Plankton community respiration, net ecosystem metabolism, and oxygen dynamics on the Louisiana continental shelf: Implications for hypoxia

We conducted a multi-year study of the Louisiana continental shelf (LCS) to better understand the linkages between water column metabolism and the formation of hypoxia (dissolved oxygen < 2 ml O(2)l(-1)) in the region. Water column community respiration rates (WR) were measured on 10 cruises during spring, summer and fall seasons from 2003 to 2007 at multiple sites distributed across the Louisiana continental shelf, overlapping the region where bottom-water hypoxia occurs. We found consistent broad scale patterns in WR rates that followed depth and salinity gradients across the shelf. Observed WR rates were highest at low salinity inner shelf stations ( < 30 m depth) and decreased with increasing water depth. Surface waters had higher WR rates than bottom waters, a pattern most pronounced near the Mississippi river during spring and early summer. Surface water WR rates were highest in eastern transects and decreased westward; a trend that was not evident in bottom waters. WR tended to be higher in spring and summer compared to fall months, but overall the seasonal variability was small. We combined the WR rate measurements with contemporaneous measurements of phytoplankton productivity rates (reported in Lehrter et al., 2009, Continental Shelf Research, 29: 1861-1872) to estimate net water column metabolism. There was consistent evidence of net heterotrophy, particularly in western transects, and in deeper waters ( > 40 m depth), indicating a net organic carbon deficit on the LCS. We offer a simple scale argument to suggest that riverine and inshore coastal waters may be significant sources of organic carbon to account for this deficit. This study provided unprecedented, continental shelf scale coverage of heterotrophic metabolism, which is useful for constraining models of oxygen, carbon, and nutrient dynamics along the LCS. Published by Elsevier Ltd.