Abstract

A net budget for fine-grained sediment (<63 pm) has been computed for the Königshafen tidal area; a small tidal bay covering an area of some 4 km², located in the Wadden Sea at the north-eastern end of Sylt. The net budget for fine-grained sediment has been separated into the following sediment sources: (1) Supply from the North Sea, (2) Salt marsh erosion, (3) biological primary production and (4) atmospheric deposition. The total net accumulation within the area has been calculated by geomorphological mapping combined with estimates of fine-grained accumulation rates determined from 2!O_Pb core dating. The net accumulation was calculated to 1400 t/y based on 2in_Pb dating. The salt marsh erosion is measured to supply 16% of this sediment, and from literature studies primary production and atmospheric deposition are estimated to supply 7% and 1%, respectively. The remaining 76 % between the three above mentioned sediment sources and the total net accumulation originates from the North Sea and from fluvial input to the adjacent tidal area. A minimum of 70 % of the total supply of fine-grained sediment accumulated mulatedin Königshafen originates from the North Sea. It is shown that on a yearly basis an average of 0.5 g/m³ is filtered out of the water exchanged between Königshafen and the North Sea. Although this is a small quantity compared to other tidal areas, the trapping efficiency is shown to be relatively high compared to other areas. It is suggested that this difference may be explained by the influence ofbenthic organisms on the aggregation of fine-grained suspended sediment.

Keywords

210Pb dating, fine-grained sediment, sediment budget, salt marsh
erosion, turbidity, trapping efficiency.

Michael Larsen, Morten Pejrup & Karen Edelvang: Institute of
Geography, University of Copenhagen, Øster Voldgade 10, 1350
Copenhagen K., Denmark.

Danish Journal of Geography 96: 1-10, 1996.

Normally, the concentration of fine-grained sediment suspended in the Wadden Sea region increases from the North Sea through the tidal inlets reaching a maximum in the inner part of the tidal area. Typical concentrations of finegrained suspended sediment in the North Sea are 0-2 mg/1 (Eisma 1981), in the tidal inlets of the Wadden Sea about 10 mg/1 (Postma 1981), and in the inner parts of the Wadden sea 50-600 mg/1 (Pejrup 1980; de Haas & Eisma 1993). This longitudinal distribution of concentrations may indicate that the fine-grained suspended sediment in the inner part of the Wadden Sea originates from sediment sources within the area. However, Bartholdy & Madsen (1985) found that 85% of the fine-grained sediment deposited in the Grådyb tidal area comes from the North Sea.

Accumulation of cohesive sediment in estuarine environmentsis
influenced by a number of different processes.
Van Straaten & Kuenen (1957, 1958), and Postma (1967)

tainingfine-grainedsediment during inundation period
(Pethick 1984; Frey & Basan 1978).

Based on morphological mapping, 210_Pb core dating, am grain size analysis, this paper presents a net budget fo fine-grained sediment from a small tidal bay with lov concentrations of suspended sediment but relatively higl accumulation rates of fine-grained sediment.

Study Ares

The island of Sylt is situated in the northern part of the Wadden Sea. Königshafen is a small tide-influenced baj located at the northern end of Sylt (fig.l). The morphology of the area as it appears today was formed from the middl< of the 17th century to the end of the 18th, according to ok maps and geological findings (Bayerl & Higelke, 1994).

Figure 2 shows the geomorphology of the bay. To the west and north it is protected from the North Sea and the prevailing westerly winds by sand dunes. To the south it is protected by the core of the island, making it exposed onl> to the east. The tide in the area is semi-diurnal with £ mean tidal range of 1.8 m. The total area potential!} influenced by the tide is approximately 4 km², of which the sandy tidal flats cover 2.9 km², and muddy tidal flats and tidal channels 0.2 km² each. The supratidal salt marsh areas cover the remaining 0.7 km².

The surface sediments in Königshafen are described by Austen 1., (1994). She found that the sandy tidal flats contain less than 10% of fine-grained sediment and that the muddy tidal flats contain 10 - 50% fine-grained sediment. The salt marsh contains 93-95% fine-grained sediment.

Methods

210 Pb Core Dating

The net accumulation of fine-grained sediment can be determined by using the 210_Pb dating technique validated on fine-grained sediment by e.g. Madsen & Sørensen (1979). This dating method has been used in earlier studies on the accumulation of fine-grained sediment in the Grådyb tidal area (e. g. Madsen 1981; Bartholdy & Madsen 1985).

Along the profile-line in figure 2, three sediment cores were collected. On the salt marsh two sediment cores were taken. One core was taken close to mean high tide level, and another was taken at a slightly higher level in the

central part of the salt marsh. To avoid compaction, these cores were cut out of the side of a hole digged in the salt marsh. The core on the tidal mud flat was taken by penetrating an aluminium tube into the sediment (figure 3). Before pulling out the tube, it was filled with water and sealed with an expandable rubber plug. The plug creates a vacuum within the tube thus enabling it to be pulled out leaving the sediment almost undisturbed inside the tube.

The cores from the salt marsh were 30-40 cm long, which is about the maximum depth possible for 210_Pb-dating with an average deposition rate of 1-2 mm/year. The core from the tidal mud flat was 68 cm long with a compaction of 4 cm. By averaging the compaction throughout the whole core each cm represents 72/68 cm = 1.06 cm, which makes the error made by the compaction insignificant.

The three cores were divided vertically into equal halves.
Each half was cut horizontally into 1 cm pieces. One half
was archived for grain-size analyses. The activity of 210Pb

was measured on selected subsamples from the other half. 210_Pb is a natural, radioactive isotope. The content of 21()_Pb in the sediment can be divided into two different parts; supported and unsupported 2K)_Pb. Both parts originate from the decay of 226_Ra, which is a natural component of the sediment. The supported 210_Pb originates from the decay of 226_Ra within the sediment. The unsupported 210_Pb originates from the decay of the gas 222_Rn, which is a daughter isotope of 226_Ra emitted from the continental crust. The unsupported 210_Pb is washed out of the atmosphere with precipitation. When introduced into the marine environment it adheres to the surface of cohesive sediment particles. When the sediment is deposited, further adhesion of 210_Pb stops. The age of different levels in the sediment can then be calculated by measuring the vertical activity profile of the unsupported 210_Pb, knowing the half-life of 2l°Pb to be 22.3 years. The background activity of the supported 210_Pb was determined by measuring the activity deep in the core, where the unsupported 210_Pb had decayed totally.

To calculate the age of different levels in the cores, the constant rate of supply method (C.R.S.) was used. This method assumes that the flux of unsupported 21()_Pb is constant in time. Even in cases where this condition is not strictly fulfilled, the C.R.S. method can be used ( e.g. Appleby & Oldfield 1978; Madsen & Sørensen 1979; and El-Daoushy 1988). This is because mixing caused by physical and biological processes causes differences in the upper part of the sediment core to be levelled out as the contributions from several years are mixed before the vertical profile of the 210_Pb is established beneath the mixing layer.

Sediment Analyses

Subsamples from the second half of the core were used for grain-size analyses. The samples were divided into two fractions by wet-sieving the sediment through a 63 urn sieve. The fraction >63 jam was weighed and sieved at 1A ø intervals.

The fine fraction (< 63 urn) was suspended in 0.002 M Na₄P₂O₇ and stirred for 2 minutes with ultrasound. The grain-size analysis was carried out by the use of a Sedigraph Model 5100. The Sedigraph calculates the fine fraction of the sediment in equivalent Stoke diameters. The sediment is suspended in a sedimentation cell and the dampening of a soft X-ray beam is measured during sedimentation.

Measurement of Salt Marsh Erosion

Erosion of the salt marsh was measured by use of two dif-

ferent methods. One was to establish iron sticks in the erosional salt marsh cliff. Five iron sticks were established in Königshafen along a representative part of the erosional salt marsh. The measuring site is marked in figure 2.

The iron sticks are l m long with a diameter of 12 mm. They were initially driven 0.9 m into the salt marsh cliff. In this way, the erosion of the cliff could be surveyed directly by measuring the length of the stick exposed by erosion. Measurements were carried out every 2-3 months throughout the year. The average height of the salt marsh cliff was determined, and the total erosion was computed by multiplying the length of the representative cliff, the height of the cliff, the erosion measured by the iron sticks, and the dry bulk density of the salt marsh sediment. The bulk density of the salt marsh was determined by taking an undisturbed sediment sample of 100 cm³ with an aluminum ring at the same site as the erosion measurements were carried out.

The other method for measuring cliff erosion was to survey the edge of the salt marsh at the measuring site with a theodolite Topcon BS 3. This theodolite uses infrared light to measure the horizontal distance between the optic and a reflector. The reflector was placed at points along the edge of the salt marsh. The distance between two points was at the most 1 m. The edge-lines from 1992, 1993 and 1994 are shown in figure 6. The area between the lines was measured, and the erosion volume between two measurements could be calculated when knowing the average height, length, and dry bulk density of the cliff.

Results

Net Accumulation

The dating of different levels in the cores is shown in figure 4a-c, and the vertical distribution of sand, silt, and clay in the profiles is shown in figure sa-c. These figures are based on results from Pejrup & Bartholdy (1991). Figure 5c shows a marked break in the grain-size distribution between 12 and 15 cm, the age of this layer is dated back to 1870 ± 68 years. This is approximately the age of the formation of the present Königshafen (Bayerl & Higelke 1994).

The total accumulation rate shown in table 1 refers to all grain-size fractions and includes organic matter as well. The average accumulation rate (mm/y) corresponds to the linear deposition rate in the top layer (0-1 cm) computed

from the bulk density at this level. The mixing depth is the thickness of the top layer that is regularly mixed, either due to resuspension caused by wave activity or mixing caused by animals living in the sediment. By dividing the mixing depth with the linear accumulation rate, it appears that it will take about 4 years at an average to get the sediment buried beneath the mixing layer on the tidal mud flat and 9 to 16 years on the salt marsh.

The percentage of silt and clay in table 1 also corresponds to the top layer values (0-1 cm). When the sediment is compressed, the linear accumulation rate will be much smaller than indicated. To avoid this problem, the accumulation rate in kg/m²/y is used instead. The accumulation of fine-grained sediment is obtained by multiplying the total accumulation rate by the percentage of fine-grained particles in the sediment. The fairly constant content of clay and silt within the topmost 10-15 cm of the cores, as appears from figures sa-c, justifies this computation.

The accumulation rates of fine-grained sediment in the different geomorphological units are listed in table 2. The areas of the units were measured on the geomorphological map (figure 2). The accumulation rates of the salt marsh area are taken as the average value of core II and 111. This is justified by the apparent one-dimensional distribution of the accumulation rate along the profile line, with the highest values close to the high-water line decreasing with increasing level of the salt marsh surface away from the erosional salt marsh cliff. Core I is taken as representative for the muddy tidal flats. In this way, the total accumulation of fine-grained sediment in Königshafen was determined to 1400 t/y. This figure is about one third lower than the total accumulation for the area presented by Pejrup and Bartholdy (1991). The higher value from the latter investigation is caused by a relatively rough estimate of the area of accumulation and furthermore, accumulation rates were not corrected for content of fine-grained sediment in the collected sediment samples.

Salt Marsh Erosion

The map of the edge-lines from the survey is shown in figure 6. The retreat of the salt marsh is clear, the average retreat of the coast line being approx. 0.25 m/y, at selected places up to 1 m/y.

The amounts of eroded material computed by the two methods are listed in table 3. It appears that the iron stick method underestimates the erosion rate 3-4 times, taking the survey method to be the most reliable. This underestimationis

mationisdue to the different numbers of points measured by the two methods. The iron sticks have caused problems because each stick has disappeared once during the two years of investigation leaving a gap in the sequence of measurements. However, this error can not account for the underestimation alone. The underestimation by a factor 3-4 by the iron stick method is also found in other areas with salt marsh erosion in the Lister Tief area (Larsen et al. 1994).

The theodolite surveys were only carried out once a year, whereas the iron sticks were measured every 2-3 months or just after major storm events. Therefore, the iron sticks are suitable to describe the seasonal variation in erosion, whereas the surveys give a much more exact quantitative measure because of the larger number of measuring points. The results from the successive surveys were used to determine the yearly erosion rate.

The investigated salt marsh to the south is 600 m long
with an average height of 0.61 m, the erosional part of the
salt marsh to the west is 1300 m long with an average
height of 0.15 m, the salt marsh to the north is protected
from erosion by sand dunes.

The content of fine-grained sediment in the salt marsh is
94% and the bulk density of the salt marsh is 1010 kg/m3.

erncliffis computed as 0.15/0.61 • 0.256 = 0.063 m3/m/y.m3/m/y.
The erosion was calculated as:

(length)-(mean erosion)-(bulk density )-(percentage offmes)

This gives a total erosion volume of the southern cliff of 146 t/y and a total erosion volume of the eastern cliff of 78 t/y giving a total salt marsh cliff erosion of 224 t/y. This corresponds to about 16 % of the total accumulation of fine-grained sediment within the area of investigation.

Primary Production

In the Königshafen tidal area, Asmus & Asmus (1985) distinguished 3 types of bottom communities: The Neireis- Corophium-belt, the seagrass bed, and the Arenicola-flat. They measured gross primary production rates for these 3 communities to be 148,473 and 152 g C/m²/y respectively.

In the Dollard estuary van Es (1977) estimated the pelagic primary production to be 7.5 gC /m²/y and values for the benthic primary production on the tidal flats to be 116 gC /m²/y. Thus, the total gross primary production was estimated to 123.5 gC /m²/y, of which at least 90% is mineralized (van Es, 1977) before final burial beneath the benthic mixing layer. This leaves a maximum value of 12 gC /m²/y to contribute to the net accumulation of finegrained sediment. This is equivalent to about 24 g/m²/y of organic matter.

In the Grådyb tidal area, Bartholdy & Madsen (1985) used a value of net primary production in the Grådyb tidal area of 6000 t/y, equivalent to 44 g/m²/y. This value was based on unpublished data and on the assumption that 90 % of the gross primary production was mineralized. Information on the relative distribution between benthic and pelagic contribution was not offered.

Today, the Königshafen tidal area is mostly characterized by the Neireis-Corophium-belt and the Arenicolaflats. Therefore, a mean value of the gross primary production of 150 g C/m²/y from these two biotopes has been used in this study. A minimum of 82 % of this productivity is caused by microbenthos and microphytobenthos (Asmus & Asmus 1985). On the assumption that 90 % is mineralized, this will give a net input of organic matter of 30 g/m²/y. This value is fairly consistent with the findings by van Es (1977). Taking the whole intertidal and subtidal area of 3.3 km² as potentially productive, this will give a net input of organic matter of 99 t/y to the Köninghafen tidal area.

According to results from the Dollard estuary (van Es,
1977) the supply from biological production on the salt
marsh only amounts to 0.6 % of the total supply of organic

C, and this contribution is therefore considered insignificant.

Atmospheric Deposition

The supply from the atmosphere to the net sediment budget originates primarily from aerosols of insoluble matter. These primarily originate from clay minerals and combustion of coal. The primary chemical constituents are Al, Ca, Fe, and Mg. Schulz (1993) presented results on the concentrations in rainwater of such chemical compounds, measured throughout the year, from Westerhever Lighthouse on Eiderstedt some 50 km south of the Königshafen area. Based on these results, a wet deposition rate of about 1.7 t/km²/y can be estimated. To this figure should be added dry deposition equivalent to a maximum of 30 % of the wet deposition (Schulz 1993). In this way a maximum value of total atmospheric input of 2.2 t/km²/y can be estimated.

These new figures can be compared to older ones presented by McCave (1973), estimating a net atmospheric deposition for the whole North Sea area to be 1.6*10⁶ t/y. The total North Sea is 575*10³ km² (McCave, 1973), which gives an atmospheric deposition rate of 2.8 t/km²/y. When using the values from Schulz (1993), an atmospheric deposition of about 9 t/y can be estimated for the Königshafen area amounting to about 1 % of the total net budget of fine-grained sediment.

Discussion

The net budget of fine-grained sediment for the Königshafen tidal area is shown in table 4. Local erosion of salt marsh is seen to contribute 16 %, whereas primary production is estimated to contribute 7 % of the net budget. Atmospheric deposition is of little significance, contributing only about 1% . The remaining 76 % originates from the North Sea. However, it cannot be excluded that a minor part of this contribution stems from river input to the Sylt- Rømø tidal area. According to Pejrup et al. (1995) the net fluvial input of fine-grained sediment to the Sylt - Rømø tidal area is about 8000 t/y. If this is evenly distributed over the tidal area, about 1/100 or 80 t/y would deposit in the Königshafen area. This is only about 6 % of the total net budget, and it can therefore be concluded that at least 70 % of the fine-grained sediment accumulating in the investigated area originates from the North Sea.

The mean tidal volume in Königshafen is obtained from the hypsographic curve in figure 7, the mean tidal range is 1.8 m, mean low water is -1.0 m and mean high water is 0.8 m giving a mean tidal prism of 3.6* 10⁶ m³. Averaged over a year, the amount of sediment settling out of each m3m³ of water exchanged between Königshafen and the adjacent North Sea can thus be calculated to 0.5 g/m³.

An investigation of the turbidity in Königshafen made by Austen G. (1994) shows that in weather conditions with strong westerly winds, the turbidity is low and varies between tween5-20 mg/1. Austen G. (1994) found that the tidal variation in hydrodynamics and turbidity in Königshafen show the same pattern as in most other tidal areas in the Wadden Sea. Therefore, the low turbidity values described in Königshafen must be caused by its position close to the Lister Tief tidal inlet. Based on the above mentioned typical suspended concentrations it can be concluded that maximum values of filtering efficiency of the water flowing in and out of the Köningshafen area will be in the range of 3-10 % of the total fine-grained suspended sediment transport.

To compare the filtering efficiency in absolute values for different tidal areas in the Wadden Sea, a relation between yearly accumulation of fine-grained sediment and yearly total tidal volume was suggested by Bartholdy & Madsen (1985):

In table 5 values of the C index from Jade, Dollard, and Grådyb are compared with the value for Königshafen. It appears that the C value for Königshafen is low compared with other areas. Although these other areas are much larger, the relative distribution between areas covered by salt marsh, intertidal flats and tidal channels are comparable. parable.Therefore, the low C value for Königshafen is probably explained by the generally low values of suspended sediment in the bay caused by its position close to the Lister Tief tidal inlet, because the actual percentage of sediment deposited from water entering the bay is fairly high.

Königshafen is densely inhabited by benthic filter feeders and deposit feeders (Reise et al. 1994). Briggs & Ho well (1984) stated that the trapping efficiency of estuaries is connected to benthic filter feeders forming faecal pellets of the fine-grained material and thereby enhancing the settling velocity of the fine-grained sediment. The measured accumulation rates of fine-grained sediment in Königshafen are comparable to accumulation rates from the Grådyb tidal area reported by Bartholdy and Madsen (1985). The comparison between C values and accumulation rates for Königshafen and Grådyb suggests that the impact of benthic organisms on the net accumulation of fine-grained sediment is much larger in Königshafen than in Grådyb.

Conclusion

Based on morphological mapping and 210Pb core dating,
the net accumulation of fine-grained sediment in the
Königshafen tidal area was determined to be 1400 t/y.

The contribution from the North Sea amounts to 70-76 % of the total supply, whereas local salt marsh erosion is estimated to account for 16 % and primary production and atmospheric depositon for 7 % and 1 %, respectively.

In spite of low turbidity values, the accumulation rates in Königshafen are comparable to other tidal areas in the Wadden Sea. This is explained by a high trapping efficiency in the bay that may be caused by the dense populations of benthic organisms.

Acknowledgements

This study was carried out as part of the large interdisciplinary project "Sylter Wattenmeer Austaush Prozesse". We are deeply indebted to the Ministerium für Forschnung und Technologie (BMFT), Bonn, for providing the funds for this project. The authors thank Jesper Bartholdy for sampling the two cores on the salt marsh used for 210_Pb dating and grain size analysis and one core on the tidal flat used for grain-size analysis. Furthermore we wish to thank Michael Ansbak Petersen, Mette Bradam Hansen, Anette Lützen Møller and Thorbjørn Andersen for helping with the field work and Charlotte Bak for doing the major part of the grain-size analyses.

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