Implications for the role of pre- versus post-depositional transformation of chlorophyll- a in the Lower Mississippi River and Louisiana shelf

Abstract

Water column particulates and sediments were collected in September 1998 and July 1999 from the Lower Mississippi River and two shelf sites, an oxic shelf site within the existing plume region and a hypoxic shelf site west of the existing plume. Sediment core samples were collected in April 2000 along a cross-shelf transect that included hypoxic waters. Photosynthetic pigments (chloropigments and carotenoids) and chlorophyll- a (Chl- a) transformation products in water column particulates and sediments were examined using high performance liquid chromatography (HPLC) on-line photodiode array (PDA) detector and HPLC on-line mass spectrometry (MS). Based on class-specific plant pigment concentrations, diatoms and cyanobacteria were found to be the major sources of organic matter buried in Louisiana (LA) shelf sediments. Pheophytin- a (17.3±6.3%), pyropheophytin- a (24.6±8.5%), chlorophyllone- a (9.6±4.9%), carotenoid chlorin esters (CCEs, 13.7±5.4%) and sterol chlorin esters (SCEs, 25.5±8.5%) were the dominant chlorophyll- a transformation products in shelf sediments. The absence of the two grazing biomarkers, CCEs and SCEs, in river sediments suggests that inputs from water column macrozooplankton grazing were not as important in river waters as compared to shelf waters. High concentrations of chlorophyllone- a in river sediments suggest that it was not likely produced through macrozooplankton grazing processes—like CCEs and SCEs. Our results indicate that chlorophyllone- a was likely to be associated with phytoplankton senescence and microbial breakdown of chlorophyll- a. Correlation analyses revealed significant correlations between SCEs, CCEs and pyropheophytin- a in shelf sediments, which indicate that their production in shelf waters was closely coupled to grazing activities on the shelf. Although many of the specific mechanisms of chlorophyll- a decay proposed in this study remain speculative, this work does provide a general foundation, using state-of-the-art analyses of plants pigments, for comparing and contrasting pre- and post-depositional carbon decay pathways in river and shelf environments.