Methods

3.4.1 SEDIMENT TRAPS

Active field work at the anthropogenic marshland of the Halligen is limited to the stormy winter season from October to March. Further specific frame conditions are harsh weather which has the ability to damage or influence permanent installations by ground frost, wave action with ice drift as well as driftwood. For that purpose two different types of sediment traps seemed to be feasible: (1) Containers buried into the ground or pending above the surface or (2) flat boards or mats that are evenly fixed to the marsh surface. For the practical field test, we decided for a combination of one litre LDPE bottles, further named trap type A and PE-synthetic turf mats (20 x 30 cm), further named trap type B (fig. 3-2). Trap type A (bottle) is 9.5 cm in diameter, 20 cm high and has a narrow opening of 5.0 cm in diameter. Trap type B (mat) consists of straight and curled PE-blades with a length of 21 mm, stitched irregular onto a gauze which is rubber coated at the bottom side. Both types of blades together are building a rather dense surface. The structure of the mat seems to be similar to those of Steiger et al. (2003) who reviewed a variety of flat sediment trap devices and finally recommended synthetic turf mats for use at riparian zones and flood plains.

Figure 3-2: Field devices used to sample inundation sediments. (A) One litre LDPE-bottle buried in the marsh soil. (B) Mat of synthetic turf, fixed with steel nails onto the ground.

3.4.2 SEDIMENT SAMPLING CONFIGURATION

Traps type A were filled with brackish water from adjacent ditches before being inserted into the ground to avoid a lifting of the bottle when seawater is flooding the marsh plain. Holes were made with a soil sampler of the same diameter as the bottle and the depth was adjusted in a way that the bottleneck protruded approximately 3 cm above the surface. Traps type B were fixed with five steel nails onto the surface. Both traps were arranged side by side in a regular grid of approximately 400 x 400 m over the whole area of each of the investigated Halligen (fig. 3-3). Due to the individual size and shape of the Halligen, 61, 36 and 12 locations (Langeness, Hooge, Nordstrandischmoor) were equipped with the described combination of trap type A and B. For each trap location the GPS position was recorded. All sampling devices were installed in the field at the beginning of the stormy season in October. Their removal had to be completed before the bird breeding season that started in early April. In between the sampling devices remained untouched for the sampling of all storm surge events, i.e. they record the cumulative sediment

Figure 3-3: Location map showing the positions of regular sediment traps (dark grey crosses), different test sites to validate bottles and mats as sediment traps (numbered black dots) and coring positions (numbered black rectangles). In total 61 (Langeness), 36 (Hooge) and 12

(Nordstrandischmoor) grid positions of a sediment trap combinations A and B, three experimental test sites (Hooge) and 12 coring locations (2 Nordstrandischmoor, 4 Hooge, 6 Langeness) were established.

CHAPTER 3 deposition of the complete winter season which we consider as representative for one calendar year (There are almost no inundations during spring and summer). During the following years, the same positions based on GPS accuracy were used.

3.4.3 METHODOLOGICAL INVESTIGATION ON SEDIMENT TRAPS

To validate the measurement devices for the purpose of calculating areal sediment depositions at the Hallig marshland, we performed methodological investigations on their general sediment sampling efficiency and how that efficiency might be affected by external factors. In detail the following investigations were performed within the project period.

Installation protrusion depth of trap type A

For trap type A the effect of different installation protrusion depths (1, 3, 5, 7, and 9 cm difference between top of the bottle and Hallig surface level) on sediment trap efficiency was tested to evaluate a possible error resulting from an improper installation. Each depth was tested with three replicates. Bottles were placed with a lateral distance of 50 cm to each other. This experiment was performed during a single storm surge event in winter 2011/12 at two different locations at Hallig Hooge. Location 1 was a short grazed meadow and location 2 a young fallow.

Efficiency of trap type A versus B

A variety of different sediment trap devices; containers, funnels as well as mats and flat boards;

have been tested under field and/or laboratory conditions (Mansikkaniemi 1985, Bloesch and Burns 1980, Gardner 1980a, 1980b, Steiger 2003). Well knowing that it would be impossible to evaluate the exact sediment trapping efficiency of a sediment trap under the rough and inhomogeneous field conditions of the Halligen, we decided not to rely on only one trap type. The simultaneous use of two trap types; bottles and mats; offers the opportunity to rate their relative sediment sampling and retain capacity in comparison to each other.

Small scale variability of sediment deposition rates

Additional six cluster (two per Hallig and year, fig.3-3) each composed of five sediment traps type A and B were established to gain knowledge about the small scale variability of sediment deposition rates in contrast to the 400 x 400 m grid. The traps were arranged crosswise with a distance of 10 m to each other.

3.4.4 THE ROLE OF ORGANIC MATTER AND BULK DRY DENSITY

To translate sediment deposition into vertical accretion on a longer time perspective, information about alteration of inundation sediments towards soil material are inevitable. Variations of bulk dry density (BDD) and the content of organic matter are important soil parameters in this respect.

A total of 12 percussion cores (6 at Langeness, 4 at Hooge, 2 at Nordstrandischmoor) with depth of 74 to 100 cm are available for the Hallig marshlands (fig.3-3). Cores were taken by driving plastic tubes (10 cm diameter, 120 cm length) into the soil.

3.4.5 PROCESSING THE TRAPPED SEDIMENT Trap type A (bottles)

The samples inside trap type A (bottles) are a mixture of water, sea-salt, clastic material, small organic particles and huge organic compounds like blades of grass. To remove water and salt and to quantify the amount of clastic material and organic particles, all samples were treated either by filtration or dialyses method. After decantation, the sample is transferred by use of a funnel into the filtration device (vacuum or high pressure, ashless cellulose paper of 100 mm in diameter, 12 to 15 μm porosity) or dialyses tubes (molecular mass of 10000 to 20000 Dalton).

Grass and flotsam is removed by a sieve (e.g. mesh size of 2.0 mm). The removal of salt was achieved by leaching the filter residual with deionized water or purging the dialyses bath until conductivity reached values < 300 µS/cm. The remaining moisture was removed by drying at 105 °C.

To determine the organic matter content the loss on ignition (LOI) method was used (German Norm 19684-3 2000, Kuntze et al. 1994). The heating time was low as 425 °C for 2 hours to minimize crystallization water losses, which could result in an overestimation of organic matter (Leong and Tanner 1999, Barillé-Boyer et al. 2003).

Trap type B (mats)

To quantify the total mass of solids (Ms) on trap type B, these were dried at temperatures which do not exceed 80 °C. Above theses temperatures the PE material of the mat is losing strength.

Afterwards the dry mats are weighted, thoroughly cleaned using a high-pressure cleaner and weighted again. Due to the cleaning process, sediments are not kept but washed away. Therefore the organic matter could only be analyzed for trap type A sediments. Organic matter values for trap type B are calculated using the value of the adjacent trap type A. Further on, the data of each sample location were checked for outliers by comparing sediment deposition values of trap type

CHAPTER 3 A and B. Extreme ratios indicate that sediment mass of either bottle or mat must have been influenced by external factors like grazing livestock or human intervention. Outliers were detected statistically via Grubbs and IQR test.

Soil sediment samples

The upper 25 cm of the core was cut in increments of 1 cm. For depth > 25 cm, the increment size was increased to 5 cm. BDD was analyzed gravimetrically by drying the soil material at 105 °C until no further change in weight was recognized. Afterwards the samples were pulverized by a motor grinder (Fritsch pulverisette, type 02.102). The determination of the organic matter content was also done by the LOI method. BDD and LOI values with respect to soil depth (fig. 3-6) were calculated as average values on the available sediment cores (Langeness n = 6, Hooge n = 4, Nordstrandischmoor n = 2). Depths were corrected by the compaction factor, resulting from percussion coring. This was calculated as the ratio between the length of the core liner (driven into the soil) and the length of the soil column within the liner.

Statistical evaluation

To test the different data collectives, the t-test for independent samples (α = 0.05) was used as well as linear regression analyses. All statistical evaluations were performed by the software packages SPSS Statistics 17.0 (SPSS, Inc.) and Grapher 8.8 (Golden Software, Inc.).