programme tuesday, july, 31 - Université de Caen Basse Normandie

Tidalites 2012, 8th International Conference on tidal environments, Caen, France – Abstract book
ANALYSIS OF SUBTIDAL HABITATS IN THE GERMAN WADDEN SEA ON THE
BASE OF HYDRO-ACOUSTIC REMOTE SENSING DATA
Alexander BARTHOLOMAE, Peter HOLLER
SENCKENBERG, Suedstrand 40, 26382, Wilhelmshaven, Germany, abartholomae@senckenberg.de,
peter.holler@senckenberg.de
Since the Wadden Sea area was awarded a World Nature Heritage the public pressure of
habit protection grows distinctly. The European Water Framework Directive requires a standard
mapping of marine habitats. Therefore quality tools to assess the recent stage and to monitor
changes in the near future are needed. For the intertidal area airborne laserscanning and
Synthetic Aperture radar (SAR) are the state of art (Bartholdy & Folving 1986, Brzank et al. 2008).
In contrast to the intertidal area, optical approaches are less successful to cover the subtidal part
of the Wadden Sea area. High suspension load coupled with less visibility reduce and /or inhibit
wide-spread used optical based techniques. To compensate this deficit, alternative techniques
have to be developed. From the pelagic area the quite well-known hydro-acoustic devices are
now more and more adapted to shallow water operation (Hughes-Clarke et al. 1996).
To work out standard procedures for sub-aquatic monitoring, three hydro-acoustic devices
were tested to their resolution, their redundancy and their value of benefit detecting habitats in a
three year lasting case study in the East Frisian Wadden Sea (Bartholomae et al 2011). In order
to find out which are the system-specific limitations like foot-print sizes and coverage, seven test
areas in water depths between 5 m to 15 m were simultaneously surveyed using singlebeam
echosounder (SBS) (200 kHz), multibeam echosounder (MBES) (455 kHz) and Sidescan sonar
(SSS) (380 kHz). Based on the principle of acoustic respond the return signals were analysed
with acoustic seabed classification tools to determine the different characteristics of seafloor
roughness. Up to five major acoustic classes have been specified. These classes cover sea
surface sediments consisting of sand, shells debris, gravel, less mud and infrequently some peat
at the seven study sites in the East Frisian backbarrier tidal flats. Repetitive surveys were carried
out to investigate system specific variations and natural changes in the complex spatial pattern of
the subtidal habitats. Based on the given technical specifications of the different acoustic devices
the influence of footprint sizes was tested by applying different grid sizes in the data analysis. In
order to generate similar footprint sizes for a water depth of 15m MBES data were calculated on a
grid sizes of 33x17 pixel (0.9m x 3.7m ; 3.3 m²), SSS data with 17x9 pixels (0.8m x 2.4m ;
1.92m²) which correspond to a SBS footprint of 2.98 m². This already limits the minimum water
depth of operation for the oblique-angled geometries.
The spatial distribution of the acoustic classes was tested by means of confidence levels of
acoustic similarities within the classification results of each system as well as between the
different acoustic devices. The fit of ground truth data to acoustic classes and the importance of
the sediment specific surface roughness was tested by multidimensional scaling (MDS) and
cluster analysis for bulk sediment composition and for the individual grain size distributions.
Seasonal up to annual scaled times series were analysed with regard to habitat dynamics in
exposed and sheltered areas of the Wadden Sea.
The results of the case study are summarized in a comprehensive report which discusses
the differences in backscatter based classification, local effects of natural and human impact to
the specific sites and the system specific differences such as working-frequencies and footprint
geometry.
In general the backscatter based systems are much more sensitive to changes in surface
roughness than of the sediment type itself. For the sediments in the Wadden Sea this sensitivity
starts getting more relevant in dependence of the footprint size, the effect of smoothing decreases
in shallower water depth.
Results of habitat distributions and their site dependent differences and the system related
limitation will be discussed in a condensed way.
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