The Functioning of a Water Body Within a Fluvio-Lacustrine System as an Effect of Excessive Nitrogen Loading—The Case of Lake Symsar and its Drainage Area (Northeastern Poland)

Abstract

Generally, in water ecosystems, it is assumed that rivers play a transport role. In turn, lakes have accumulation properties. However, in fluvio-lacustrine systems, each water body located on a river track can disrupt naturally occurring processes. One such process is the nitrogen cycle. An analysis of the nitrogen cycle, at both the global and local levels, is of extreme significance in view of the progressive degradation of aquatic ecosystems. In this study, we attempted to show that the specific properties of reservoirs located in river–lake systems contribute to an adequate reaction of these reservoirs to situations involving an excessive pollution load. Despite the intensive exchange of water in lakes, they were mainly shown to have an accumulation function. In particular, in those located in the lower part of the system, the total nitrogen load transported outside the example reservoir decreased by 4.3%. The role of these reservoirs depends on the morphometric, hydrologic, and meteorological conditions. The actual loading of the water body was shown to be more than double the permitted critical loading. The creation of conditions similar to those occurring in river–lake systems by, for example, delaying the outflow of water, may favor the protection of surface water from the last element of the system, because this limits the transport of pollutants. This study of the functioning and evolution of lakes’ fluvio-lacustrine systems, including the balance of the nutrient load, enables the prediction of the aquatic ecosystem’s responses in the future and their changes.

Influence of Cascading River–Lake Systems on the Dynamics of Nutrient Circulation in Catchment Areas

“…Weather conditions affect the heating, cooling, mixing and circulation of lake waters [37]. They do not directly influence the lake water pollution, but they indirectly influence the hydrological conditions in river-lake systems [29,38] by regulating water levels in river beds and lakes, influencing dilution and accumulation of nutrients in waters and influencing the intensity of water flow in river beds. For example, lower water abundance in individual hydrological years, resulting from lower precipitation sums, contributed to the occurrence of periodic water depletion in some watercourses in the catchment area of the river-lake system under study, especially in summer, during intensive plant vegetation and increased water demand [29].…”

“…The specific properties of river-lake ecosystems and their retention capacity contribute to the formation of their functions in river-lake systems. In addition to the bottom material, the river flowing through the reservoir may also carry nutrients from the river basin into the reservoir [29,38].…”

“…Therefore, the load on river-lake systems depends on the development of catchment areas and anthropogenic factors occurring not only in the direct catchment but also in the whole river-lake system [42]. Flowing rivers, often draining multi-hectare agricultural catchment areas, affect the quality of water in the reservoirs through which they flow, e.g., by introducing biogenic substances [29,38]. Significant loads are also introduced with waters of smaller watercourses, fed with sewage from housing estates and runoffs from rural homesteads located nearby [38,43].…”

“…Flowing rivers, often draining multi-hectare agricultural catchment areas, affect the quality of water in the reservoirs through which they flow, e.g., by introducing biogenic substances [29,38]. Significant loads are also introduced with waters of smaller watercourses, fed with sewage from housing estates and runoffs from rural homesteads located nearby [38,43]. Agricultural use of direct catchment areas of river-lake systems may result in the introduction of mineral phosphorus species into lakes [43].…”

“…Wind-less conditions and high temperatures, especially during the growing season, favour the heating of water, making oxygen conditions deteriorate and create conditions conducive to the decomposition of organic matter, disturbing the circulation processes and creating favourable conditions for algae blooms, as in the case of Lake Erie [44]. Moreover, adverse agricultural practices in the lake basin may contribute to this phenomenon [38]. Intensive agriculture and high air temperatures later in spring and early summer are conducive to the inflow of dissolved reactive phosphorus species to the reservoirs [44].…”