Concurrent reductions in sinking particle flux and its ratios of opal and organic carbon to CaCO ₃ in the oligotrophic western North Pacific Ocean during 2007–2014

Sediment trap data collected from the western North Pacific Ocean (NPO) revealed a reduction in sinking particle flux (SPF) to the mesopelagic zone (~1000 m) during 2012–2014 compared to levels during 2007–2011. Satellite-based net primary production (NPP) estimates showed no discernable decline with time, resulting in a decreasing SPF/NPP ratio. Based on a synchronized reduction in the opal/CaCO ₃ ratio, the decline in SPF can be attributed to a shift in the relative composition of phytoplankton from a group with relatively high nutrient demand (i.e., diatoms) to those with low nutrient demand (i.e., coccolithophores and other small phytoplankton adapted to a more nutrient-depleted environment). This shift could result in a decline in particle export below the mixed layer depth (MLD) and an increase in regenerated production (or nutrient recycling) within the MLD, which could be reconciled with decreasing SPF and non-decreasing NPP, as shown by the significant correlation between SPF/NPP and the opal/CaCO ₃ ratio. Environmental factors associated with nutrient availability were investigated, along with climate variability indices of the NPO. Using eddy kinetic energy, vertical Ekman transport, and vertical density profiles, the nutrient deficiency was partially explained during the seasons showing significant SPF reductions. An association between SPF and the Pacific Decadal Oscillation was identified. However, the 7-year data were not sufficient for verification. The results of this study indicate that particle fluxes to depth and ocean biological pumps are subject to interannual variability caused by various factors.