Distributions, speciation and stable isotope composition of organic matter in the southeastern Bering Sea

Seawater samples were collected from the southeastern Bering Sea for determinations of dissolved (DOC, <0.7 μm), colloidal (COC, 1 kDa–0.7 μm) and particulate organic carbon (POC, >0.7 μm), dissolved organic nitrogen (DON), particulate nitrogen (PN), and dissolved carbohydrate (DCHO) species, as well as stable isotopic composition (δ13C and δ15N) of suspended particulate organic matter (POM). DOC concentrations ranged from 61 to 92 μM (average 76±9 μM) with increasing concentrations from outer shelf to inner shelf along 166°W. DOC concentrations were significantly inversely correlated with salinity, indicating that the DOC distribution was predominantly controlled by water mixing although primary production is the ultimate source of organic matter. DON concentrations averaged 6.4±0.9 μM, ranging from 4.3 to 8.4 μM, resulting in an average DOC/DON ratio of 12±2 (ranging from 8 to 15). Total dissolved carbohydrates (TDCHO) comprised 13±3% of bulk DOC, with 35±15% of TDCHO in the polysaccharide and 65±15% in the monosaccharide fractions. Suspended POM had an average C/N ratio of 7.4±1.1, significantly lower than the average DOC/DON ratio (12±2). There was a significant correlation between chlorophyll-a and POC concentrations. However, Chl-a concentration was not closely coupled to DOC abundance. On average, POC comprised 14±5% of the total organic carbon (TOC) pool in the water column, and COC comprised 34±8% of the TOC, leaving 52±5% of the TOC in the <1 kDa low-molecular-weight dissolved phase. Values of δ13C and δ15N in the suspended POM ranged from −27.1‰ to −23.4‰ (average −24.5±0.9‰) and from 2.8‰ to 9.9‰ (average 6.0±1.9‰), respectively. Interestingly, particulate δ15N values were negatively correlated with nitrate (NO3) concentrations at low NO3 concentrations (≤5 μM) but positively correlated with NO3 concentrations when NO3 concentrations were >5 μM, indicating the effects of nutrient drawdown in surface waters and diagenesis below the euphotic zone on the isotope fractionation. POM containing lighter C and N isotopes appeared to be decomposed preferentially during their transport from upper water column to bottom sediments.