179 ISSN 1995-4255, Contemporary Problems of Ecology, 2018, Vol. 11, No. 2, pp. 179–194. © Pleiades Publishing, Ltd., 2018. Original Russian Text © E.A. Kurashov, G.G. Mitrukova, J.V. Krylova, 2018, published in Sibirskii Ekologicheskii Zhurnal, 2018, No. 2, pp. 207–224. Interannual Variability of Low-Molecular Metabolite Composition in Ceratophyllum demersum (Ceratophyllaceae) from a Floodplain Lake with a Changeable Trophic Status E. A. Kurashov a, d, *, G. G. Mitrukova a, c, ***, and J. V. Krylova b, d, ** a Institute of Limnology, Russian Academy of Sciences, St. Petersburg, 196105 Russia b Berg State Research Institute of Lake and River Fisheries, St. Petersburg, 199053 Russia c Saint Petersburg State Chemicopharmaceutical Academy, St. Petersburg, 197376 Russia d ITMO University, St. Petersburg, 197101 Russia *e-mail: evgeny_kurashov@mail.ru **e-mail: juliakrylova@mail.ru ***e-mail: galya-21@mail.ru Received July 19, 2017; in final form, October 24, 2017 Abstract⎯The regularities that shape the composition of low molecular weight organic compounds (LMWOCs) in aquatic macrophytes in response to aquatic environment alterations remain poorly character- ized. The aim of the present study consists of a comparative interannual investigation into LMWOC compo- sition in rigid hornwort (Ceratophyllum demersum L.) from a Volga-Akhtuba floodplain lake with a variable trophic state. A high variability of LMWOC composition and individual compound levels in hornwort is detected as different trophic states of the water body are analyzed. Active allelochemicals are the predominant LMWOCs in the case of a “macrophytic” mesotrophic state of the lake, with fatty acids (the free fatty acid fraction) apparently being the most important in this group. Hornwort LMWOC composition in the case of a “cyanobacterial” eutrophic type of lake development is characterized by the predomination of compounds that enhance the protective reactions (manool being the most important) under the conditions of suppression by cyanobacteria, which is also manifested as an almost twofold decrease in the overall intensity of organic- compound biosynthesis. Keywords: Ceratophyllum demersum, cyanobacteria, macrophytes, low molecular weight organic compounds, gas chromatography–mass spectrometry, allelopathy, low molecular weight metabolome DOI: 10.1134/S1995425518020063 INTRODUCTION Low molecular weight organic compounds (LMWOCs) belong to the groups of primary and sec- ondary metabolites of aquatic and semiaquatic plants and constitute a part of low molecular weight metabo- lome of these plants. These organic compounds are extremely important for hydrobiont community forma- tion and thus act as regulatory agents in aquatic-system hydrobiocenoses (Gurevich, 1978; Meteiko, 1978; Fink, 2007; Bykova et al., 2010; Kurashov et al., 2014). Many aquatic plant LMWOCs possess high biological activity and thus have considerable potential for use in many branches of economy (ecological engineering, medicine, pharmacology, food and cosmetics industry, etc.), since aquatic macrophytes represent rich and renewable underused natural resources (Hu and Hong, 2008; Kurashov et al., 2015; Kurashov et al., 2016). The phenomenon of a change in trophic state and ecosystem type in shallow lakes is well known: a lake can be a pure water body with well-developed aquatic vegetation or a water body characterized by low trans- parency, high turbidity, and intensive phytoplankton development (Scheffer et al., 1993; Scheffer, 2001; Scheffer et al., 2001; Jackson, 2003). Small shallow lakes are suitable research objects that provide excel- lent opportunities for studies of this interesting phe- nomenon due to the distinct character of lake ecosys- tems and relative simplicity of collecting raw material from these objects (Jackson, 2003). Floodplain lakes of the Volga-Akhtuba floodplain occupy a distinct position among small water bodies, since they are filled with river water during flood and are thus “born again” every year. A currently unchar- acterized combination of environmental factors can direct the development of these lakes along one of two possible pathways that involve the rapid development of aquatic vegetation or intensive phytoplankton development. The same water body can be either a eutrophic/hypereutrophic lake characterized by low