An investigation of size-fractionated organic matter from Lake Superior and a tributary stream using radiocarbon, stable isotopes and NMR

摘要

This study investigated the concentration and isotopic composition of different size fractions of organic matter (OM) in Lake Superior and in one of its many tributary streams and rivers (Amity Creek, Duluth, Minnesota, USA). Structural compositional drivers of the △~(14)C of high molecular weight (HMW, >1 kDa) dissolved organic carbon (DOC) in the lake were also evaluated. Low molecular weight (LMW, <1 kDa) DOC was the fraction containing the largest proportion (68–88%) of organic carbon (OC) in the lake. Particulate organic carbon (POC, >0.7 lm) was generally ~(13)C-depleted (-29 ± 1.2‰) relative to “bulk” (<0.7 lm) DOC (-26.4 ± 0.7‰), “init” (<0.2 lm) DOC (-26.6 ± 0.8‰), HMW DOC (-26.9 ± 0.3‰) and LMW DOC (-26.5 ± 0.9‰), and had more variable ~(14)C content (△~(14)C of -94‰ to 53‰; 735 years BP to modern) than the other size fractions. Init DOC (△~(14)C of 17–59‰), HMW DOC (△~(14)C of 23–64‰) and LMW DOC (△~(14)C of 16–62‰) all reflected contemporary (modern) radiocarbon signatures. Bulk DOC (△~(14)C of -19‰ to 57‰; 90 years BP to modern) had modern radiocarbon values in the offshore sites (△~(14)C of 2–57‰) with pre-aged samples (△~(14)C of -8‰ to -19‰) seen at the nearshore site. HMW DOM was relatively N-poor (C:N of 12–19) compared to particulate organic matter (POM, C:N of 8–10) revealing either a more diagenetically altered state or contrasting sources. ~(13)C NMR data showed that biochemical composition of HMW DOC in Lake Superior was dominated by carbohydrates (53–65%) with only trace aromatic components (2–4%). Structurally complex components such as heteropolysaccharides (HPS), amide/peptides and amino sugars (AMS) constitute 75–84% of HMW DOC whereas carboxylic-rich alicyclic molecules (CRAM) made up 16–25% of HMW DOC in the lake. Combined HPS and AMS, O-alkyl carbohydrate carbon, and total carbohydrate carbon contents were significantly positively correlated to the △~(14)C of HMW DOC suggesting they contribute a contemporary ~(14)C-enriched component to the HMW DOC in the lake. In contrast, CRAM and aliphatic carbons were significantly inversely correlated with △~(14)C of HMW DOC implying that these represent ~(14)C-depleted (older) components of HMW DOC in the lake. At Amity Creek, storm events led to larger loads of bulk DOC and POC, which were both ~(14)C-enriched (modern). However, in baseflow conditions ~(14)C-depleted bulk DOC and POC were exported. LMW DOC from the creek was considerably ~(14)C-depleted compared to concurrent HMW DOC.