Size-fractionated C:N:P:Si stoichiometry of particulate matter in the subtropical Western North Pacific

Abstract

Particulate C:N:P:Si ratios and their variations in the upper ocean are crucial for understanding carbon export and its coupling with nutrient dynamics and phytoplankton community composition associated with nutrient limitation. Here, we present the first dataset of size-fractionated biogenic particle concentrations and their elemental ratios, including particulate organic carbon (POC), particulate nitrogen (PN), particulate phosphorus (PP), and biogenic silica (BSi), in the upper 500 m of the water column in the subtropical western North Pacific. The highest POC, PN, and PP concentrations consistently occurred in surface water and then decreased with increasing depth, whereas BSi concentration was frequently highest in the subsurface chlorophyll maximum layer at ∼ 120 m. The small size fraction (SSF, 0.8 or 1–51 μm) dominated the total pool of POC, PN, and PP but contributed less to the total BSi pool than the large size fraction (LSF, >51 μm). This feature was accompanied by lower C:N, C:P, and N:P ratios and higher C:Si ratios in the SSF than in the LSF. In the euphotic zone, total particulate C:N, C:P, N:P, and C:Si ratios averaged 8 ± 1, 146 ± 30, 17 ± 3, and 120 ± 48 mol:mol respectively, and exceeded the canonical Redfield and Brzezinski ratio. The three former ratios aligned with cellular C:N:P ratios of Prochlorococcus and Synechococcus. Moreover, these ratios exhibited observable latitudinal gradients; they were generally higher in the gyre center than in its southern boundary occupied by the North Equatorial Current. Below the euphotic zone down to 500 m, there was a distinct increase in SSF molar C:N, C:P, and N:P ratios with depth, while total particulate molar C:Si ratios remained relatively constant. Combined with data collected in the subtropical eastern South Pacific and North Atlantic, our results demonstrate that the composition of the phytoplankton community primarily controls particulate molar C:N:P:Si stoichiometry in the euphotic zone of ocean gyres, in particular in the SSF, below which preferential remineralization of various bioelements plays an important role.