In the year-to-year dynamics, higher concentrations of carbon in the DOM were observed in Blagodati and Golubichnoe Lakes in 2011–2012 (7.5–8.4 mg/l). The concentrations were lower due to their dilution in the high-water year 2013, and the lowest concentrations (4.5 mg/l) were observed in the dry year 2014, apparently due to a large-scale removal of terrigenous DOM reserves during floods in 2013.
The concentration of particulate matter (PM) in the small lakes varied from 1 to 14 mg/l. The lowest content of particulate matter was found in Golubichnoe Lake, 2–4 mg/l, and it reached as high as 11 mg/l in Vaskovskoe Lake. The content of particulate matter increased in all lakes in the bottom layers due to its subsidence. In Vaskovskoe Lake, which is the deepest lake, the smallest content of particulate matter was recorded in the middle of vertical profile (3 m). The quantity of particulate matter depends on the hydrological regime, the type of the drained soil, rock composition and geomorphology of the catchment. Apparently, the contribution of the latter was the most significant factor for Vaskovskoe Lake: during the extreme high water conditions in 2013, the content of suspended matter in the water doubled (compared to the average) thanks to the steep ridges limiting its catchment from the west and east.
The contribution of the POM in the overall pool of organic carbon did not exceed 10–30%. The largest share (25–30%) of carbon in the POM was found in Blagodati Lake. Particulate matter is mainly represented by plankton and detritus, making Blagodati Lake likely the most productive one. During the sampling in July, 2014, the water in Blagodati Lake was turbid, and flakes of foam were present on the water surface and on the shore, which apparently evidenced ongoing, intense production/destruction processes. According to Agatova et al. (2004) the main component of particulate organic matter is protein, and its presence is associated with bacterial activity.
The content of carbon in particulate matter varies greatly, with average values ranging from 5 to 46% of the particulate matter mass. The water of Blagodati Lake mainly contains particulate matter enriched with organic carbon (up to 42–46%) from diatom phytoplankton (Medvedeva, 2001). Even visually, the particulate matter from the lake placed onto filters has a notable bright-greenish colour associated with the presence of large amounts of phytoplankton. The particulate matter of Golubichnoe Lake is enriched in organic carbon at 20–29%. Vaskovskoe Lake mainly contains less organic particulate matter: the concentration of organic carbon is approximately 14% on average (Table 1).
Based on structural studies of DOM and humic substances of various origins, it has been demonstrated that spectroscopic characteristics can be used for a relative assessment of the degree of aromaticity of the structure and molecular weight (MW) of water DOM (Peuravuori and Pihlaia, 1997 and Weishaar et al., 2003). A higher SUVA254 value indicates a higher contribution of allochthonous humic substances with a higher proportion of condensed carbon and a higher molecular weight.
The SUVA254 value varied from 1.60 to 4.96 l/mol·m in the lake water. The highest SUVA254 values were usually characteristic of influent streams and the Golubichnaya River. The year-to-year spatial and temporal dynamics of the SUVA254 values follow the hydrological conditions (Fig. 1) because precipitation removes DOM from the humus layers of the soil.