A gradient of dissolved organic carbon and lignin from Terrebonne–Timbalier Bay estuary to the Louisiana shelf (USA)

Abstract

Here we report on spatial and temporal changes in the concentration and composition of total dissolved organic carbon (DOC) and dissolved lignin-phenols (sum 8) in surface and bottom waters in estuarine and inner shelf off the Louisiana coast (USA). DOC samples were collected at 6 stations on 3 cruises (April 2008, July 2008) for organic analyses along a transect that spanned from inside Terrebonne–Timbalier Bay estuary, Louisiana (7 m water depth) to the outer-most station on the inner Louisiana shelf (18.5 m water depth). An additional set of samples was taken for DOC and dissolved lignin-phenols in April 2009 in the upper marshes of the estuary in water depths typically ca. 1 m. Hydrographic properties of shelf waters in April 2008 were drastically different than in July 2008 because of the higher river discharge from the Mississippi River (MR) and the opening of man-made water diversion structures in Louisiana in April. There were no significant differences in DOC concentrations between the inner bay and shelf stations, likely due to extensive mixing between the bay and shelf waters. However, there were significantly higher concentrations of dissolved lignin (sum 8) and Λ 8 at the marsh stations ( x̄ = 0.28 ± 0.15 mg L − 1 and x̄ = 0.69 ± 0.35), compared to the inner shelf ( x̄ = 0.16 ± 0.06 mg L − 1 and x̄ = 0.24 ± 0.12) stations, respectively. Low S/V and C/V ratios observed in Terrebonne–Timbalier Bay estuary were likely the result of selective degradation of syringyl and cinnamyl over vanillyl lignin-phenols, and not inputs of woody gymnosperms. A gradient of decreasing S/V ratios along a transect from the upper marshes to the shelf was likely the result of photochemical breakdown of lignin and dilution effects from marine waters near the outer reaches of the estuary. Inputs of photochemically-altered DOC from estuaries such as Terrebonne–Timbalier Bay to the inner Louisiana shelf may provide an additional source of OC for microbial food webs in the northern Gulf of Mexico.