UC Irvine

El Niño Southern Oscillation (ENSO) influences multi-year variation in sea-surface temperature and the intensity of upwelling in many Pacific regions. However, it is currently unknown how El Niño conditions will affect the concentration and elemental ratios of particulate organic matter (POM). To investigate this, we have quantified POM weekly for 6 years (2012 to 2017) at the MICRO time-series station in the Southern California Bight. We found a strong influence of the 2015 El Niño on sea-surface temperature and phosphate concentration but to a lesser extent on nitrate availability. The 2015 El Niño also resulted in a short-term depression in POC and POP concentrations, whereas PON concentrations displayed an independent long-term decline regardless of the El Niño event. Reduced POM concentrations resulting from the 2015 El Niño occurred in parallel to high C:P and N:P ratios. Following the changes in PON, C:N continued to climb reaching ~9.4 at the end of our sampling period. We suggest that an Eastern Pacific- vs. a Central Pacific-type El Niño as well as a switch in the Pacific Decadal Oscillation phase significantly altered the local response in POM concentrations and ratios.