Associating particulate essential fatty acids of the x3 family with phytoplankton species composition in a Siberian reservoir NADEZHDA N. SUSHCHIK,* MICHAIL I. GLADYSHEV,* OLESIA N. MAKHUTOVA,* GALINA S. KALACHOVA,* ELENA S. KRAVCHUK* AND ELENA A. IVANOVA † *Institute of Biophysics of Siberian Branch of Russian Academy of Sciences, Akademgorodok, Krasnoyarsk, Russia † Krasnoyarsk State Agricultural University, Krasnoyarsk, Russia SUMMARY 1. We studied variation in the composition of fatty acids in the seston of a small freshwater reservoir with changes in phytoplankton composition during four growth seasons. We focused on the dynamics of the x3 fatty acids because of their potential importance for zooplankton nutrition. 2. Total diatoms were related to the 20:5x3 fatty acid (eicosapentaenoic, EPA) content in seston. Among two dominant diatom genera, Cyclotella was not associated with EPA content. In contrast, there was a significant correlation between Stephanodiscus and the percentage contribution and content of EPA throughout the study. Hence, freshwater diatoms can differ strongly in content of the essential EPA. 3. We considered abundant cyanobacteria as a potential source of 18:3x3 fatty acid (linolenic, ALA) to aquatic food webs. Among four dominant cyanobacteria species, two (Anabaena flos-aquae and Planktothrix agardhii) showed significant correlation with the ALA content of the seston, while the other two (Aphanizomenon flos-aquae and Microcystis aeruginosa) did not. 4. Dinophyta had a relatively high level of 22:6x3 (docosahexaenoic, DHA) for freshwater species and can be also a source of EPA to aquatic food webs. 5. Our results show that various species of diatoms as well as cyanobacteria can be of contrasting nutritional value for zooplankton because of their different content of the essential PUFAs. Diatoms, which are low in EPA, could not be considered as a valuable food, while some field populations of cyanobacteria might be valuable sources of essential ALA. Keywords: cyanobacteria, diatoms, fatty acids, food quality, species composition Introduction Fatty acids in particulate matter and sediments are widely studied as biomarkers of the sources and transfer of organic matter in aquatic food webs (Claustre et al., 1989; Ederington, McManus & Harvey, 1995; Desvilettes et al., 1997; Volkman et al., 1998). Some of them, namely polyunsaturated fatty acids of the x3 family (x3 PUFA), are now recognised as essential for the nutrition of a wide range of animals (Sargent et al., 1995; Ahlgren et al., 1997; Gulati & DeMott, 1997; Arts, Ackman & Holub, 2001). The x3 PUFAs in freshwater ecosystems are produced mainly by micro-algae, with their syn- thesis depending strongly upon genetic (Cobalas & Lechado, 1989; Gugger et al., 2002) and environmental factors, such as temperature, mineral nutrients, Correspondence: Nadezhda N. Sushchik, Institute of Biophysics of Siberian Branch of Russian Academy of Sciences, Akademgorodok, Krasnoyarsk, 660036, Russia. E-mail: labehe@ibp.ru Freshwater Biology (2004) 49, 1206–1219 doi:10.1111/j.1365-2427.2004.01263.x 1206 Ó 2004 Blackwell Publishing Ltd