The effect of the 2013–2016 high temperature anomaly in the subarctic Northeast Pacific (the “Blob”) on net community production

Abstract

A large anomalously warm water patch (the Blob) appeared in the NE Pacific Ocean in the winter of 2013–2014 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 dissolved inorganic carbon (DIC) using data from a profiling float and a surface mooring. Year-round oxygen mass balance in the upper ocean (0 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 from 2012 to 2016, with a mean value of 1.7±0.7 mol C m−2 yr−1). ANCP determined from the DIC mass balance has a mean value that is similar within the errors as that from the O2 mass balance but without a 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 1 month while the CO2 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 CO2 does not. Phytoplankton pigment analysis from the upper ocean shows lower chlorophyll a concentrations and changes in plankton community composition (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 the temperature-induced changes in biological processes.