This study was supported by the Federal Ministry for Education, Science, Research and Technology, Bonn, {BMBF) as part of the "Ecosystem Research in the Schleswig-Holstein W a d d e n Sea". The authors want to express their gratitude to all those involved in this project who h a v e contributed to this paper by way of data, e x c h a n g e of knowledge or discussions. Special thanks are due to Klaus Bayerl and Dagmar Lackschewitz for providing most of the data used for the sedim e n t map. This is publication no. 269 of the project "Ecosystem Research Wadden Sea".
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