U.S. Environmental Protection Agency
EPA Science Advisory Board
Background Information

Document NameSeasonal Phosphorus Limitation on the Louisiana Shelf: A Result of Nitrogen Loading from the Mississippi River?
Document AuthorAmmerman, J.W.
Dortch, Q.
Gaas, B.M.
Nelson, D.M.
Quigg, A.
Sylvan, J.B.
Tozzi, S
Short DescriptionPresented at the EPA Mississippi River Basin Nutrients Science Workshop, St. Louis, Oct. 4-6, 2005
CategorySubgroup 1: Characterization of the Cause(s) of Hypoxia
Publication Year2005

Abstract: Oxygen-depleted subsurface waters occur on the Louisiana Continental Shelf when the uptake of oxygen by respiration exceeds its resupply. Measurements of shelf water samples demonstrate that geographic extent of oxygen-depleted waters has increased since 1985.

We use the relative abundance of three low-oxygen-tolerant benthic foraminifers (Pseudononion atlanticum, Epistominella vitrea, and Buliminella morgani = PEB index) in sediment cores as a proxy for extending the record of low-oxygen bottom-water conditions on the Louisiana shelf back in time. The PEB index in four sediment cores provides evidence for low-oxygen events that pre-date the start of extensive use of commercial fertilizer in the Mississippi Basin (~1950). Fluctuations in the PEB index between 1817 A.D. and 1910 A.D. may correspond with increased discharge/flooding events in the Mississippi River drainage basin.
Our chronology for one long core from the Louisiana shelf (core PE0305-GC1) indicates low-oxygen events occurred periodically on the Louisiana shelf for at least the last 350 years. High PEB values similar to values found in the last 50 years occur in the early 1700s.

Carbon stable-isotope analyses of sedimentary organic matter have also been completed for core PE0305-GC1. In the upper 100 cm of the core, negative excursions in δ13C generally correspond to increases in PEB. Below the 100 cm core depth, the correspondence of δ13C and PEB is more variable. The very negative δ13C values found in the core most likely represent incorporation of biomass from anerobic microbial recycling communities to the bulk sediment and support the interpretation that high PEB values represent low-oxygen bottom-water conditions.

Fluctuations in the PEB index, supported in part by δ13C, indicate low-oxygen bottom-water events developed periodically on the Louisiana shelf before the extensive use of commercial fertilizer. The PEB proxy indicates that low-oxygen conditions near the Mississippi Delta in the early 1700s were as severe as conditions associated with hypoxia events of the last 50 years. Our results suggest that development of low-oxygen bottom waters on the Louisiana Shelf is a complex natural process that has been altered by human activities.

The abstract and Power Point slides to this presentation are available in PDF. The link to the PDF is here: