Discussion

The study area in the Inner Jade is a very heterogeneous environment. The western part and the old navigation channel are anthropogenic disturbed areas due to the conducted dredging activities, while the eastern part is relatively undisturbed. Many studies demonstrated that acoustic seabed classification using SSS is a suitable tool for the detection of benthic habitats in various environments (Brown et al. 2004b, Ehrhold et al. 2006, Zajac et al. 2003, Franklin et al. 2003, Brown and Collier 2008). However, in such a heterogeneous area as the Inner Jade benthic habitat mapping is a challenge.

Brown et al. 2004a described the problem of generalization (lumping) versus separation (splitting) of acoustic classes in a heterogeneous study area. The high degree of sediment heterogeneity was problematic for the identification of discrete boundaries between the physical habitats (Brown et al. 2004a). The acoustic classification reflected various dredge marks in the western area, which was dredged for the JWP (classes B, E, F, G, H, J) and in the regularly dredged old navigation channel (class F). Only in the western classes A and D

relevant seabed features were absent. In contrast, in the undisturbed eastern area (classes C and I) natural bedforms dominated, which were already reported by Capperucci and Bartholomä (2012) and Kubicki and Bartholomä (2011). Thus, the split classification represented the dredge marks and natural bedforms, but not the full heterogeneity of sediments in the Inner Jade.

2.6.1 Hydroacoustic classification versus sediment distribution

Strong links between sediments and acoustics have been demonstrated, but the relationship between sediment and backscatter is not always clear (Ehrhold et al. 2006). Sedimentological factors (e.g. grain size, volumetric heterogeneity, fine-scale roughness or surface sediment) and significant slope variation may play an important role in the acoustic response (Urick 1983). In this study, low backscatter intensities (class A) were detected in areas, where muddy sand and slightly gravelly sand dominated in front of the bulkhead and in the southern area. In contrast, shells are known as strong and characteristic acoustic reflectors (Wienberg and Bartholomä 2005), therefore the north eastern area was clearly identifiable as high backscatter region (class I). The medium backscatter classes (B-H) were dominated by various sediments (slightly gravelly sand, sandy mud, muddy sand, slightly gravelly sandy mud). Thus, no significant relationship between the acoustic classification and the sediment classes could be expected in this heterogeneous study area. Brown and Collier (2008) concluded that in special environments it will not be possible to extrapolate substrate maps into habitat maps based on acoustic signatures. Due to the high anthropogenic impact in combination with natural variability the Inner Jade seemed to be an example for such a certain environment.

2.6.2 Hydroacoustic classification and sediments versus macrofauna community structure

The low, but significant correlation between the resemblance matrices of the acoustic classification and the macrofauna community structure stressed the heterogeneity in the Inner Jade. In this study, the low average similarity of macrofauna communities (4-49%, Table 2.6) indicated a high level of spatial heterogeneity also within the species distribution. This spatial heterogeneity of species was probably linked to the high heterogeneity of sediments (Brown et al. 2004a) as indicated by the low, but significant relationship between the macrofauna

heterogeneity in the sediment composition and macrofauna community structure in the shore-face connected ridges north of the island Spiekeroog in the German Bight of the southern North Sea. The authors could explain the small scale spatial distribution of the macrofaunal affinity groups by a heterogeneous surface sediment pattern resulting from local hydrodynamics, which also influences the food availability. However, only few taxa with known sediment preferences were characteristic taxa in the Inner Jade. These were Anthozoa which settle on gravel and shells in the undisturbed north-eastern area (communities III and IV). In contrast, Corophium volutator and Petricolaria pholadiformis prefer fine sediments (Fenchel et al. 1975; Tebble 1976) and were found in the regularly dredged old navigation channel and at some stations in the undisturbed south-eastern area (communities I and II).

Scoloplos (Scoloplos) armiger (communities III, IV, V) is more cosmopolitan without a real sediment preference (Coosen et al. 1994) and the opportunist Pygospio elegans (communities II, III, IV, V) has also a wide habitat tolerance (Bolam and Fernandes 2003). Therefore, these taxa cannot be used as indicators for a particular sediment type, but probably for tolerance of disturbance. The impoverished macrofauna abundance in the areas, which were dredged for the JWP, was most likely a result of the physical disturbance by the conducted dredging activities and not on sedimentary characteristics of the bottom. In general, the macrofauna data proofed that quick re-colonisation after the cessation of dredging activities is possible in highly dynamic areas (Borja et al. 2010), such as the Inner Jade. Nevertheless, community V showed still the characteristics of an early succession state (low taxa number, low abundance and dominance of cosmopolitan and opportunistic taxa).

2.6.3 Environmental factors structuring the macrofauna communities

The number of days after the last dredging/dumping activities for the JWP was the most important parameter structuring the variability of macrofauna communities in the Inner Jade, followed by sediment characteristics (content of sand, mud and shell debris). Depth, high and low backscatter grey values, and dredging intensity (expressed as the number of dredging days for the JWP) played also a significant role. Gravel content was less important, because only community IV appeared on undisturbed stations with elevated gravel content.

Community V was the biggest group and occurred mainly in the area, which was dredged for the JWP, and which was also the deepest site of the study area and was dominated by sand.

Community II showed an affinity to elevated mud contents, which mainly prevailed in and close to the old navigation channel. In contrast, the spatial distribution of community III was

best explained by the presence of shell debris in the undisturbed north-eastern area. However, the total degree of variation explained by all these variables was rather low, at 40%, indicating that there where additional forces active in the study area.

In the adjacent Jade Bay, Schückel et al. (2015) found that the species composition was best explained by the variability of tidal current velocity and depth, followed by sediment characteristics (mud, total organic carbon, gravel and median grain size). Schückel et al.

(2015) could also only explain 30% of the total variability in the macrofauna community structure by using these natural parameters. Therefore the authors suggested that variables related to food availability (chlorophyll a content), predation or topographical characteristics could be responsible for the unexplained variability. Additionally, the unknown dredging intensity in the old navigation channel in the Inner Jade could be helpful information to explain the re-colonisation in that area.