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381 ␥ overturning. Therefore, the zone is expressed as the past frequency of hypoxia occurrence at a particular location since the initiation of shelfwide monitoring (Rabalais et al. 1999) (Fig. 23.1). The survey data demonstrate that the size of the area affected by hypoxia has been increasing since 1985. However, no routine shelfwide measurements of the extent of subsurface-water oxygen content were made before 1978 (Rabalais et al. 1999; Turner et al. 2005). In this chapter, we discuss the use of low-oxygen– tolerant benthic foraminifers and geochemical measurements in sediment cores from the Louisiana shelf as proxies for extending the observed record of low-oxygen bottom-water conditions back through time and space. Low-Oxygen Faunal Proxy Both living and fossil calcareous benthic foraminifers have proven to be valuable tools for assessing oxygen conditions of bottom water (Bernhard 1986; van der Zwaan and Jorissen 1991; Kaiho 1994) and as a proxy for bottom-water hypoxia in the Santa Barbara Basin, Adriatic Sea, and elsewhere (Bernhard et al. 1997; Bernhard and Sen Gupta 1999; Duijnstee et al. 2004). Only a few studies have investigated the faunal sedimentological record near or within the Louisiana hypoxia zone. Blackwelder et al. (1996) investigated core BL10 from near the Mississippi delta (Fig. 23.1). Core BL10 (sediment accumulation rate = 0.55 cm/yr), collected in 1993, has a 210 Pb chronology covering ~90 years and provides evidence for Oxygen-depleted subsurface waters occur on the Louisiana continental shelf when the uptake of oxygen exceeds its resupply. When oxygen concentrations fall below 2 mg/L, it is operationally defined as hypoxic. Measurements of Louisiana continental shelf waters demonstrate that the size of the hypoxic zone has increased since 1985 (Rabalais et al. 1999; CENR 2000; Rabalais and Turner 2001) (Fig. 23.1). The likelihood of hypoxia occurring is increased with enhanced nutrient loading and the onset of greater water-column stratification (van der Zwaan 2000; Rabalais 2002). Increased nutrients can stimulate marine surface phytoplankton blooms whose organic matter, upon death, may sink to the bottom and decay. The modern recurrent hypoxia on the Louisiana shelf and expansion of the area of the hypoxic zone has been attributed to the increased use of fertilizer in the Mississippi River basin (Dinnel and Bratkovich 1993; Nelson et al. 1994; Rabalais et al. 1994; Rabalais et al. 1996; Rabalais et al. 1999; Goolsby et al. 2001). The increased frequency and expansion of hypoxic conditions has become an important economic and environmental issue to commercial and recreational fisheries in this region of the Gulf of Mexico (Malakoff 1998; Diaz and Solow 1999). Beginning in 1985, systematic summertime surveys have measured the oxygen content of subsurface waters across the Louisiana shelf (Rabalais et al. 1999). Because the hypoxia exists on an open shelf as opposed to a closed basin, the temporal and spatial extent fluctuates throughout the year but is concentrated in the warmer summer months when thermal stratification inhibits water column 23 Over 300 Years of Anthropogenic and Naturally Induced Low-Oxygen Bottom-Water Events on the Louisiana Continental Shelf Lisa E. Osterman, Richard Z. Poore, Peter W. Swarzenski, David Hollander, and R. Eugene Turner 382 ~ Osterman, Poore, Swarzenski, Hollander, and Turner Louisiana continental shelf and slope and showed that the distribution of 3 benthic foraminiferal species can be used to characterize the hypoxia zone. The cumulative percentage of 3 foraminifers, named the PEB index (P. atlanticum, E. vitrea, and B. morgani), was higher in the surface-sediment samples collected in the Louisiana hypoxia zone than elsewhere along the Texas–Louisiana shelf (Osterman 2003). Benthic foraminiferal assemblages in surface samples from the eastern part of the hypoxia zone (Platon and Sen Gupta 2001; Platon et al. 2005) confirm that the PEB index species are the most common living foraminiferal species in the Louisiana hypoxia zone. The PEB species also live in slightly deeper water (30– 70 m water depth) than the taxa used in the A–E hypoxia index (Sen Gupta et al. 1996), and thus the PEB index can extend the application of foraminiferal hypoxia proxies over a wider area of the continental shelf. The PEB species are opportunists that inhabit nutrient-rich environments, tolerate low-oxygen conditions, and are believed to be epifaunal (Blackwelder et al. 1996; Austin and Evans 2000; Gooday and Hughes 2002) or migrate to epifaunal habitats in nutrient-rich, low-oxygen environments (Jorissen et al. 1992; Ernst and van der Zwaan 2004). Total loss of oxygen (anoxia) will result in death for many...

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