Kam says ...

ECOLOGICAL  IMPACTS  OF  THE  TSUNAMI  ON DOTILLA INTERMEDIA IN LAEMSON, THAILAND

There  have  been  several  studies  that  have  described  changes  to  the  topology  of beaches following the impact of the tsunami in. Distinct  changes  to  the  slope  of  beaches were also  reported  along  the Chennai  coast  in  India. The  ecological work undertaken in this thesis demonstrated that the location of the area inhabited by Dotilla  intermedia  has  a  very  close  relationship  to  the  gradient  of  the  beach. One direct  consequence  of  sediment  accumulation  or  removal  caused  by  the  tsunami would be to alter the beach gradient, and thus shift the position of the Dotilla zone on the shore.

In  addition  to  altering  the  topology  of  beaches,  the  tsunami  created  a  large change  in  the patterns of grain  size distribution on exposed beaches  throughout  the 162 Laem  Son.  There  was  a general  coarsening  of  sediment  resulting  from  the overturning of  the beaches by  the  tsunami wave, and  the  sediment on  the  tsunami-impacted beaches had yet to reach pre-tsunami conditions by April 2005. Changes in sediment grain size created by the tsunami would be expected to have a  large  impact on Dotilla  intermedia. Although sediment grain size was not found to be significant in controlling the zonation D. intermedia of on beaches within the Laem Son, it is hypothesised that sediment grain size dictates which beaches are available  for  habitation  by  D.  intermedia.  Sediment  of  a  particular  size  range  is necessary to sift through the mouthparts of the crabs when feeding .  However,  the  granulometric  analysis  of  sediments  was based on measuring the overall granulometry of a sediment volume, and did not focus on the thin surface layer processed by the crabs. Sections through the sediment showed strong patterns of layering consistent with considerable resuspension  and  settlement,  with  the  coarse  material  overlain  by  finer  material. Therefore the grain size of the surface sediment may not have been outside the size range required by D. intermedia to feed upon The coarsening of  the sediment may have had more  impact on  the ability of Dotilla  intermedia  to burrow  through  the sediment. There have been several studies 163 on effect of differing sediment types on the burrowing of the mud crab Helice crassa. The depth to which H. crassa burrows is related to the relative proportion of the silt-clay  fraction  in  the  sediment, with  the  crab  burrowing deeper in muddier sediments than sandier sediments. The tsunami-related coarsening  of  the  sediment may  have  resulted  in D.  intermedia  only  being  able  to construct shallower burrows, which in turn may have had important implications for the effectiveness of  the burrow as a  refuge  from desiccation and predation. However, considering  the  speed  at which  the  populations  of D.  intermedia recovered  to  their pre-tsunami numbers  in  the Laem Son,  it may be  that  there was little or no impact on the burrowing ability of the crabs. The changes in the sediment granulometry and beach topology created by the tsunami  would  slowly  begin  to  revert  to  pre-tsunami  conditions  over  time  by sediment movement caused by  tidal action. Sediment resuspension and resettlement would  slowly  move  coarse  particles  deeper  into  the  sediment,  whilst  sediment removal or accumulation would shape the slope of beaches. When compared to other large-scale  marine  disturbances  such  as  tropical  storms  and  cyclones,  tsunami-generated disturbance can be seen  to have a similar  level of  impact.  In both storms and  tsunamis,  the majority of disturbance on  the near-shore marine environment  is physically derived from wave action. Along the Andaman coast of Thailand most of these  storm events occur  in  the winter months, with a peak  in November, meaning that the 2004 tsunami occurred during a period when large scale disturbances would be predicted. As a  result of  the historical precedence of winter storms,  the exposed shore fauna that occupy the Andaman coast would be expected to have evolutionary adaptations to overcome disturbance during this time period, so the overall impact of the  tsunami may  have  been moderated  by  virtue  of  its  timing.  Bearing  this  in  mind,  it  is  perhaps  not  surprising  that  the  populations  of Dotilla  intermedia were  able  to  recover  relatively  quickly  after  the  impact  of  the tsunami.