RICERCA ITALIANA     

Impact of castrophic events on benthic soft-bottom fauna of the infralittoral and intertidal zones: the case of the coastal area of Khao Lak, Andaman Sea, Thailand.

Università degli Studi di Milano-Bicocca

Abstract

The study of the environmental evolution concomitant with the 26 December 2004 tsunami disaster exhibits a great deal of interest in terms of both purely scientific research and environmental recovery and protection. In this respect, the present project considers the upper infralittoral and intertidal bottoms between Khao Lak and Ban Chao Nam, along the Thai coast of the Andaman Sea. This coastal area, stretching about 40 km, includes tracts that were heavily hit by the tsunami, and other tracts where, according to preliminary observations, the effects were moderate or even locally absent.

The most prominent features of the area are as follows. 1) Erosional coastal regime characterized by strong hydrodynamic conditions. 2) Presence of some river mouths forming coastal lagoons and wetlands, and delivering sediment load to the nearshore marine bottoms. 3) Presence, primarily alongside estuaries, of mangrove forests that have played a protective role and whose actual state is worthy to be considered. 4) Soft bottoms of various kind in terms of textural and compositional features, allowing to compare the effects that have occurred (for the same energy level) depending on the type of substrate. 5) Presence of a 1.5 km wide coral reef some 2 km offshore and roughly parallel to Khao Lak coastline. 6) Dense human development in the coastal zone, primarily for tourism purposes, whose effect on marine benthic communities prior to the tsunami event is to be taken into account.

The research, carried out following actuopaleontological methods, focuses on the assemblages of shelled (having a better chance of preservation as fossils) benthic organisms, primarily foraminifers, bryozoans, molluscs, echinoderms, ostracods, barnacles and tube-worms, along with sedimentological, taphonomic and geochemical properties of sedimentary bodies forming the seafloor in the considered area. It is stimulated by the fact that, so far, knowledge is very scanty about 1) the effect of tsunami events on marine benthic communities dwelling shallow soft-bottoms, 2) the magnitude of driven changes, 3) the rates of recovery of the benthic fauna subsequently to the catastrophe, and 4) the composition of the new communities that settle (similar to the pre-event ones or totally different). The proposed investigation aims to gather a series of data adequate to bridge that gap, with a twofold purpose, i.e., 1) to verify the possibility of building an evolution model of the pre-impact – impact – post-impact sequence, largely based on biological evidence, that can be used to better interpret layers recording very high energy conditions, that occur within the sedimentary successions, and 2) to provide baseline criteria to the environmental managers concerned with the recovery and rehabilitation of the infralittoral and intertidal environments devastated by the tsunami. <<<

Principal Investigator

Elio ROBBA Università degli Studi di MILANO-BICOCCA

Research Objectives

The area addressed by this research project extends for approximately 40 km, from Khao Lak to Ban Chao Nam, along the Thai coast of the Andaman Sea (Phang Nga Province). It is depicted in the northern sector of Sheet 353 "Ban Thai Muang to Chong Pak Ko" of the 1:60000 nautical map issued by the Royal Thai Navy Hydrographic Department.
The study of the environmental changes consequent to the tsunami event of December 26th 2004 will focus on the following objectives and will require sea-based investigations.

A – A detailed bathymetric map encompassing the coastal area to the depth of 15 m will be produced. This is intended as an update of the original surveys (1968), and considered to be a fundamental tool in an inter-tropical area with high sedimentation rate. The map will allow a detailed knowledge of sea bottom morphology in order to locate erosion and deposition sectors. Such background information will be the starting point for the following surveys.

B – Definition and description of the environmental conditions of the intertidal zone and upper infralittoral fringes one year after the catastrophic event. Referring to the bathymetries reported on the presently available nautical map, it is considered feasible to extend the surveys from the coastline to a depth of approximately 15 m. This objective will be accomplished through the following activities.
1. Water and sediments chemical and physical characterization.
2. Sediment sampling for grain size and geochemical analyses; preparation of a digitalized map showing the distribution of the different sediment kinds encountered in the surveys. This map will represent the textural features of the substrate deposited after the tsunami event.
3. Benthic assemblages survey and composition evaluation. The water-sediment interface will need to be carefully sampled in order to verify one of the following situations:
i) structured assemblages testifying an absence of the impact; ii) exclusive or prevailing presence of larval stages or juvenile specimens, suggesting that a significant impact has occurred and that the seafloor is undergoing an incipient recolonization; iii) absence of fauna, indicating that the conditions of the area are not favourable to recolonization. The studied taxa are foraminiferans, molluscs, tube polychaetes, crustacens (ostracods, cirripeds and decapods), bryozoans and echinoderms. Data obtained from this investigation, should hypotheses ii) and iii) be actual, provide the basis to evaluate patterns and timing of benthic assemblages recovery, and to envision which assemblage will settle (similar or different from the pre-tsunami one), also according to the substrate characters. Results from this part of the project will be, obviously, of high relevance to environment conservation policy makers.

C – Study of sedimentologic aspects and biologic shelled components of the tsunamite found within the above mentioned depth range. The objective is twofold: 1) to identify the sedimentologic characteristics and to compare them with those of other tsunamites described in the literature, and 2) to investigate and define the taphonomy of skeletal remains within the tsunamite. This latter step requires undisturbed samples. The accomplishment of objective C will requires the following activities.
1. Tsunamite sampling, when found, at all the planed sampling stations.
2. Definition of textural features of the sediment and close examination of sedimentary structures, if any; geochemical analyses whose results will be compared with those listed above at B 2.
3. Skeletal remains identification to the species level and determination of their origin. It will be ascertained whether the skeletons belonged to the pre-existing local assemblage, or they came from other assemblages whose components, reworked by the tsunami, have been transported and then deposited again.
4. Definition of the emplacement dynamics of skeletal remains on the basis of their preservation, as well as their position and distribution within the sediment.
Results from points 3 and 4 will be used to define of a taphonomic model, applicable in case of very high-energy events reworking a significant sediment thickness.

D – Definition and description of the pre-tsunami environmental settings. Pre-tsunami assemblages of the sea bottom between the coastline and the 15 m isobath will be characterised. The definition of such assemblages along with their structure and relationships with the edaphic settings is of particular interest and importance, especially considering that the existing information on the fauna inhabiting the considered area is limited to checklists. Moreover the this kind of data will allow to compare the pre- and post-impact situations. The accomplishment of this objective will require the following operations.
1. Identification of the level reliably representing the pre-impact situation and sampling of sediment and thanatocoenoses at the various sampling stations.
2. Determination of textural characters of the sediment and preparation of a digitalized map showing the distribution of different pre-tsunami substrata. This map will be compared with the one mentioned at point B 2, in order to detect changes that might have occurred.
3. Reconstruction of the biofacies present in the area prior to the tsunami event in terms of composition and trophic structure of skeleton bearing invertebrate assemblages. This information will represent the reference data used to evaluate the tsunami effects on the benthic biota.

E – Reconstruction of the pre-impact – impact - post-impact sequence based on the integration of the results from the previous points B through D. This final phase of the research will lead to a reference model that can be used for similar situations occurring in the geological record.

F – Publication of 1) atlas covering all the mollusc species identified from the study area, giving for each one the updated taxonomic framing, illustration, description, geographic distribution and habitat preference; 2) similar atlas covering foraminifers, ostracods and decapod crustaceans.

G – Formation of a database available to the international and local scientific community on which guidelines for environmental managers could be based for restoration and sustainable use of the resources. The database will include, for each station, the following information: a) digitalized GPS location, b) bathymetry, c) surface and bottom values of temperature, salinity, pH and dissolved oxygen, d) textural features and composition of the sediment, e) thanatocoenoses and living community composition.

H – Layout and publication of thematic maps presenting: a) sea bottom morphology (isobaths), b) temperature, salinity, pH and dissolved oxygen values distribution, c) pre- and post-impact sedimentary facies distribution, d) pre- and post-impact biofacies distribution. <<<

Timescale

24 months

National and international background

The 26th December 2004 tsunami heavily hit the Southeast Asian coasts. An evaluation of the possible impact, and its extent, on the marine benthic fauna of the soft bottom most prone to the effects of the catastrophic event, i.e., the intertidal and upper infralittoral fringe, is needed. Damage to this bathymetric range is expected to have been extensive and intensive, especially where coastal ecosystems had already experienced human disturbances such as urbanization, mangrove forest destruction, aquaculture etc. Reports from the Phuket Rajabhat University mention massive marine benthos strandings, thus suggesting a strong mobilization of soft bottoms facing the coastline. Hence, besides the scientific interest for the event and its consequences for marine benthos, the minimization of the seaquake effects and the promotion of the damaged areas restoration is of the foremost importance. In order to do so, however, a suitable database is needed. Thus, the assessment of the present state of the coastal ecosystems and biota bears high priority and represents the starting point for results to be given to environmental managers.

The marine area facing the coastline between Khao Lak and Ban Chao Nam was struck by the violent impact of the 26th December 2004 tsunami. This area is located approximately 180 km south of Ranong in the Phang Nga province, and is of great interest both for basic scientific research and environmental protection and management. The most prominent features of the area, relevant to this proposal are the following. 1) Erosive coastal regime characterized by strong hydrodynamic conditions. 2) Presence of river mouths determining the formation of coastal lagoons and delivering sediment load to the sea bottom in front of them. 3) Presence of mangals, especially developed in the estuarine areas where they have functioned as natural defences and whose present conditions deserve to be evaluated. 4) Soft substrates diversified in terms of textural features and sediment composition, thus presenting the possibility of an evaluation of the tsunami effects as a function of the substrate type. 5) Presence of a coral reef, approximately 1.5 km long and crossed by a wide channel, at approximately 2 km from the Khao Lak coastline and approximately parallel to the latter. 6) Intense urbanization of the coastal area, especially for tourism purposes, whose possible effects on the benthic fauna, even before the catastrophic event took place, need to be carefully taken into account. Two important notes are to be made.
1) The study of the taphocoenoses found in the tsunamite and in the level immediately underlying it, along with the study of the taphonomic processes which have led to their formation will allow: i) to reconstruct the faunal scenario that have existed prior to the catastrophe, ii) to characterize the tsunamite taphonomic aspects, adding new information to the sedimentological ones already examined in other studies and other geographic locations. 2) Sediment and water geochemical analyses allow the characterization of the environments where benthic fauna is recovering (possibly contaminated by run-off from land, with large quantities of wastes and pollutants, debris, soil and organic matter).

The coastal area between Khao Lak in the south and Ban Chao Nam in the north, selected because of the characters exposed above, is believed to be the most suitable for the project proposed herein since, extending for approximately 40 km and including three major enbayments, it seems of the proper size for this study and offers several logistic facilities (easily accessible coast, mooring areas and locally available goods). This area includes portions of coast heavily impacted by the tsunami, alternating with others where preliminary surveys reported no major damages; these latter may represent reference areas in order to evaluate damages to the marine benthic fauna. Moreover, the soft-bottom benthic macrofauna has been documented in various publications by the "Phuket Marine Biological Centre". More than 600 intertidal and infralittoral mollusc species have been reported from the area, and checklists are matched with short information on substrate preference and depth ranges of taxa (Tantanasiriwong, 1978, 1979). This dataset is a relevant information framework as regards the assemblages ascertained to be in the area prior to the catastrophic event. Information on morphology and coastal deposits is also available (Dheeradilok, 1995).

The reasearch, using an actuoplaeontological approach, will focus on shelled (potentially preservable as fossils) organisms assemblages (primarily foraminifers, molluscs, tube-dwelling polychaetes, crustaceans, bryozoans and echinoderms) as well as on the sedimentological aspects and the geochemical properties of the sedimentary deposits to be investigated. Such research follows previous studies carried out in the Gulf of Thailand, that represent a consolidated experience on Thai shallow marine environments and benthic assemblages, and that have led to a series of taxonomic and paleoecologic scientific achievements (Robba et al., 2002, 2003, 2004; Di Geronimo et al., 2003a; Di Geronimo et al., 2003b; Chaimanee et al., 2004; Di Geronimo et al., in press; Robba et al., in press). Previous scientific research in Thailand was carried out in synergy with the Environmental Geology Division staff, Department of Mineral Resources (Bangkok, Tailandia) under the direction of Dr. Niran Chaimanee. Such collaboration represents a scientific and logistic reference point for the study presented herein.

Number of studies do exist on tsunami-induced coastal geomorphology modification and on sedimentologic features of the sedimentary bodies deposited, both onshore and offshore by tsunami events (viz., Dominey-Howes, 1996; Minoura et al., 1997; van den Bergh et al., 2003; Besana et al., 2004). Less detailed and more sparse information on coral reefs was published. Conversely, no information is available about 1) the effects on soft bottoms benthic assemblages in areas facing the tsunami-hit coastlines, 2) the entity of induced modifications, 3) the patterns and timing of recovery, and 4) the composition of newly settled communities (whether similar or different from the original ones). The proposed research project intends to create a detailed database aiming to fill the just mentioned knowledge gap, with a twofold objective. 1) To verify the feasibility of building an evolution model of the "pre-impact – impact – post-impact" sequence, based also on biological evidences. Such model could find applications in the interpretation of layers that record very high-energy conditions, found to occur in sedimentary successions. 2) To support environmental managers through a set of guidelines for the restoration of seaquake-struck intertidal and infralittoral environments.

Given the mentioned background, any results from this project will represent an innovation.
Three aspects deserve to be highlighted. 1) The research is at the interface between Biosciences and Geosciences since, by means of methodologies pertaining to applied marine paleoecology, it integrates information on the present environments with that concerning the very recent past, and places them in a temporal scale, focussing on changes that have occurred. 2) In the Thai coastal areas hit by the 26th December 2004 tsunami event, Government Agencies and local Universities have started projects on the tsunami effects on land. None of such projects takes the sea into account, and the resulting knowledge gap could be filled by the presented project. 3) Tsunami effects on shallow soft bottom assemblages, even those surrounding reefs, could be systematically studied for the first time. These assemblages, though fundamental for the chemical, physical, sedimentologic and trophic balances in tropical seas, are usually overlooked, while much more attention is paid to coral reefs. <<<