Horizontal dinoflage11ate cyst distribution, sediment characteristics and benthic flux in Manila Bay, Philippines

The lateral variation of sediment properties and associated cyst content of sediment in Manila Bay were determined and their possible role/s in the occurrences of Pyrodinium bahamense Plate var. compressum (Bohm) Steidinger, Tester et Taylor toxic blooms were assessed. Manila Bay's surface sediment was determined to be silt dominated. Clay generally increased towards the coast, probably as a result of flocculation and rapid deposition upon entry of sediments from the rivers. High sand content characterized the southeastern part of the bay attributed to the greater sand inputs and relatively strong currents in this area. Bulk densities were lower in the eastern side of the bay from dilution by high organic load from sewage and urban areas. Benthic flux calculations, particularly NH3, suggest more than 50% nutrient contribution comes from sediments. In general, dinoflagellate cyst density increased from the center of the bay towards the coast, except in Pampanga Bay where it deceased near the coasts. A maximum of 23 dinoflagellate species were identified: 5 were autotrophic (Lingulodinium polyedrum (Stein) Dodge, Gonyaulax spp., Pyrophacus steinii (Schiller) Wall et Dale, Protoceratium reticulatum (Claparede et Lachmann) Butschli, and Pyrodinium bahamense var. compressum), and the rest were predominantly composed of Protoperidinium spp. and Diplopsalis spp. Heterotrophs comprised about 70% of the total cyst counts. Pyrodinium counts increased towards the northwestern part of the bay where it was the dominant autotroph species. Negative correlations were observed for live Pyrodinium cyst density and N flux, P flux, ratio of N to P and total organic carbon (TOC) content. However, areas with high N:P ratio contain abundant Pyrodinium live cysts.