ABSTRACT
Keywords: reservoirs, eutrophication, stable isotope
So far, the most common method of identifying the origin of organic matter in bottom sediments has been the analysis of the changes in organic carbon content in relation to nitrogen (the ratio C:N). It is characteristic, that the values C:N in the organic matter, derived from sedimentation of terrigenous substances are higher, such as in cellulose, which is a component of terrestrial plants. Autochthonous-plankton matter-is characterised by lower values of the ratio, approximately 6:1, as the remains of algae contain more nitrogen than the terrestrial matter (Gu et al. 1999, de Junet et al. 2005). In recent years, the increasingly used method for assessing the origin of bottom sediments of surface waters is the content analysis of stable isotopes of carbon and nitrogen (Lehmann et al. 2002, de Junet et al. 2005, Brenner et al. 2006, Koszelnik 2009, Gąsiorowski & Sienkiewicz 2013, Oczkowski et al. 2014). These methodologies draw on the fact that the deposition of organic matter in sediments is the final step in a series of transformations of carbon and nitrogen, whose nature is different in terrestrial ecosystems than in aquatic ecosystems. The result is a variety of different isotope fractionations in the process of this transformation, which results in different values of the isotope ratios of 15N:14N and 13C:12C in the bottom sediments of various origins. The chemical composition of the organic matter contained in the bottom sediments of reservoirs and lakes is an
1 INTRODUCTION
Cultural eutrophication is one of the important causes leading to deterioration of water ecosystems quality (Chislock et al. 2013, Neverova-Dziopak & Kowalewski 2014). This problem applies also to small retention reservoirs on streams or rivers due to the localisation of strongly anthropogenic areas and supplied by external organic and mineral matter loads (Wiatkowski et al. 2015). In that case, the primary problem is silting and consequential decrease of the reservoir volume (Michalec & Tarnawski 2008). The main reason for that decrease is external inflow of debris, but the organic matter derived from the production within the ecosystem, resulting from the excessive availability of nutrients can have an important contribution to the amount of sediment deposited in reservoirs of stagnant water. Various studies indicate that up to 35% of organic matter produced in the euphotic layer of eutrophic water reservoirs enriches the surface layer of bottom sediments (de Junet et al. 2005, Vreča & Muri 2010). The formation of the sediment, associated with the mineralisation leads on a global scale to the burial in the sediment of only 0.1% of net primary production (Lehmann et al. 2002), although paleo-limnological studies indicate that the process of accumulation of sediment is directly proportional to the increase of primary production (Tyson 2001, Zilius et al. 2015).
