Molluscan associations from the Pak Phanang Bay (SW Gulf of Thailand) as a record of natural and anthropogenic changes

Recent environmental changes in the Pak Phanang Bay (SW Gulf of Thailand) are investigated by means of mollusc assemblages. The present-day water depth within the bay slightly exceeds 2 m at low tide and the sea floor is almost entirely muddy, except for the outermost part of the embayment which is directly influenced by the longshore drift that is building the Laem Talumpuk sand spit. A multivariate analysis of the molluscan fauna recovered at 16 sampling stations within the bay delineates three thanatofacies and two biofacies. The Bay Mouth thanatofacies, including several infralittoral species, is distributed around the bay entrance; the Tidal Flat thanatofacies, characterized by few brackish and freshwater taxa, occurs in the inner part of the bay; the Channel thanatofacies includes a mixed fauna and is found along the long axis of the bay. All thanatofacies are not older than a few decades. The two biofacies are significantly less diverse than their dead counterparts, and are simply identified as Bay Mouth biofacies and Inner Bay biofacies. The faunal evolution, combined with bathymetric and sedimentological data, confirms that the embayment is undergoing a con finement process. The inner bay has evolved into an undifferentiated tidal flat hosting an oligospecific fauna. The confinement trend and the consequent siltation of the bay, mainly due to natural geomorphologic processes acting since centuries, are likely to have sped up in recent years by interaction with some human activities; among these, the deforestation in the upper Pak Phanang basin and the construction of Uthokaviphatprasit Watergate