Benthic Lipids of Narragansett Bay Fatty Acids and Hydrocarbons
Abstract
The intent of this investigation was to expand our knowledge of the biogeochemistry of fatty acids and hydrocarbons in recent sediments. Several techniques for extracting fatty acids from sediments were evaluated and saponification was found to give the best yield. Samples of sediments, sewage effluents, and benthic animals were extracted to obtain the lipid fraction. The lipid extracts were analyzed for fatty acids and hydrocarbons by gas chromatography. Lipolytic activity in the sediments was demonstrated using tributyrin and olive soil assays.
Results confirmed that consistent sampling and extraction techniques should be applied when comparing fatty acid distributions in sediments from different depositional environments. The “free” fraction of fatty acids extracted by organic solvents exhibited different relative abundance patterns than the “bound” fraction of fatty acids extracted by saponification of the sediment residue from organic solvent extraction. Analysis of core sections showed the total fatty acids in sediments decreases rapidly with a 76 to 90 per cent less in 1000 years. The unsaturated 16:1 and 18:1 fatty acids decrease rapidly with increasing depth relative to the saturated fatty acids 16:0 and 18:0. The comparison of fatty acid distribution in the “free” and “bound” fractions, the analyses of the fatty acid distribution with increasing depth in recent sediments, and the demonstration of lipolytic activity in the sediments allowed the construction of a model for fatty acid diagenesis in recent sediments.
The comparison of stations sampled on the basis of the concentration of organic carbon, fatty acids, and hydrocarbons and the ratio of the hydrocarbon concentration to organic carbon divided the sampling area into three depositional environments – Providence River, Upper Bay, West Passage. Hydrocarbons which appear to be of petroleum origin by altered by degradation were present in the Providence River and Upper Bay sediments and could still be detected in the station farthest towards the mouth of the bay. A hydrocarbon thought to be a C21 branched olefin from natural sources was found in the West Passage sediments.
Sewage effluents discharge approximately 430,000 liters of hydrocarbons annually to the river as calculated from analysis of the effluents. Some of these hydrocarbons appear to be partially degraded by the microorganisms present in the sediments due to the availability of sufficient nutrients discharged in the treated sewage.
The hard shell clam samples (Mercenaria mercenaria) from the river appear to be heavily contaminated with oil. Hard shell clams obtained in the West Passage are less contaminated. The Providence River, Upper Bay, and some West Passage areas of Narragansett Bay appear to be chronically polluted with hydrocarbons from petroleum products.
The ratio of the 18:1/18:0 fatty acids of Nepthys incisa, an infaunal worm, obtained from a polluted area was significantly lower than that ratio for Nepthys from two clean areas. The fatty acid composition of Yoldia limatula is also reported. It contains a high percentage of a fatty acid which has been tentatively identified as 22:2.