Comparative Analysis Of Fatty Acids Composition Of Phytoplankton Communities In Different Trophic Water-Bodies

Trophic level, phytoplankton community and the fatty acids composition of seston were investigated in 2002 in Liuxihe Reservoir, Feilaixia Reservoir, Xiannu Lake and Zhongxin Lake. Xiannti Lake and Zhongxin Lake were eutrophic, while Liuxihe Reservoir and Feilaixia Reservoir were mesotrophic. In Xiannu Lake and Zhongxin Lake, phytoplankton was predominated by Cyanophyta, and the dominant species were some eutrophic indicators, such as Merismopedia sp. and Phormidium sp.. High phytoplankton densities were also found in the two lakes with 9.77× 106cells·L-1and 34.81 × 106cells·L-1, respectively. However, phytoplankton densities were much lower in Feilaixia Reservoir and Liuxihe Reservoir with 0.24 × 106cells·L-1 and 0.12× 106 cells·L-1, respectively. The predominate species in the two reservoirs was Bacillariophyta.The total fatty acids concentration of seston in the four investigated water bodies was between 2.0 μg·L-1 and 36.0 μg·L-1. In these water bodies, polyunsaturated fatty acids ranged from below 0.01 μg·L-1 to 7.6μg·L-1 and predominated by C18:2, while saturated and monounsaturated fatty acids were from 1.7 μg·L-1 to 32.6 μg·L-1 and from below 0.01 μg·L-1 to 17.8 μg·L-1, respectively, and dominated by C16:0 and C18:1, respectively. The content of total fatty acids and polyunsaturated fatty acids per unit filtered water in the two lakes (Zhongxin and Xiannu) was remarkably higher than that of the two reservoirs. However, the content of two types of fatty acids per unit biomass of algae in the latter was higher than that of the former. Meanwhile, the content of total fatty acids and polyunsaturated fatty acids per unit biomass of algae was evidently correlated with that of TN (-0.646, -0.655) and TP (-0.511, -0.400) . Since saturated fatty acids were usually rich in cynabacteria, the cynabacterial cell density is highly correlated with the content of saturated fatty acids (r=0.954). Among the polyunsaturated fatty acids, the content of EPA per unit biomass, one of the criteria to estimate the nutrient value of algae, was also negatively correlated with that of TN. According to the literatures, EPA mainly derives from Cryptophyta and Bacillariophyta in water bodies. Our results suggest that the concentration of EPA ispositively correlated with the abundance of Cryptophyta only rather than with that of Bacillariophyta and other algae.