Fatty acids in the Ria Formosa Lagoon, Portugal

Fatty acids in surface sediments were analysed from 59 sites in the Ria Formosa Lagoon, Portugal as part of a wider study on trace organic matter in this system. A total of 170 different fatty acids were quantified as their methyl esters using GC-MS techniques. The TOC in the sediments ranged from 0.15% in sandy areas to 8.3% in muddy regions. The total fatty acids in the sediments ranged between 0.3 to 67.5 μg•g−1 the latter accounting for 0.5% of the TOC. Individual compounds, ratios and multivariate statistical methods were used to identify the sources and their spatial variations. The sediments in the region near Armona were rich in phytoplankton biomarkers (e.g. 16:1ω7/16.0 and 16 carbon PUFAs), those near Faro and Olhão were comprised of odd chain length fatty acids, their branched derivatives, 18:1ω7 and 3-OH compounds; these suggest bacterial biomass linked to the known sewage discharges in these areas. Limited regions of terrestrial influence could be seen through use of the long chain (>22 carbon) saturates. The concentrations of 20:4ω6 and 20:5ω3 were high in regions of marine production. Multivariate statistical techniques (PLS, PCA and cluster analysis) were used to interpret the data. PCA was found to be the most useful in this case and major axes representing phytoplankton, terrestrial organic matter and bacterial biomass were extracted from the data matrix. The spatial variation of each component could be linked to observations of sewage and phytoplankton input.