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Plankton community respiration, net ecosystem metabolism, and oxygen dynamics on the Louisiana continental shelf: implications for hypoxia
MURRELL, M. C., R. S. STANLEY, J. C. LEHRTER, AND J. D. HAGY. Plankton community respiration, net ecosystem metabolism, and oxygen dynamics on the Louisiana continental shelf: implications for hypoxia. Continental Shelf Research. Elsevier BV, AMSTERDAM, Netherlands, 52:27-38, (2013).
Determine linkages between water column metabolism and formation of hypoxia in the Louisiana continental shelf
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 mg L-1) in the region. Water column community respiration rates (WR) were measured on 10 cruises during spring, summer and fall seasons from 2003-2007 at multiple sites distributed along the Louisiana continental shelf, overlapping the region where bottom-water hypoxia occurs. We found consistent broad scale patterns in WR that followed depth and salinity gradients across the shelf. The highest metabolic rates were observed at low salinity inner shelf stations (< 30 m) and decreased with increasing water depth. Surface waters had higher heterotrophic metabolism than bottom waters, a pattern most pronounced near the Mississippi River during spring and early summer. Surface water WR was highest in eastern transects and decreased westward; a trend that was not evident in bottom waters. WR tended to be highest in spring and summer compared to fall months, but overall the seasonal variability was small. We combined the WR measurements with contemporaneous measurements of phytoplankton production (reported in Lehrter et al. 2009, Continental Shelf Research, 29: 1861-1872) to estimate net water column metabolism. While variable, there was consistent evidence of net heterotrophy, particularly in western transects, and in deeper waters (>40 m), indicating a net carbon deficit on the LCS. We suggest that riverine and inshore coastal waters likely represent significant net sources of organic matter 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.