Stoichiometry of dissolved organic matter dynamics on the continental shelf of the northeastern U.S.A.

Abstract

Dissolved organic carbon (DOC), nitrogen (DON), phosphorus (DOP), inorganic nitrogen and inorganic phosphorus were measured from the Georges Bank region during spring 1993 and summer 1994. Concentrations of DOC in surface waters ranged from 65–92 μM C and decreased with depth to about 50 μM in shelf slope water at a depth of 1500 m. DON and DOP concentrations showed a similar pattern with depth, being higher in surface water and lower in deep water (∼5 to 3 μM N and 0.17 to 0.02 μM P). There was an inverse relation between inorganic nutrient concentrations and DON and DOP. While DOC concentrations were elevated in regions of highest primary production, there was little evidence of corresponding elevations for DON and DOP. Slopes of regression lines relating total dissolved N and P to inorganic N and P indicated that 19 and 15% of remineralized N and P that accumulates in deeper water is derived from the export and decomposition of dissolved organic material (DOM). While overall inorganic N and P dynamics followed Redfield, A. C. (1958) ( American Scientist, 46, 205–221) stoichiometry (C:N:P, 106:16:1), the C:N:P stoichiometry of dissolved organic matter deviated substantially. Near-surface stoichiometry of DOM was 400–800:1 for DOC:DOP, 11–15:1 for DOC:DON and 24–55:1 for DON:DOP. Relationships between DOC, DON and DOP indicated that N and P were preferentially remineralized relative to carbon resulting in C:N: P ratios for deeper water DOM that deviated even further from Redfield. Decomposition studies revealed that DOC decomposition rate was related to initial DOC concentration. The higher the DOC concentration the more quickly it was mineralized. DOC first order decay coefficients ranged from undetectable for deep slope water with an initial DOC concentration of 50 μM to 0.0024 day −1 for water collected from the Georges Bank shoals with an initial DOC concentration of 92 μM. DON and DOP decomposition occurred concomitantly with DOC remineralization. Loss rates ranged from undetectable to 0.00064 day −1 for DON and from undetectable to 0.0023 day −1 for DOP. During DOM decomposition we noted increases in the C:N, C:P and N:P ratios, which indicated preferential remineralization of N and P relative to C. Vertical concentration gradients and slow DOM turnover times relative to water residence times in the Georges Bank continental shelf region indicate the potential for export of DOM to deeper water. DOM export to depth and preferential remineralization of N and P relative to C may be important mechanisms for pumping C to depth in the ocean.