Chemical composition of particulate and macromolecular dissolved organic matter in the Delaware estuary and experimental diatom blooms: Sources and reactivity patterns

To advance our understanding of source contributions and reactivity of carbon within coastal systems, particles and high molecular weight dissolved organic matter (>1 kDa; UDOM) were isolated along a transect from freshwater to the coastal ocean of Delaware Bay for chemical characterization. To examine the composition of UDOM released in the absence of bacteria, an axenic culture of Skeletonema costatum, the dominant species in the Delaware Estuary, was grown to late log-phase. A second culture was grown to senescence in a mesocosm and placed in darkness for 51 days with multiple samples collected during growth and decay. DOM was fractionated by ultrafiltration into high (1–30 kDa; HDOM) and very high (30 kDa-0.2 μm; VHDOM) molecular weights.; Carbohydrates and total hydrolyzable amino acids (THAA) were the major components of dissolved and particulate fractions. In Delaware Bay, carbohydrates comprised 12–43% of POC, 30–56% of VHDOM-C, and 7.5–19% of HDOM-C. THAA comprised 17–38% of POC, 5–12% of VHDOM-C, and 1.5–4% of HDOM-C. Lipids constituted up to 0.3% of HDOM-C, 1.6% of VHDOM-C and 10% of POC; the majority of which were fatty acids. Stable carbon isotopes, fatty acids, sterols and lignin content indicated shifts in sources with terrigenous and planktonic material in the river and turbid region of the estuary and a predominantly algal/planktonic signal in the lower estuary and coastal ocean. Terrigenous organic matter comprised 6.6% of HDOM in the coastal ocean to 34% of HDOM in the turbid region. The flux of terrigenous DOM from the Delaware Estuary to the coastal ocean was estimated at 1.2 × 1010 g OC y−1 and suggests that temperate estuaries can be significant sources of terrigenous organic matter.; Diatom cultures revealed that 28–33% of primary production is released as DOM. Similarities in HDOM and VHDOM composition among estuarine samples and diatom cultures highlight the impact that the source of DOM has on composition versus secondary factors such as grazing and microbial degradation. The lower lipid, amino acid and carbohydrate content and higher β-alanine content in HDOM from the estuary and diatom experiments suggest that this fraction is more degraded relative to POM and VHDOM. This provides geochemical evidence that VHDOM is closer in composition to its biological origin and supports the size-reactivity continuum hypothesis which proposes that lability generally increases with size. Previous work did not examine the range of organic components analyzed in this research nor the subtle changes which occur to POM and DOM as material flows from the river to the ocean.