Biochemical Composition of Particles and Dissolved Organic Matter Slong an Estuarine Gradient: Sources and Implications for DOM Reactivity

The chemical composition of high molecular weight dissolved organic matter (DOM) and particulate organic matter (POM) was examined along the salinity gradient of the Delaware Estuary. DOM was collected and fractionated by tangential-flow ultrafiltration into 1–30 kDa (HDOM; high molecular weight) and 30 kDa to 0.2 mm (VHDOM; very high molecular weight) and compared to particles collected in parallel. Polysaccharides comprised 12–43% of particulate organic carbon (POC), 30–56% of VHDOM carbon, and 7.5–19% of HDOM carbon. Hydrolyzable amino acids comprised 17–38% of POC, 5.4–12% of VHDOM carbon, and 1.5–4.2% of HDOM carbon. Only 7–43% of dissolved organic nitrogen in VHDOM and HDOM consisted of amino acids, indicating that organic nitrogen is highly modified within the dissolved pool or an unidentified pool of dissolved organic nitrogen exists. The composition of amino acids and distribution of polysaccharides are consistent with enrichment of structural biopolymers from algae and vascular plants within DOM. Proteinaceous matter released during the growth of an axenic diatom culture contains similar amino acid distributions across size fractions as in Delaware Bay samples. The source of organic matter appears to be as important as microbial processing in determining amino acid content and composition of DOM. Shifts in amino acid composition point to contrasting sources and extent of degradation for organic matter along the estuarine gradient and among size fractions. The lower amino acid and carbohydrate content and higher b-alanine content in HDOM suggests that this fraction is more highly degraded relative to POM and VHDOM and provides geochemical evidence in support of the size-reactivity continuum hypothesis. Spatial patterns in reactivity of organic constituents were also evident with more degraded organic matter in the turbid middle estuary and the release of fresh DOM from diatoms in the lower estuary. Dissolved organic matter (DOM) is the largest reservoir of organic carbon in the ocean and is comparable to the amount of CO2 in the atmosphere (0.66 3 1018 g C; Hedges 1992). The sources and cycling of DOM is thus a key component of the global carbon cycle, but its dilute concentration within an ion rich solution and chemical heterogeneity have resulted in much of the DOM (46–87%) to be unidentified at the molecular level (e.g., Buffle 1988). Radiocarbon measurements appear to reflect this chemical complexity. Surface ocean DOM is on average .1,000 yr old (56% ,30 yr and 44% .6,000 yr), and deep ocean DOM is extremely old, ;6,000 yr old, and thus appears highly refractory (Williams and Druffel 1987). Results from estuarine, continental shelf, and slope waters suggest molecular size is an important property with .10 kDa DOM being contemporary in age (post-1950) with residence times of 1–30 d, whereas the 1–10 kDa fraction is much older (.1 kDa is 380–4,500 yr old; Santschi et al. 1995). These observations suggest that macromolecular DOM cycles rapidly within estuarine and 1 Corresponding author ([email protected]).