Bulk Organic Matter and Lipid Biomarker Composition of Chesapeake Bay Surficial Sediments as Indicators of Environmental Processes

Abstract

Seasonal measurements of lipid biomarker (fatty acid and sterol) composition along with organic carbon and nitrogen elemental and stable isotopic signatures were made in surficial sediments collected along the salinity gradient of the Chesapeake Bay mainstem. These data along with water quality information including chlorophyll and dissolved oxygen concentration were used to assess temporal and spatial variations in organic matter (OM) composition and the processes that control its distribution. While the amount of OM in sediments was largely related to sediment surface area and exhibited very little seasonal variability, OM lipid composition was spatially and temporally variable. Principal components analysis (PCA) identified three suites of lipid compounds that encapsulate these elements of variability. The first, representing allochthonous versus autochthonous OM identified the Northern Bay as the major site of terrestrial OM deposition. The greater contribution of terrestrial OM in this region was supported by elemental C:N and stable isotope data. The second was identified as a seasonal component of lipid composition and indicated the deposition of labile, primarily diatom-derived OM in the spring and degradation of this OM through the summer and fall. This component was particularly enriched in Southern Bay sediments relative to other portions of the Bay and varied with tributary water inflow. A third component of OM composition represented microbially-derived OM which, although most abundant in the Mid-Bay, represented the greatest fraction of OM in the Southern Bay. Sediments of the Mid-Bay were particularly enriched in flagellate-derived OM in the summer. Sediment OM composition was not influenced by water-column dissolved oxygen concentration. The combination of lipid biomarkers and PCA proved a more sensitive indicator of sediment OM sources and reactivity than bulk elemental or isotopic data and presents a picture of the estuary as a trap for both allochthonous and autochthonous OM. The high degree of spatial and temporal variability in estuarine sediment OM composition may influence the distribution of benthic communities and the long-term sediment record.