In an aquatic environment different compounds are incorporated within or absorbed on mineral matter depending on the physical, chemical, and biological processes that may change substantially the sediment texture. Extra difficulties arise in analysing fine-grained materials (clay and silt). Fine-grained sediments in an aquatic environment may aggregate into larger, porous aggregates commonly called flocs (Van Rijn, 1993; Roberts et al., 1998; Kim et al., 2005). These sediments are cohesive by definition (Hayter & Pakala, 1989; Paterson, 1997) and their composition and structure are temporally very changeable. The organic matter as well as different micro- and macrocomponents in the aquatic environment will be closely associated with suspended mineral particles, e.g. adsorbed on single particles, forming complexes with metal (usually iron) oxides on the surface of particles, become aggregates and are deposited or transported in this form within the lake. The large specific surface area, surface energy, and electrostatic charge of small and colloidal particles mainly drive coagulation (Gu et al., 1996).
Thus sediment texture gives very diverse information about the sedimentation environment. Depending on the aims of research, different methods are used to describe sediment texture. For example, electron microscopic research enables to fix the three-dimensional structure of particles or aggregates (Konert & Vandenberghe, 1997; Kim et al., 2005). Light microscopic research enables primary estimation of sediment composition and selection of appropriate pretreatment methods, as well as monitoring the progress of processes. Recent researches show that due to the fragile nature of aggregates, sometimes their properties have to be determined in situ (Mikkelsen & Pejrup, 2001; Thonon et al., 2005).
The grain-size spectrum is influenced by authigenic minerals such as carbonates or diatom valves of diameter range 5-200 [micro]m (Round et al., 1990) and by their fragments. The proportion of diatoms may vary largely, being even up to 50% of the bulk sediment, and it is source material for diatomite (Kadey, 1983).
The methods used for the pre-treatment of samples and grain-size analysis depend on the aim of studies. If we are interested in the distribution of allochthonous siliclastic matter in sedimentation processes, it is necessary to have for analysis purified material, that is the disaggregate flocs and sorbed materials (organics, carbonates, etc.) have to be removed from the grains. The problem is quite easily solved in the case of medium- or coarse-grained particles by using the sieving method for grain-size analysis (Last, 2001a). Complicated problems arise in the case of fine-grained material where secondary side-effects in the sedimentation environment as well as during the pre-treatment process (flocculation, damaged grains, etc.) could seriously affect the reliability of the obtained grain-size spectrum.
The majority of sediment grain-size studies are associated with marine, loess, fluvial, and aeolian sediments (Lu & An, 1997; Buurman et al., 2001; McCave et al., 2006; Almeida et al., 2007), which are usually poor in organic matter. Also these researches commonly study coarse-grained sediments (sand, gravel, etc.), formed in the water environment with a high ion concentration, and the sediments are generally rather well sorted.
The aim of this work is to compare the impact of different pre-treatment methods on the results of granulometric analysis of fine-grained lacustrine sediments, rich in organic matter (ca 40-50%).
To determine the influence of sediment pre-treatment methods on the grain-size spectrum of different lacustrine sediments obtained with a laser particle sizer, two lakes of different sedimentation environment were selected: typical South Estonian eutrophic Lake Tundre (L. Tundre) (sampling site: 57[degrees]57'19? …