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Spatially Variable Physical Energy and Bioturbation Drive the Biogeochemical Seascape in the Louisiana Continental Shelf Hypoxic Zone
Citation:
Devereux, R., J. Lehrter, G. Cicchetti, D. Beddick, D. Yates, B. Jarvis, J. Aukamp, AND M. Hoglund. Spatially Variable Physical Energy and Bioturbation Drive the Biogeochemical Seascape in the Louisiana Continental Shelf Hypoxic Zone. BIOGEOCHEMISTRY. Springer, New York, NY, 143(2):151-169, (2019). https://doi.org/10.1007/s10533-019-00539-8
Impact/Purpose:
It is unknown how sediments on the Louisiana Continental Shelf vary spatially, both inshore to offshore, and from the Mississippi River delta westward. Sediments are important determinants of the extent and duration of bottom water hypoxia that occurs seasonally in this region of the Gulf of Mexico. This study provides valuable information over a large area of the shelf on variation of benthic macrofauna communities, changes in how organic matter is processed, and the biological and physical forcings that determine rates of OM degradation in sediments. Knowing the spatial variation in sediment processes will now aid the development of tools to guide decisions aimed to reduce hypoxia on this shelf and in other continental shelf regions around the globe where hypoxia is becoming a significant concern.
Description:
Sediment biogeochemical properties and benthic macrofauna were characterized at twelve stations along four transects spanning 320 km across the LCS hypoxic zone in the season when bottom-waters return to oxic conditions but before the arrival of major winter storms. Macrobenthic community condition was assessed using sediment profile imaging (SPI) which showed communities dominated by small infauna across the sampled area, with greater abundance in deeper water. Evidence of larger fauna was found at only one station. Chemical analyses and images together provided spatial information on three primary pathways of microbial degradation of OM: sulfate reduction in anoxic sediments; metal oxide cycling in sediments beneath oxic waters with iron and magnesium as electron receptors; and faunal bioturbation delivering oxygen deeper into the sediments as the terminal electron receptor or oxidant. Stations nearest shore had lower amounts of oxidized iron and higher sulfate reduction rates, seen also in the images as black sediments, than the deepest stations. High concentrations of porewater Mn and Fe2+ and solid phase oxidized iron along the easternmost transect (20 – 40 km west of the Mississippi River delta) and imaged sediment structure are suggestive of metal oxide cycling potentially linked to carbon oxidation pathways. Sediment profile images showed oxidized iron as red and orange sediments, and provided evidence of physical energy and sediment resuspension, conditions that support metal oxide cycling. Chemistry and faunal abundance in images also showed areas where bioturbation likely mixed oxygen into sediments, enabling aerobic microbial pathways that contribute to OM mineralization. These results provide a larger spatial assessment of benthic macrofaunal communities and sediment biogeochemistry, show areas with potentially different pathways of OM degradation, and demonstrate the importance of both bioturbation and mixed muddy sediments in processing large amounts of OM on the LCS. This work has particular significance given the massive sediment discharge between the Mississippi and Atchafalaya Rivers onto the LCS.
URLs/Downloads:
DOI: Spatially Variable Physical Energy and Bioturbation Drive the Biogeochemical Seascape in the Louisiana Continental Shelf Hypoxic Zone
ORD-023717 FINAL MANUSCRIPT.PDF (PDF, NA pp, 2839.919 KB, about PDF)