Time-series study of the biogeochemistry of the North East subarctic Pacific: reconciliation of the Corg/N remineralization and uptake ratios with the Redfield ratios

Abstract

Abstract Ten years of a time-series study at Station P or OSP (50°N, 145°W) in the NE subarctic Pacific are presented in this paper. Measurements of nutrients, primary production, dissolved inorganic carbon (DIC), total alkalinity (TA), dissolved organic carbon and nitrogen (DOC and DON), and sinking organic and inorganic particulate fluxes (derived from free-drifting sediment traps positioned every 100 m in the upper 1000 m) were made. One-dimensional C and N budgets were calculated in the mixed layer and in 4 layers below 50 m down to 1000 m. The remineralization and uptake stoichiometric ratios were determined, with an emphasis on the C org /N ratios. Three independent approaches were used: (1) nutrient vertical gradients in the 100–1000 m, (2) nutrient seasonal drawdown in the 0–50 m upper layer, and (3) nutrient regeneration from sinking particles. Analysis of the nutrient vertical gradients showed that the P/N/C org /−O 2 remineralization molar ratios were 1/16.6/107.9/−170.7 and that the average C org /N was about 6.5 in the upper 400 m. In contrast, the seasonal DIC:NO 3 − drawdown molar ratio varied widely from 6.9 to 12.1 and averaged 9.3. However, when corrections were made for CO 2 gas exchange, dissolved organic matter (DOM) production and CaCO 3 production, the true average C org /N uptake ratio was estimated to be 6.1. This study suggested that the elevated DIC:NO 3 − depletion ratios were mostly due to calcification, and that the net effect of gas exchange and DOM production was small at station P. Furthermore, whereas the POC/PON flux (particulate organic C and N fluxes) ratio ranged from 8 to 12 (mole ratio) and increased with depth, there was a linear relationship between the POC and PON fluxes, with a slope of 7.2±0.2. This indicated that the C org /N remineralization ratio from sinking particles was relatively low and that there was a methodological problem with free-drifting sediment traps.