Ho Bugt Research Articles

Aeolian sand movement and relative sea-level rise in Ho Bugt, western Denmark, during the `Little Ice Age'

This study documents an extensive aeolian sand sheet buried in the salt marshes of the Ho Bugt embayment in western Denmark. The extent of the sand sheet was mapped from c. 60 cores and sections, which were also used to reconstruct late-Holocene changes in relative sea level based on diatom analyses. Chronology was provided by optically stimulated luminescence (OSL) and radiocarbon dating. Our study shows that since 2000 cal. yr BP relative sea level in Ho Bugt has risen by approximately 1.5 m. The overall rise was punctuated by two periods with higher than average rates of sea-level rise, the first between 2000 and 1400 cal. yr BP and the second commencing c. 500 cal. yr BP. Around 700 cal. yr BP the evolution of the Skallingen Spit may have been responsible for a reduction in the local tidal range and the formation of an extensive `black layer' in the western part of the embayment. OSL analyses date the sand sheet to between AD 1460 ± 40 and AD 1550 ± 30 (490 ± 40 and 400 ± 30 cal. yr BP), consistent with a period of increased storminess, coastal dune building, saltmarsh formation and increased relative sea-level rise during the early part of the LIA. Other studies have ascribed formation of coastal dunes along western European coastlines during the `Little Ice Age' (LIA) to a combination of increased storminess and low relative sea level. In contrast, we conclude that during the LIA in western Denmark, storminess, transgressive conditions and relative sea-level rise, rather than a sea-level low stand, were important contributing factors to coastal sand movement and dune formation.

Salt-marsh diatom distributions in Ho Bugt (western Denmark) and the development of a transfer function for reconstructing Holocene sea-level changes

Modern diatom samples and associated environmental variables were collected from the Kjelst Enge and Moservå salt marshes in the northern part of Ho Bugt, western Denmark. The relationship between diatom assemblages and elevation was investigated in order to assess the potential use of diatoms as sea-level indicators. Diatoms were found to be well preserved across the salt-marsh surface. A total of 54 diatom taxa were found with relative abundances greater than 3%. Cluster analysis reveals the presence of three vertical assemblage zones. The relationship between diatoms and environmental variables (elevation, pH, salinity) was explored using Canonical Correspondence Analysis (CCA). A series of partial CCAs shows that 24% of the explained variance can be attributed to elevation relative to tidal levels. Associated Monte Carlo permutation tests demonstrate that this relationship is significant ( p = 0.014). We developed weighted averaging (WA) and weighted-averaging partial least squares (WA-PLS) transfer functions from 40 surface diatom samples. The WA-PLS transfer function was found to perform best ( r 2 jack = 0.90, root mean square error of prediction jack = 0.14 m) and was subsequently applied to reconstruct salt-marsh surface elevations from the diatom assemblages in a core from Kjelst Enge salt marsh. The results indicate that with appropriate chronologic control it will be possible to establish a high-precision record of changing sea level from this core.

Wind influence on tidal flat sediment dynamics: Field investigations in the Ho Bugt, Danish Wadden Sea

Sediment dynamics on a mixed microtidal flat in a sheltered backbarrier position are shown to vary strongly on short term, tidal, and monthly timescales. Bed level changes were ± 1 mm per tidal period in the absence of winds. Erosion of 10 to 20 mm within a few hours was possible during high combined current and wave shear stress. Mean grain size in the upper 1 cm bed sediment varied from 34 to 155 μm and was consistently coarser than the mean size of trapped suspended matter (7–10 μm). Threshold for resuspension was low at 0.03 N m − 2 . No clear correlation was found between resuspension activity and suspended matter concentrations as these concentrations also depend on current induced advection. Due to tidal asymmetry, the net suspended sediment flux was flood directed. On average it was 14 kg m −2 per tidal period in the summer and 3 times higher during November when wave activity generally was stronger. Bed micro topography, was followed on digital photos taken every 1.5 h during the two month study period, showed that (1) ripples started to develop when wave shear stress exceeded 0.15 N m − 2 , and (2) their wavelength varied with shear stress.

Late Holocene sea-level changes and isostasy in western Denmark

Cores and exposed cliff sections in salt marshes around Ho Bugt, a tidal embayment in the northernmost part of the Danish Wadden Sea, were subjected to 14C dating and litho- and biostratigraphical analyses to reconstruct paleoenvironmental changes and to establish a late Holocene relative sea-level history. Four stages in the late Holocene development of Ho Bugt can be identified: (1) groundwater-table rise and growth of basal peat (from at least 2300 BC to AD 0); (2) salt-marsh formation (0 to AD 250); (3) a freshening phase (AD 250 to AD 1600?), culminating in the drying out of the marshes and producing a distinct black horizon followed by an aeolian phase with sand deposition; and (4) renewed salt-marsh deposition (AD 1600? to present). From 16 calibrated AMS radiocarbon ages on fossil plant fragments and 4 calibrated conventional radiocarbon ages on peat, we reconstructed a local relative sea-level history that shows a steady sea-level rise of 4 m since 4000 cal yr BP. Contrary to suggestions made in the literature, the relative sea-level record of Ho Bugt does not contain a late Holocene highstand. Relative sea-level changes at Ho Bugt are controlled by glacio-isostatic subsidence and can be duplicated by a glacial isostatic adjustment model in which no water is added to the world's oceans after ca. 5000 cal yr BP.

Optically stimulated luminescence dating of young estuarine sediments: a comparison with 210Pb and 137Cs dating

A sediment core taken from a tidal mudflat in Ho Bugt in the northernmost part of the Wadden Sea in Denmark is used to explore the application of optically stimulated luminescence (OSL) dating to young fine-grained estuarine sediments, using 210Pb dating and the 137Cs Sellafield peak from 1980 as independent age control. Despite the anticipated difficulties of weak luminescence signals and incomplete resetting of residual radiation dose prior to deposition, the OSL ages from the sediment core ranged from 7.0±1.5 at the surface to 305±16 years (68 cm depth). OSL- and 210Pb-dates were in good agreement back to ∼1975, and even as far back as ∼1945 using the CRS-model. The average OSL age of 9±3 years for the surface mixing zone indicates that the OSL signal of the quartz grains was well zeroed at deposition. The OSL ages give new information on past deposition regimes on the Ho Bugt mudflat: fine-grained sediment started to deposit at ∼0.5 mm year −1 approximately 300 years ago, when a terrestrial reed-swamp was transgressed. About 100 years ago, the sedimentation rate increased to ∼4 mm year −1. Approximately 40 years ago, the sediment became more fine-grained, perhaps because of emergence of the tidal flat; the accretion rate at that time was already well above the local rate of sea-level rise of ∼1 mm year −1. The post-1980 accretion rate was found to be ∼16 mm year −1 with OSL dating and ∼9–12 mm year −1 with 210Pb and 137Cs dating. Advantages of the OSL method include applicability over a much larger time-span (∼125 ka compared to ∼120 years for 210Pb) and a lack of significant dependence on changes in mean grain size or 210Pb supply. With this method, it is also possible to date sediment cores from sand flats, providing a new approach to the problem of evaluation of stability and calculation of sediment budgets for estuaries and coastal lagoons.

Salt-marsh foraminifera in Ho Bugt, western Denmark, and their use as sea-level indicators

Twenty-seven surface samples were collected from the Kjelst Enge salt marsh in Ho Bugt, Jutland, to investigate the potential use of salt-marsh foraminifera as sea-level indicators in Holocene intertidal deposits. Previous work from many coastlines around the world has shown that the vertical distribution of salt-marsh foraminifera is strongly controlled by the duration of tidal flooding (or height relative to tide level). Concentrations of foraminifera in Kjelst Enge salt marsh are low, especially in the high marsh, but a strong correlation was found between relative abundances of foraminiferal species and tidal elevation for 16 samples containing more than 30 specimens per 5 cc of sediment (r2 = 0.80, root mean square error = 0.16 m). Sediment pH also has a strong influence on the distribution of the foraminifera, but salinity appears to be insignificant.

Large-scale trends in some mass physical properties of Danish Wadden Sea sediments, and implications for organism-sediment interactions

Mass physical properties (wet/dry bulk densities, mud/sand contents and concentrations, water content, shear strength) were determined on 132 surficial sediment samples from intertidal flats in Ho Bugt and north of Langli Island between the Hjerting Løb tidal channel and the Skallingen Peninsula, Danish Wadden Sea. Except for shear strength, all parameters were found to be highly intercorrelated (r<0.97), and the 12 equations thus generated can be used conveniently for regional modelling purposes. Investigations which rely on small sample numbers can be optimised by prior pilot studies in which average conditions are defined along environmental gradients and, once suitable in-situ calibrations have been performed, the method can be expanded to other parameters. Water content is a particularly useful predictor because it is a universal master variable by which differences in compaction between different environments can be normalised. Shear strength values show large variations at low mud contents, which decrease sharply towards higher mud contents. Nevertheless, shear strength appears to decrease with increasing mud content, a trend observed in spatial plots of comparative data from the German Wadden Sea. This suggests that the data comprise both spatially coherent and incoherent values. Similarly to the German Wadden Sea, counter-intuitive convex mud concentration curves were recorded along sand-mud gradients, peaking at intermediary mud contents in mixed sediments. Based on a growing body of published literature, interpretations of these novel trends suggest far-reaching implications for organism-sediment interactions.

Tidal and meteorological induced nutrient (N, P) dynamics in the micro-tidal Ho Bugt, Danish Wadden Sea

Tidal and meteorological induced variations in nutrient dynamics are important at all depth intervals in the Ho Bugt estuary. In the tidal channels, nitrate concentrations may vary ±20% and total nitrogen concentrations even more due to sedimentation/resuspension effects during the tidal cycle. On tidal flats, nutrient and carbon contents of the deposited material depend strongly on meteorological conditions and may vary 0.7% TC, 0.1% TN and 0.4%c TP from week to week. Trapped material 0.65 m above the bottom of the tidal flat always contained more nutrients than tidal flat sediments indicating that fine-grained material does not stay for long on the bottom. On salt marshes, the nutrient content of deposited material increases from the salt marsh edge (5 g N kg−1 and 1.5 g P kg−1) to the distal part (9 g N kg−1 and 2.5 g P kg−1), whereas seasonal variations appear to be small. Total (tidal flat+salt marsh) deposition of nutrients for the area is estimated to 426 ton N y−1 and 117 ton P y−1. This may be compared to a terrestrial input of 2200 ton N y−1 and 48.5 ton P y−1 indicating that the estuary acts as a filter for the terrestrial supply to the open sea and that import from the open sea plays cm important role in the nutrient budget especially for the particle bound P.

Parameters affecting fine-grained suspended sediment concentrations in a shallow micro-tidal estuary, Ho Bugt, Denmark

Many shallow estuarine environments contain considerable quantities of fine-grained sediments. It is difficult, however, to determine theoretically the concentration and transport of fine-grained sediments in such environments, because the fine particles flocculate and because the suspended sediment concentration is dependent on a number of different parameters. The most important of which are: (1) bottom shear stress (current velocity), (2) wind speed and direction, and (3) salinity. In this study the relative importance of these parameters is evaluated in relation to a shallow micro-tidal environment, Ho Bugt, Denmark. Furthermore, a mathematical model explaining the variation in the concentration of fine-grained suspended sediments at a station within this area was created and calibrated to measurements of wind speed and direction, salinity and tidal current velocity. The model describes about 80% of the variance of the suspended sediment concentration at the site of observation.

The intertidal sediments

Jacobsen, N. Kingo: The intertidal sediments. Geografisk Tidsskrift 86: 46–62. Copenhagen, June 1986. Cost 647 Intertidal sedimentary subgroup: Long termed studies of ecological and physical parameters especially within the Danish Wadden Sea. The following items have been dealt with: 1) baseline studies concerning zoo-biology (1980–85) and some physical factors (1983/85). 2) Studies in the Ho Bugt area as such dealing with the bottom sediments and the suspended matter. Further studies of the landscape elements, the sediments, and the dynamics have been dealt with through remote sensing. The working group settled the importance of the local physical parametres co-ordinated with the benthic base-line studies. Further the long termed trends in hydrology and climatology were emphazised. The fundamental object of the project is to obtain data on those scales of natural, temporal variability which is attributable to climatic/hydrographic factors. As to the discussion of the sediments it is in this relation essential to relate the character of the sediments to the dynamics of the environment.

Transport of suspended matter in a bar-built Danish estuary

The river Varde Å discharges into the bay of Ho Bugt on the western coast of Jutland forming a small, bar-built estuary. This paper deals with tidal fluxes of water and sediment in the Varde Å estuary. The inflowing water at flood tide is part of a turbidity maximum in the northern part of the bay. At high tide slack water the suspended material deposits inside the estuary. During ebb-tide it is resuspended, and the estuary bottom is washed clean coinciding with the influx of relatively pure freshwater from the drainage area. From one station in the estuary mouth, current velocities and concentrations of suspended material have been measured during 10 tidal periods covering all four seasons. It is shown how these data can be used in a quantitative calculation of the transport of water and suspended material through the cross-section of study. A model has been formulated which—based on half-tidal periods—quantifies the transport of water and suspended material through the estuary mouth. The model is calibrated on the basis of measurements made during the above-mentioned 10 tidal periods. The rather small number of measurements is to some extent compensated for by a carefully pre-arranged selection of tidal periods. The model is discussed in relation to the prediction of net suspended transport through the estuary mouth in different weather and tidal situations.

Bottom sediments in Ho Bugt—a wadden sea environment

Pejrup, Morten: Bottom sediments in Ho Bugt—a wadden sea environment. Geografisk Tidsskrift 81: 11–16, Copenhagen, June 1981. Twenty eight samples of bottom sediment from Ho Bugt, located at the west coast of Jutland, have been anlysed, and the variations within the fine and the coarse fractions described in detail. A method for classifying estuarine- and wadden-sea sediments is presented. The classification together with special characteristics found for the coarse fraction is used as a basis for a sediment chart for the northern part of Ho Bugt.

THE TURBIDITY MAXIMUM IN THE NORTHERN PART OF HO BUGT

Pejrup, Morten: The Turbidity Maximum in the Northern Part of Ho Bugt. Geografisk Tidsskrift 80: 72–77, Copenhagen, June 1980. Ho Bugt contains in its northernmost part a turbidity maximum. Its extension, textural composition, and concentration levels have been investigated and the share of fluvial-suspended material estimated. Finally, the different concentration processes are discussed, and it is suggested which of them are the most active in the maintenance of the turbidity maximum.

SEDIMENTS AND DYNAMICS IN THE VARDE Å ESTUARY

Bartholdy, Jesper: Sediments and Dynamics in the Varde A Estuary. Geografisk Tidsskrift 80: 64–71. Copenhagen, June 1980. The Varde A estuary is supplied with material from two sources: the drainage area and the turbidity maximum in the N-part of Ho Bugt. The first supplies bed load and suspended material, whereas the latter supplies fine-grained, flocculated material only. The sediments and the dynamics related to sediment transport in the area will be discussed on the basis of investigations carried out in the period from June 1976 to July 1977.

Spartinas kolonisation og udbredelse langs Ho Bugt

Spartina sp. (“townsendii” and anglica) was introduced in the Ho Bugt area at Esbjerg in 1931. The investigation is based on a study of a sample area on the peninsula Skallingen during the years 1957, 1960, 1965 and 1970, on litterature and on a survey of the whole bay in 1971. The colonization in the different parts of the bay is described. “Spartina shows an extremely plastic morphological response to the varying conditions of tidal submergence and exposure on different parts of the marsh. This enables it to occupy a wide range of zones occupied in other areas by a series of different plant communities.” (Ranwell, 1964). “It has, in fact, not only taken possession of a vacant ecological niche, but has spread from this to form a serious competition to other salt marsh species of long standing.” (Goodman a.o., 1959).