Historical trends of hypoxia on the Louisiana shelf: application of pigments as biomarkers Nianhong Chen a , Thomas S. Bianchi a, *, Brent A. McKee b , John M. Bland c a Institute for Earth and Ecosystem Sciences, Department of E.E. Biology, Tulane University, New Orleans, LA 70118, USA b Institute for Earth and Ecosystem Sciences, Department of Geology, Tulane University, New Orleans, LA 70118, USA c Southern Regional Research Center, USDA, ARS, New Orleans, LA 70148, USA Abstract Increases in the deposition of phytoplankton-derived organic carbon resulting from increases in nutrient inputs through the Mississippi±Atchafalaya system since the early 1950s has been speculated as the primary reason for the occurrence of hypoxic events in this region (Rabalais, N.N., Wiseman, W.J., Turner, R.E., Sen Gupta, B.K., Dortch, Q., 1996. Nutrient changes in the Mississippi river and system responses on the adjacent continental shelf. Estuaries 19(2B), 386±407). However, due to the lack of long-term measurements of oxygen in this region it is unclear if hypoxia events occurred prior to anthropogenic inputs of nutrients from the Mississippi river. In this study, we used naturally occurring radionuclides and plant pigment biomarkers to document changes in hypoxia events over the past 100 years. Speci®cally, we used pigments derived from the anoxygenic phototrophic brown-pigmented green sulfur bacteria Chlorobium phaeovibroides and C. phaeobacteroides. In sediments, at a hypoxic site west of the Mississippi plume, we observed high concentrations (52 nmol/g OC) of bacteriochlorophyll-e along with the speci®c decay product homo- logues of bacteriopheophytin-e (15 nmol/g OC). The down-core distribution of bacteriochlorophyll-e and bacter- iopheophytin-e homologues (in particular the more stable bacteriopheohytin-e) indicated that the highest concentrations occurred between 1960 and the present, coinciding with increased nutrient loading from the Mississippi river. These bacteriopigments were not detected prior to the early 1900s. These results are consistent with the view that increases in riverine nutrient loadings is likely the major cause of increasing trends in hypoxic events along the Louisiana coast over the past 50 years. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hypoxia; Anoxygenic phototrophic bacteria; Green sulfur bacteria; Mississippi river plume; Louisiana shelf; Carbon cycling; Pigment biomarkers 1. Introduction Hypoxic/anoxic waters have existed throughout geo- logic time, but their occurrence in shallow estuarine and coastal areas appears to be increasing through the last century, most likely due to the oversupply of nutrients from human activities. Eutrophication caused by human activities such as land-use, anthropogenic emissions, and enhanced use of fertilizers has been found to be a major cause of hypoxic conditions in areas such as the Baltic Sea (Cederwall and Elmgren, 1990), Adriatic Sea (Justic, 1991; Justic et al., 1993), and estuaries such as Long Island Sound (Welsh and Eller, 1991) and Chesa- peake Bay (Malone, 1991, 1992; Zimmerman and Canuel, 2000). The largest zone of oxygen-depleted coastal water in the US and the entire western Atlantic is in the northern Gulf of Mexico (Louisiana/Texas shelf) (Rabalais et al., 1998). Sporadic records of bot- tom oxygen depletion in this area began in the middle 1970 0 s (Ragan et al., 1978; Turner and Allen, 1982), with more systematic measurements of oxygen begin- ning in 1985 (Rabalais et al., 1991). Enhanced nutrient input through the Mississippi±Atchafalaya system has been proposed as the primary cause of hypoxia events in this region (Turner and Rabalais, 1994), however, the 0146-6380/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0146-6380(00)00194-7 Organic Geochemistry 32 (2001) 543±561 www.elsevier.nl/locate/orggeochem * Corresponding author. Tel.: +1-504-862-8000, ext. 1557; fax: +1-504-862-8706. E-mail address: tbianch@tulane.edu (T.S. Bianchi).