ISOTOPIC COMPOSITIONS OF LIPID BIOMARKER COMPOUNDS IN ESTUARINE PLANTS AND SURFACE SEDIMENTS

We examined the isotopic compositions of fatty acids, sterols, and hyd rocarbons isolated from three coastal macrophytes (Zostera marina, Spa rtina alterniflora, and Juncus roemerianus) in order to investigate th e relative contribution of these vascular plants as sources of organic matter in coastal sediments such as Cape Lookout Eight, North Carolin a. On average, lipid biomarker compounds extracted from the plants wer e depleted in C-13 by 3-5 parts per thousand relative to delta(13)C to tal organic carbon (TOC). However, individual compounds within each li pid class varied by up to 5.6 parts per thousand. Trends in the isotop ic compositions of lipids were consistent with delta(13)C(TOC); compou nds obtained from Z. marina were the most enriched in C-13 and those f rom J. roemerianus were the most depleted. The range in isotopic abund ances and molecular compositions of the sediments was greater than tha t obtained from the plants, indicating that additional, presently unid entified sources of organic matter contribute to the Cape Lookout Eigh t sediments. Similarity between the signatures for suspended particula te matter and the sediments indicates that much of the sedimentary org anic matter is derived from algal and bacterial sources. Bacterial sou rces of organic matter are Likely greater during summer/early fall, an d incorporation of C-13-enriched bacterial biomass may contribute to o bserved seasonal shifts in delta(13)C(TOC) in the surficial sediments.