Lipids as indicators of eutrophication in marine coastal sediments

Total organic carbon (TOC) and sedimentary lipid contents were investigated in the Bunnefjord, the most inner part of the Oslofjord (Norway). The Bunnefjord is an intermittent anoxic basin and has undergone major eutrophication since the early 1800s. A core from this fjord was collected at 100 m depths under anoxic remnant waters.

The first 15 cm corresponding to deposits from 1500 to present were considered for analysis. Lipid classes were quantified by TLC-FID and the molecular composition of selected lipid classes was investigated by GC and GC-MS. Lipids were dominated by two main classes, phospholipids and hydrocarbons. The hydrocarbons represented up to 7.4% of total lipids in the sediment layers covering the period when the most extensive cultural eutrophication took place (1900 to 1970).

The higher fluxes of organic carbon produced during this period may have controlled hydrocarbon inputs into the sediments, due to the hydrophobic character of these pollutants. The hydrocarbon concentration reversed toward pre-industrial levels in the more recent layers, which suggests an improvement of the water quality, possibly in response to improved treatment of the sewage in the cities around Bunnefjord.

The second most abundant pool of lipids consists in phospholipids, mostly contributed by bacteria. Even though the concentration decreased with depth, their relative proportions to total lipids remained high, mainly in the deepest layers (>80% of total lipids). A rapid decrease of the polyunsaturated fatty acid methyl esters (FAME) from the phospholipid fraction in the upper 4 cm suggests a rapid biodegradation of planktonic inputs and meiofauna.

Odd branched fatty acids were more probably contributed by bacteria linked to the high sedimentary hydrocarbon content. The down core distribution of 16:1omega7, 18:1omega7, 18:1omega5 esterified to phospholipids suggests a vertical zonation of the microbial community in relation to redox conditions and available organic matter.

In addition to bacterial sulphur biomass, the presence of hopanoic acids in the phospholipids fraction suggests the contribution of bacteria growing on methane. According to the sterol composition, dominated by 4alpha(H)-methylsterols, dinoflagellates represent the major contributors to the organic matter produced in the water column, particularly during the period of extensive eutrophication.

Long-chain diols (1,13-C(26), 1,15-C(30) and 1,15-C(32)) and long-chain keto-ols (1,15-C(30) and 1,15-C(32)) are reported for the first time at high latitudes. Their relative distributions (diol and keto-ol indexes of Versteegh et al. [Org. Geochem. 27 (1997)]) have allowed depicting a particular event during the eutrophication period, a freshwater intrusion with inputs of land-derived organic matter.

This is in accordance with the downcore distribution of freshwater/terrestrial markers as sitosterol, dehydroabietic acid and iso- and anteiso-pimaric acids. The diol and keto-ol indexes have also underlined the general transition trend from marine to more brackish waters in the Bunnefjord. These last observations provide confidence into the use of these compounds in paleoenvironmental reconstruction.