Leonardo de Silva, and Edward Lucero

National Institute of Geological Science, University of the Philippines, Diliman, Quezon City


Eleven surface sediment samples were collected from Subic Bay, a ~ 100 km2 north-south embayment of the south central coast of Luzon, to study the occurrence and distribution of living benthic foraminifera.  Bottom water hydrographic properties such as temperature, salinity, dissolved oxygen as well as pH of the sediment water interface were also measured and correlated with resulting benthic foraminiferal assemblages.

                A Ponar type of sediment grab sampler was used for bottom sediment collection and sampling depths were determined using an electronic depth meter.  Bottom water temperature, salinity, and dissolved oxygen were measured onboard using a multimeter.  For uniformity, a five (5) cm diameter plastic cylinder was used for the collection of samples for benthic foraminiferal analysis.  Three (3) sub-samples were collected at 0-1 cm, 1-3 cm and 3-5 cm below the sediment surface to determine the distribution of the benthic foraminifera within the upper 5 cm sediment layer.  All samples were stained using Rose of Bengal to distinguish benthic foraminiferal tests with living protoplasm (“living”) during the time of collection.

More than one hundred species and varieties of benthic foraminifers were identified in the study representing 30 families and 65 genera.  Initial data on the occurrence and distribution of living benthic foraminifers within the upper five cm sediment layer reveal no general nor species trend.  This may be indicative of active mixing with the upper 5 cm.  Homogenization of the top sediment layer is expected to be more pronounced in the samples from shallow depths.  At least three distinct zones were delineated from spatial distribution of living benthic foraminifera in Subic Bay.  The shallow water zone (< 10 feet) is characterized by the maximum relative abundance of Ammonia beccarii, Coscinospira acicularis, Elphidium depressulum, Milliolinella suborbicularis, Peneroplis planatus and triloculinella chiastocytis.  Maximum relative abundance of the species Calcarina calcar, Cellanthuc craticulatus, Elphidium crispum, Margarinopora vertebralis, Pararotalia domantayi and Spirolina arietina occur within the reef related zone. The deep water zone, on the other hand, is characterized by maximum relative abundance of Bolivina vadescens, Discorbinella bodjongensis, Nonion subturgidum and Heterolepa subhaidingeri.  Heterolepa praecincta appears to be restricted to the deeper and open marine setting of Subic Bay.  In addition, species diversity and absolute abundance generally increases towards the deeper and open waters.

                 Temperature, dissolved oxygen and salinity generally co-vary with depth.  Distribution of living benthic foraminifera generally follows the depth gradient except for local patches related to reef environments.  The presence of several rivers that empty into the bay results to local variations in salinity as well as sedimentation rates.  Lowest species abundance and diversity are associated with sample locations with very high sedimentation rates.

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