Dagat-dagatan (a Tagalog word for miniature sea) is a brackish-water lagoon of 163.45 hectares on the fringe of Manila Bay about 10 kilometers north of Manila. In official charts this small body of water is labeled as Dagat-dagatan Lagoon. The Navotas-Malabon River and other estuarine tributaries such as the Batasan and Tonsuya on the north, the three rivulets on the east, Maypajo and Marala Creeks on the south, all empty into this lagoon. The municipality of Navotas borders the lagoon on the west (text fig. 1). The lagoon is on a tidal gradient zone, the highest tide level reaching six feet. The run-offs from the adjoining lowlands, tidal streams and tributaries enrich the water of the lagoon with nutrients that enable the lagoon to support an abundant growth of plankton. Diatoms commonly taken as food by oysters such as Mastoglorci, Plew'osigma, Surillci, Chaetoceros, BidduU phia, Nitzchia, Melosira, Navicula, and Coscinodiseus are found in Dagat-dagatan.