The present study is the first bay wide assessment of Masinloc-Oyon Bay Marine Reserve using biological and environmental parameters. Tintinnids were collected through vertical and horizontal tow during July 2017. Night~ and day-time collection was conducted to verify the diel characteristic of tintinnid species. Environmental parameters (temperature, salinity, pH, DO, TDS) were measured using YSI 6600 multiparameter. The nine identified tintinnid species in this study are first records in Masinloc-Oyon Bay (MOB). The warm water species Helicostomella longa dominated the tintinnid assemblage during wet season of July (29-33°C). Triplots from multivariate analysis showed that stations (1, 6-7) far from Marine Protected Areas (MPAs) create a different cluster compared with the stations (2-5) within MPAs. Station 1 is inside the power plants outfall and consistently had the lowest abundance and diversity. The power plant thermal discharge has a negative effect on abundance and diversity of tintinnids assemblage. No significant difference was observed between night and day-time abundances using horizontal tow. A significant P-value (.000, P\u3e.05) was recorded between night and day-time abundances using vertical tow. The said initial result suggest that tintinnids deviate from the generalized diel characteristic of zooplankton community. The diel and spatial tintinnid variation in this study presented the uniqueness and diversity of MOBs marine environment. Ecological tools such a tintinnids can be used as a bio-indicator to assess the water masses and water quality of such complex site
