ARTICLE Salt-plug estuarine circulation in Malampaya Sound, Palawan, Philippines Olivia C. Cabrera* 1 , Cesar L. Villanoy 1 , Gil S. Jacinto 1 , Lawrence Patrick C. Bernardo 1,2 , Charissa M. Ferrera 1,3 , Imelda B. Velasquez 1,4 , and Rhodora V. Azanza 1 1 Marine Science Institute, University of the Philippines, Diliman, Quezon City, Philippines, 1101 2 Department of Civil Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan, 152-8552 3 Department of Mechanical and Environmental Informatics, Graduate School of Information Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan, 152-8552 4 Hill Laboratories 101c Waterloo Road, Hornby, Christchurch New Zealand M alampaya Sound is an enclosed bay in Pala- wan, Philippines. A salinity maximum or salt plug was discovered in the middle of the Sound by a field survey in May 2005 and suc- cessfully simulated using a Delft3D numerical model of the sound, forced with tide and freshwater discharge at the lateral boundaries, and evaporation at the surface. Different conditions to simulate the major monsoonal regimes of 2005 (dry inter-monsoon, southwest-wet, and northeast-dry) indicate that the salt plug persists, although its relative position changes with the magnitude of freshwater discharge. The salt plug effec- tively limits the exchange of water between the Inner and the Outer Sound, which may account for differences in nutrient lev- els and the phytoplankton assemblage during the same survey. Such a thermohaline circulation has important consequences for sediment transport, phytoplankton bloom formation, and accu- mulation of pollutants in the Sound. Vol. 7 | No. 2 | 2014 428 Philippine Science Letters *Corresponding author Email Address: olive@msi.upd.edu.ph Submitted: October 29, 2014 Revised: December 6, 2014 Accepted: December 6, 2014 Published: December 26, 2014 Editor-in-charge: Kevin S. Rodolfo INTRODUCTION Salt-plug estuaries, relatively uncommon among the world’s estuaries, include the Gulf of Fonseca in Honduras (Valle- Levinson and Bosley 2003) and the Bay of Guaymas in Mexico (Valle-Levinson et al. 2001). Characteristically, they each have two vertical thermohaline cells: an inner typical freshwater- estuarine circulation and a seaward reverse estuary. High evapo- ration rates coupled with long residence times produce a salt plug, or surface salinity maximum in the estuary. Formation of a reverse estuary in the seaward portion of an estuary modifies its transport pattern, reducing flushing of its inner portion and modifying the transport of nutrients and other substances within it. Malampaya Sound is a shallow embayment in northwestern Palawan, Philippines that opens northwestward to the South China Sea (Figure 1). The bay narrows in its middle portion, which also contains a group of small islands that demarcates the shallow Inner Sound from the deeper outer portion. Draining into the Sound’s southeast portion is the Abongan River, esti- mated to discharge about 200 x 10 6 m 3 annually or at a rate of 6.3 m 3 s -1 (Alejandrino et al. 1976). November to April is the dry season of the northeast monsoon, and the wet southwest mon- soon lasts from June to October. Malampaya Sound is a unique estuary in the Philippines. Specimens of the rare Irrawaddy dolphin have been reported in it (Dolar et al. 2002). In July 2000, it was proclaimed a protected area for its rich waters and potential for shellfish production and KEYWORDS Salt-plug estuary, thermohaline circulation, numerical model, Malampaya Sound