A large anomalously warm water patch (the “Blob”) appeared in the NE Pacific Ocean in the winter of 2013–14 and persisted through 2016 causing strong positive upper ocean temperature anomalies at Ocean Station Papa (OSP, 50° N, 145° W). The effect of the temperature anomalies on annual net community production (ANCP) was determined by upper ocean chemical mass balances of O2 and DIC using data from a profiling float and a surface mooring. Year-round oxygen mass balance in the upper ocean (surface to 91–111 m) indicates that ANCP decreased after the first year when warmer water invaded this area and then returned to the “pre-blob” value (2.4, 0.8, 2.1, and 1.6 mol C m −2 yr −1 with a mean value of 1.7 ± 0.7 mol C m −2 yr −1 ). ANCP determined from DIC mass balance has a mean value that is similar within the errors as that from the O2 mass balance but without significant trend (2.0, 2.1, 2.6, and 3.0 mol C m −2 yr −1 with a mean value of 2.4 ± 0.6 mol C m −2 yr −1 ). This is likely due to differences in the air-sea gas exchange, which is a major term for both mass balances. Oxygen has a residence time with respect to gas exchange of about one month while the CO 2 gas exchange response time is more like a year. Therefore the biologically induced oxygen saturation anomaly responds fast enough to record annual changes whereas that for CO 2 does not. Phytoplankton pigment analysis from the upper ocean show lower chlorophyll- a concentrations and greater relative abundance of picoplankton in the year after the warm water patch entered the area than in previous and subsequent years. Our analysis of multiple physical and biological processes that may have caused the ANCP decrease after warm water entered the area suggests that it was most likely due to changes in plankton community composition.