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

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. Specifically, 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 specific decay product homologues of bacteriopheophytin-e (15 nmol/g OC). The down-core distribution of bacteriochlorophyll-e and bacteriopheophytin-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.