Historical trends of hypoxia on the Louisiana shelf: application of pigments as biomarkers

Increases in the deposition of phytoplankton-derived organic carbon resulti ng from increases in nutrient inputs through the Mississippi-Atchafalaya sy stem since the early 1950s has been speculated as the primary reason for th e 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 measureme nts of oxygen in this region it is unclear if hypoxia events occurred prior to anthropogenic inputs of nutrients from the Mississippi river. In this s tudy, we used naturally occurring radionuclides and plant pigment biomarker s to document changes in hypoxia events over the past 100 years. Specifical ly, we used pigments derived from the anoxygenic phototrophic brown-pigment ed green sulfur bacteria Chlorobium phaeovibroides and C. phacobacteroides. 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 specific decay product homologues of bacteriopheophytin-e (15 nmol/g OC). The down-core distribution of bacteriochlorophyll-c and bacteriopheophytin- e homologues (in particular the more stable bacteriopheohytin-e) indicated that the highest concentrations occurred between 1960 and the present, coin ciding with increased nutrient loading from the Mississippi river. These ba cteriopigments were not detected prior to the early 1900s. These results ar e consistent with the view that increases in riverine nutrient loadings is likely the major cause of increasing trends in hypoxic events along the Lou isiana coast over the past 50 years. (C) 2001 Elsevier Science Ltd. All rig hts reserved.