The lability and source of particulate organic matter in the northern Gulf of Mexico hypoxic zone

Decomposition of particulate organic matter (POM) plays a key role in the formation of hypoxia in subsurface waters of coastal ocean, yet little is known about the lability and transformation of POM in the hypoxic zone. Suspended particles were collected from surface waters to overlying waters (similar to 30 cm above the sediment-water interface) along the shelf of northern Gulf of Mexico (nGOM) in late spring/early summer of 2010-2013. Total hydrolyzable amino acids (THAA) and pigments were measured in these particulate samples to trace organic matter lability. The degradation indices, derived from the THAA and chloropigments, were positively correlated with dissolved oxygen (DO) concentrations in the shelf region, suggesting that decomposition of POM contributed greatly to DO utilization. Bacterial degradation appears to be the major pathway for POM decomposition on both inner and mid shelves, while zooplankton grazing played a minor role. POM samples in the overlying water on the inner shelf were the most degraded from the THAA and pigment results, and they also had high C/N ratios (9-14) and depleted delta C-13 values (-29 parts per thousand to -24 parts per thousand), pointing to a source of terrestrial C3 plant material. This distinct terrestrial signal of POM in the overlying water suggests strong selective degradation of marine-sourced organic matter, but how the terrestrial organic matter is settled to this layer and its ultimate fate remain unclear. Taken together, these data offer new angles looking into the lability and degradation pathways of POM, and mechanisms of hypoxia formation in coastal waters.