Nitrogen retention (Nret) and denitrification were studied in the mesotrophic Solina reservoir (south-east Poland) between spring and autumn in 2002 and 2003. Nret was calculated on the basis of the input-output mass balance. The rate of denitrification was measured using the 15N isotope pairing technique in two types of areas; in deep areas, where the bottom sediment makes contacts with the hypolimnion (Dprof), and in the unstratified shallows (Dlitt). The calculated daily load of removed nitrogen (Dtot) varied somewhat from 11.0 to 19.1 mg N m–2 day–1 with a standard deviation of 3.0 mg N m–2 day–1. The limited oscillation noted for Dtot results from the major participation of the deep areas of the reservoir, which are characterised by stable Dprof, as distinct from Dlitt, which varied seasonally from 5.8 mg N m–2 day–1 in November to 109.9 mg N m–2 day–1 in August. The primary factor limiting the denitrification rate was temperature, the other being availability of substrates, mainly organic matter. Nitrogen retention ranged seasonally between –25.0 and 175.9 mg N m–2 day–1. The contribution of denitrification to Nret ranged from 9.0% to 49.8% (mean 16.4%). On average, only 4.9% of the nitrogen external load was denitrified (range 2.6–7.9%, s.d. 1.5%). After comparing obtained data with others available in the literature, we concluded that reservoirs, estuaries and highly-loaded lakes in a river system experience a more limited contribution of Dtot to Nload than is the case in natural lakes.
The estimated diffusion fluxes of methane (CH4) and carbon dioxide (CO2) at the sediment–water interface in the Rzeszów Reservoir in southeastern Poland are presented. The relevant studies were conducted during 2009, 2010, and 2011. Calculated fluxes ranged from 0.01 to 2.19 mmol m−2 day−1 and from 0.36 to 45.33 mmol m−2 day−1 for methane and carbon dioxide, respectively. While the values for calculated diffusion fluxes of methane are comparable with those reported for other eutrophic reservoirs, much higher values were obtained here for carbon dioxide. The resulting values of δ13C-CH4 and the fractionation coefficients between methane and carbon dioxide (αCH4-CO2) suggest that methane in the sediment of the Rzeszów Reservoir is produced by acetate fermentation, while the hydrogenotrophic methanogenic process is of successively greater importance with increasing depth. In the top layer of the sediment, 24–72 % of CO2 came from methanogenesis, while the contribution made by the degradation of organic matter by methanogenesis to CO2 was greater in the deeper layer.
The studies concerned the ecosystem of the Solina-Myczkowce cascade of reservoirs. While the elemental C:N ratio was higher in the Solina Reservoir branches and in the Myczkowce Reservoir (>10), the values for stations located near the Solina Dam were low (<10). Markedly, the lowest values for δ 13 C were noted in the Myczkowce Reservoir (-28.5‰), and these contrasted with the higher values found in the Solina Reservoir (ca. -27‰). δ 15 N varied from 2‰ in the upper Solina to above 3.5-4‰ downstream. While the data obtained indicate the autochthonous origin of sedimentary matter, primary production influences the properties of the bottom sediments in the lower part of the Solina reservoirs.
A reference study on both the nitrogen content in waters and nitrogen and oxygen isotopic signatures characterising nitrate from different sources was conducted within the San River catchment area. Three kinds of catchments were studied: (1) forested and uncultivated; (2) artificially fertilised with nitrate; and (3) fertilised with manure and sewage. Moreover, atmospheric water was studied. The obtained values were found to be similar to others in the literature, with the exception of nitrate from the atmosphere, in regard to which influence reflecting the local conditions was to be noted. The isotopic signature of nitrate in the studied water results from the biogeochemical transformation of nitrogen compounds rather than from the mixing of different sources. The obtained results were statistically distinct and can be used as end-member values in further modelling studies connected with the management of nitrate in river waters, especially under middle-eastern European conditions.
Denitrification rates in the bottom sediment of the Rzeszów Reservoir (southeastern Poland) were measured using the isotope pairing technique. At the stations studied rates were reported in the range of about 26 to 610 μmol N 2 m -2 h -1 , with the figures generally highest in summer. Simple and multiple regression analyses of relationships with selected abiotic factors gave rise to a model that revealed a statistically significant influence on rates of denitrification in the bottom sediment of the Rzeszów Reservoir due to concentrations of nitrate in overlying water, temperature, and the organic matter content in the sediment. The present study confirms that nitrate concentration in the overlying water is the main factor controlling sedimentary denitrification.
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