Microplanktonic Polyunsaturated Fatty Acid Markers: a Mesocosm Trial

The fatty acid composition of total lipid was determined in microplankton (<200 μm) from control, diatom-dominated and coccolithophore-dominated mesocosms. There was a good correlation between the abundances of chlorophyll aand total fatty acids in the three mesocosms. Analyses of fatty acid markers characteristic of diatoms (16:1n−7, 16:2n−3, 16:3n−3, 16:4n−1 and 20:5n−3) and of coccolithophores (18:5n−3 and 22:6n−3) established that in the diatom-dominated mesocosm, an increase in coccolithophore biomass, preceded the diatom bloom. Similarly, an increase in the levels of diatom fatty acids preceded the coccolithophore bloom in the coccolithophore-dominated mesocom. These transient increases in microplanktonic marker fatty acids levels were not evident from measures chlorophyll a, or estimated carbon contribution derived from cell counts. There was appreciable 22:6n−3 derived from coccolithophores in the diatom-dominated bloom suggesting that the amount of this key nutrient at the base of the marine food-web may often be underestimated, with important consequences for higher trophic levels.