Organic matter sources and their use by bacteria in the sediments of the Altamaha estuary during high and low discharge periods

Abstract

To understand the influence of river discharges on the biogeochemical cycling of organic matter (OM) in estuarine sediments, we conducted chemical and isotopic analyses of surface sediment samples collected from the Altamaha estuary (Georgia, USA) in March 2002 (high discharge period) and October 2002 (low discharge period). Chemical and isotopic analyses included bulk parameters (TOC, TN, δ 13C TOC and δ 15N TN), chlorophyll- a (Chl- a), lipids (fatty acids, alcohols, and sterols) and lipid carbon isotopic compositions. Bulk parameters were apportioned with a three-end-member mixing model and biomarkers were processed using principal component analysis (PCA). The modeling indicated that OM from C3 terrestrial and marine sources dominate (>80%) at most sediment sites during the two periods while C4 salt marsh plants contribute a large fraction (∼40%) of OM at one site during the low discharge period. The PCA demonstrated that more allochthonous OM deposits in the high discharge period while more autochthonous OM accumulates in the low discharge period; OM in the high discharge period is relatively fresh while that in the low discharge period is highly degraded. Distributions of bacteria-specific fatty acids in the estuarine sediments followed the same patterns as Chl- a, algal lipids, and mixed lipids but were not correlated with terrestrial lipids, suggesting that the microbial community in the sediments depends largely on OM from phytoplankton rather than from terrestrial higher plants. When salt marsh plants became an important input into the sediments, bacteria efficiently used this OM, indicated by the coincident changes between molecular isotope ratios of bacteria-specific fatty acids and bulk δ 13C TOC.