Peculiarities of the Dynamics of Some Elements of Hydrochemical Regime in Small Water Bodies of Urban Territories: Nutrients and Organic Matter

Abstract

Results of the study of spatial and temporal dynamics of inorganic forms of nitrogen (NH4+, NO2-, and NO3-), phosphorus, silicon, and dissolved organic matter in the water of the system of Opechen lakes are given in the paper. It has been found that the concentration of dissolved oxygen essentially influences relationship between inorganic forms of nitrogen. The coefficient of correlation between oxygen content and the contribution of ammonium, nitrite, and nitrate accounted for respectively -0.79, 0.60, and 0.78 (at the level of significance of 0.01). At the concentration of dissolved oxygen of ≤ 4 mg/L, the contribution of NH4+, NO2-, and NO3- accounted for respectively ≥ 80, ≤ 6, and ≥ 14% Ninorg, at the concentration of oxygen of 4-8 mg/L - 60-80, 6-12, and 14-28% Ninorg, whereas at a further increase in oxygen content up to 16 mg/L their contribution was 18, 24, and 58% Ninorg. In wintertime during the period of reverse temperature stratification, the difference between the content of inorganic nitrogen and its different forms, and also between the content of inorganic phosphorus, in the surface and near-bottom layers of water was insignificant, or it was not more than 2.5 times. During the period of the formation of direct temperature stratification from spring until autumn, the difference between the concentration of the above-mentioned forms of nitrogen and inorganic phosphorus in these layers was rather significant. The concentration of NH4+ and Pinorg significantly increased in the layer located below thermocline. The upper three lakes (Minske, Lugove, and Ptashyne) proved to be most contaminated by the compounds of inorganic nitrogen and phosphorus. A decrease in their concentration in the surface layer of water was conditioned by the assimilation of these substances by the biota, whereas the increase in the concentration of the above mentioned compounds in the near-bottom layer of water - by their influx from bottom sediments under conditions of oxygen deficiency accompanied by the formation of anaerobic conditions. Based on the averaged indices, N : P molar ratio decreased from winter until autumn within the range between 16.0 and 5.7, which is indicative of the increase in the concentration of phosphorus relative to nitrogen, and also of the formation of favorable conditions to the development of blue-green algae. The concentration of dissolved silicon in the water of the lakes varied from 0.03 to 6.4 mg/L. A decrease in its content in the surface layer of water in spring was conditioned by silicon assimilation by diatoms. The total content of dissolved organic matter and its individual groups changed from season to season and in space. The contribution of readily oxidized organic compounds in the water of the upper three lakes accounted for 15.8-55.3%, whereas in the water of the lower three lakes - 29.1-81.6% of the total content of dissolved organic matter. The maximum indices were registered in summertime. The upper three lakes proved to be most contaminated by organic matter. The concentration of natural groups of dissolved organic matter, including humic substances, carbohydrates, and protein-like substances, in the water of the lakes accounted for 4.6-8.0, 0.66-5.43, and 0.26-1.09 mg/L. Their contribution to the total balance was 37.6-55.5%. The rest was made up by other groups of dissolved organic matter (probably of anthropogenic origin).