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Sediment-water fluxes of dissolved inorganic carbon, O2,nutrients, and N2 from the hypoxic region of the Louisianacontinental shelf
Citation:
LEHRTER, J. C., R. DEVEREUX, D. L. BEDDICK, D. F. YATES, AND M. C. MURRELL. Sediment-water fluxes of dissolved inorganic carbon, O2,nutrients, and N2 from the hypoxic region of the Louisianacontinental shelf. BIOGEOCHEMISTRY. Springer, New York, NY, 109:233-252, (2012).
Impact/Purpose:
The purpose of this research was to quantify the role of the sediments as both a source and sink of nutrients and as a site of respiration of organic matter in the hypoxic region of the Louisiana shelf. This study represents the most comprehensive evaluation to date of sediment respiration, nutrient fluxes, and denitrification and addresses several important gaps in our understanding of sediment processes on this shelf. The results also indicate that revisions to the current conceptual model describing how sediments contribute to the onset and maintenance of hypoxia are necessary.
Description:
Sediment fluxes of dissolved inorganic carbon (DIC), O2, nutrients, and N2 (denitrification) were measured on the Louisiana Continental Shelf during six cruises from 2005 to 2007. On each cruise, three to seven stations were occupied in regions of the shelf that experience summer bottom-water hypoxia. Station average DIC fluxes (range of 14.6 to 19.9 mmol m-2 d-1) did not vary significantly either spatially or as a function of bottom-water O2 concentration. Sediment O2 fluxes, however, did vary with changing O2 concentrations, and below 16 mmol O2 m-3 sediment O2 fluxes were below detection. Based on the invariance of DIC fluxes as a function of O2 concentration, anaerobic processes were the dominant metabolic pathways in the sediments. In contrast to previous estimates of sediment respiration accounting for approximately 75% of the sub-pycnocline respiration, we observed the sediment respiration to be 19 to 31% of the sub-pycnocline respiration at our four sites. Similar to DIC fluxes, nutrient and N2 fluxes did not exhibit spatial patterns. Average fluxes of dissolved inorganic N, P, and Si potentially supported 2-7%, 0-4%, and 9-26% of phytoplankton N, P, and Si demand, respectively. Station average N2 fluxes ranged from 0.4 to 1.4 mmol N2 m-2 d-1. Scaled to the area of the shelf, this N sink represents roughly 39% of the N load from the Mississippi and Atchafalaya Rivers. The magnitudes of fluxes and lack of spatial patterns observed challenge previous conceptualizations of the role of the sediments in Louisiana shelf carbon, oxygen, and nutrient cycling.