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Habitat dynamics in response to constructional impacts (Jade-Weser-Port, Germany - Tauranga Harbour, New Zealand): the geological approach

Introduction and state of the art
In recent years, environmental case studies of highly impacted coastal areas have become more relevant for understanding both short- and long-term human impact on the geo- and biosphere and to predict the effects of such pressures on the ecosystem (Winter and Bartholomä, 2006; van der Veen and Hulscher, 2008). In fact, most human activities – i.e. fisheries, sea-food farming, coastal construction, dredging and dumping operations – are continuously increasing the socio-economic disturbance of such a vulnerable environment (OSPAR, 2009).
As emphasized by the OSPAR Commission (Convention for the Protection of the Marine Environment of the North-East Atlantic), there are still gaps in the knowledge of environmental effects of land reclamation and dredging activities, and no comprehensive reports are available (OSPAR, 2004; 2008). A review of published and grey scientific literature shows that research about port construction/dredging activity effects mainly focused on studying the biological disturbance by means of point sampling (Kenny and Rees, 1994; 1996, Piersma et al., 2001; Newell et al., 2004), and on understanding the hydrodynamic modifications using bathymetric data and numerical or physical modelling (Limpenny, 2002; Bale et al., 2007). Other studies evaluated grain-size sediment changes driven by man-made structures (Flemming and Nyandwi, 1994) or in response to the use of different dredging methods (Kaplan et al., 1975; van der Veen, 1995; Desprez, 2000). The study of the impact of aggregates extraction mainly focused on offshore areas, where most of the licensed activities are located (ICES, 2009).
New techniques of underwater acoustic seabed detection, mapping and classification can help to define and classify underwater habitats and their natural dynamics on various spatial and temporal scales. Hydroacoustic systems for habitat mapping are becoming a standard tool when coupled with seafloor ground truthing samples (Coggan et al., 2007).
Among them, a new generation of semi-automatic Acoustic Ground Discrimination Systems (AGDS) is nowadays widely used for seabed and habitat mapping. They present the advantages of being time and cost effective, in regard to their coverage and spatial resolution. They have been tested over different environments since the last decade (Brown et al., 2011).
Nevertheless, there are still drawbacks and, therefore, room for more research. In particular, there is no agreement about standards and methods to be used for a reliable and repeatable mapping. In fact, most of the scientific literature is based on single systems deployed a single time over a specific area. It is a common practise to ground-truth the acoustic data using samples collected in different time frames or surveys. The accuracy in the positioning of the sampling is often not even mentioned, as well as the natural heterogeneity of the seafloor. Thus, the reproducibility of classifications, the relationship between the frequency/geometry used and the reliability of sampling procedure and ground-truthing have not been completely understood yet. This is particularly important for environments where natural forces (e.g., tidal cycles) and anthropogenic disturbance (e.g., on-shore constructions, dredging/dumping activities, underwater structures) have a direct impact on the natural conditions of the seafloor and its changes.

Research question, areas and methods
The application of AGDS to extremely dynamic environments like port construction and maintenance areas can be used:

  • - To assess and quantify the changes occurring to the seabed features (e.g. grain-size, sediment distribution, sea-bottom morphology, benthic communities);
  • - To relate those changes to human disturbance, thereby improving our understanding of the response of the natural environment to human impacts;
  • - To study the role played by sediments and seabed morphologies in the classification process of an acoustic signal.
The general aims of this project were:
  1. To test the response of AGDS in different environments, studying the interaction between different acoustic signals (sound-wave frequency and geometry of the transducer/receiver) and the seabed features (e.g., sediment composition and distribution, sediment patchiness, natural seabed morphologies, dredging features, bio-communities);
  2. To assess the short- and long-term impacts of harbour construction and maintenance on sedimentological and biological changes, by means of AGDS coupled with sediments and macrofauna data.
Two areas were initially selected for the purpose: the new JadeWeserPort (JWP) in Germany and the Tauranga Harbour in New Zealand.

The time-plan for the construction of the German new deep water harbour was favourable:
  • To assess the response of AGDS and sampling techniques in a highly- recently-disturbed environment (simultaneously with the construction phase);
  • To document the short-term habitat changes by means of a time-series comparison (before, during and after the port was realized).
Three surveys were carried out in the area (May 2010, July 2010, and January 2011) deploying different acoustic devices (Singlebeam echo sounder SBES, Multibeam echo sounder MBES, and Side-scan sonar dual frequency SSS) and collecting samples. Sediment and biological data were available before the port construction (2002); it was not possible to have access to any acoustic dataset for the same time-frame. In addition, delays in the harbour development did not allow completing the original survey plan, thereby forcing to a slight change in the PhD topic, more focused on the methodological approach and on changes in macrofauna and sediments in the area. The following research questions were faced:
  1. What is the variability of repeated sediment samples in a highly heterogeneous environment?
  2. How does the positioning error/uncertainty of sediment samples affect the ground-truthing process?
  3. What drives the seabed classification in the different acoustic systems?
  4. Which are the main changes in sediment and macrofauna composition and distribution over the JWP area and what drives these changes?
The Tauranga Harbour represented a good laboratory for documenting the environmental long-term reaction to the presence of a port and related activities. The dredging works were focused on the fairways since the last decades, while a new expansion phase was already approved. The intertidal and subtidal areas represented therefore an optimal testing site where to analyse the changes due to indirect human disturbance. Previous studies (e.g., Healy, 1985; De Lange, 1988; Brannigan, 2009) had shown the presence in the Tauranga Harbour of a combination of distinctive seabed types (in particular shell lags) and seafloor morphologies (e.g., bedform fields), providing the background for a time-series comparison. All considering, the area was chosen to deploy the same equipment used in Germany and compare the results. Besides, the original plan included a second deployment of similar acoustic systems for assessing the differences in seabed classification related to the specifics of the AGDS.

The first acoustic and sampling campaign was conducted in March 2011. Underwater videos were collected during a second survey in June 2011. Technical issues did not allow to run a third campaign (planned for July 2011), leading to minor changes in PhD topic.

The following research questions were addressed:
  1. What is the spatial variability of seabed types and how can it be resolved by acoustic systems and point sampling vs. continuous sampling (underwater video) techniques?
  2. What is the indirect influence of regular dredging activities on sediment dynamics and habitat changes in a long term context (time-series approach)?
The research in the JWP area was carried out in close collaboration with Ruth Gutperlet and Dr. PD. Ingrid Kröncke (from INTERCOAST Project IC6 and Senckenberg Institute for Marine Research, Biological Department) to integrate the macrofauna information into the sedimentological and acoustic classifications.

Results and main outcomes
Results from the JWP surveys clearly shown an extreme spatial variability (patchiness) in sediment composition and distribution over the whole area (not only the disturbed one), enhanced by the works related to the port construction. Several stations were sampled in different tidal-moments in order to check the tidal influence on sediment composition (thin temporary layers of different grain-size material), acoustic response and ground-truthing process.
In conclusion, repeated sampling clearly demonstrated that:
  • Replications presented significant differences in sediment composition, not related to the tidal moment;
  • There was not clear relationship between positioning and similarity of the samples;
  • Less disturbed areas presented the same variability of directly impacted ones; therefore there was no link between anthropogenic disturbance and heterogeneity.
The acoustic classification was independent from the tidal-moment (Capperucci and Bartholomä, 2012).

SBES classification reveals a highly heterogeneous seabed texture, being likely controlled by the distinctive patchiness in sediment distribution (=sediment roughness). Nevertheless, the extreme variability of sediment composition did not allow any interpolation and/or real ground-truthing. Thus the final classification could hardly be translated into sedimentological information. On the contrary, swath-based backscatter (MBES and SSS) seemed to be largely dependent on seabed topography for their classification, with acoustic classes that matched the general division into morphological domains (= topographic roughness). The angle between the acoustic lines and the seabed features was also crucial for the final acoustic classification. In fact, regularly shaped and spaced features, like dredging marks, could lead to significantly different results.
The study about the impact of the JWP on habitats indicated complex sediment and macrofauna changes. Sediment comparison (2002-2010) showed a general coarsening trend all over the area, with the exception of the old navigation channel, where fine material was mapped, marking a significant difference with the existing literature. An increase of opportunistic and stress tolerant species was also determined, with no strict link to the sediments. A higher amount of macrofauna communities characterized the area in 2010.
Two sites were surveyed in the Tauranga Harbour area. Processing and analysis were conducted only on one of them (Western Channel). Results demonstrated a good agreement between acoustic classifications from different AGDS. They seemed to be mainly driven by the sediment distribution, with a distinctive fingerprint given by shells and shell fragments. Nevertheless, the presence of relevant topographic features (i.e. large bedforms fields) influenced swath-looking systems (SSS and MBES), confirming the outcomes of the JWP study (Capperucci et al., 2013). The point sampling techniques resulted to be a limit both as a stand-alone description and mapping of the sediments and for a reliable ground-truthing of the acoustic classifications. In fact, despite there was generally a good agreement between sediment samples and corresponding seabed surface as shown in the video transects, the sampling techniques is not able to qualify and quantify the special variability of the environments.
The comparison with existing sediment and seabed type maps indicated changes in the distribution of surface sediments, bedforms and shell lag. Although a general sedimentary pattern could be recognised over the time series data, a reduction in the shell coverage and the shallowing of the lower Western Channel could be related to an adjustment of the hydrodynamic conditions due to the dredging activities in the shipping channel nearby.

Members

Proponents:Dr. Alexander Bartholomä University of Bremen
Prof. Dr. Dierk Hebbeln
Dr. Ingrid Kröncke
:Dr. Willem de Lange University of Waikato
PhD Candidate:Ruggero Capperucci University of Bremen

Publications

Capperucci, R.M., A. Bartholomä, S. Renken and W. De Lange, (2013). Natural vs human-induced changes at the Tauranga Harbour area (New Zealand): a time -series acoustic seabed classification comparison, Geophysical Research Abstracts Vol. 15, EGU2013 Proceedings, in print.

Bartholomä, A., P. Holler and R.M. Capperucci, (2012), Monitoring vonsublitoralen Habitaten – kartierung mit hydroakustischen Methoden. – Forum der Geoökologie, 23 (3), 14- 21.

Capperucci, R.M. and A. Bartholomä, (2012a), Sediment vs Topographic Roughness: Anthropogenic Effects on Acoustic Seabed Classification, Proceedings Hydro 2012, Rotterdam, 47-52.

Capperucci, R.M. and A. Bartholomä, (2012b.) High resolution seabed classification of human impacted areas: does it work? GeoHab Conference Proceedings, Orcas Island, 17-18.

Miscellaneous

Research stay at the University of Waikato, Hamilton: 01.03. - 11.09.2011 and 06.02. - 06.03.2010