Abstract
Eutrophic lakes of all types are marked by oxygen shortage in the bottom waters during the summer season, which results in excessive release of phosphorus load. Therefore, numerous restoration activities (chemical precipitation, aeration) are being taken based on bottom-up control, which consists of limiting the nutrient pool available to lower trophic levels. The present study provides an efficiency analysis of pulverising aeration of waters in two stratified and five unstratified lakes located in Poland. The assessment was based on monitoring data (dissolved oxygen concentration (DO), % oxygen saturation (DO%), visibility of the Secchi disc (ZSD), the concentrations of: chlorophyll a (CHL), total phosphorus (TP) and total nitrogen (TN)) obtained before and after installation of the aerator on the lakes. The study was conducted during the spring and the summer seasons. Throughout the study period, the stratified lakes exhibited oxygen saturation <0.1%. Having completed the restoration activities, TN:TP ratio was found to gradually increase in all analysed lakes, which indicates that aeration may decrease phosphate content in the water column. In three lakes, the ratio was > 17, so phosphorus became the most growth-limiting nutrient. The restoration activities on four unstratified lakes had no significant effect on the changes of the Carlson trophic state indices (TSI). Either individual values of the TSI(TP), TSI(CHL) and TSI(ZSD) were comparable, or the value of TSI(TP) was higher than one or both of the remaining indices for all of the lakes.
Highlights
Intensification of agricultural production and global population growth has resulted in the enhanced fertility of aquatic ecosystems
One meter below the water table, the following concentrations were measured: chlorophyll a (CHL), total phosphorus (TP), and total nitrogen (TN), by means of spectrophotometry methods: CHL with acetone as extraction solvent, TP by molybdenum method after mineralization to orthophosphates in nitric and sulfuric acid, and TN calculated as sum of nitrite nitrogen, which was measured with sulphanilic acid and 1-naphtylamine; nitrate nitrogen was measured with sodium salicylate, and total nitrogen with the use of Kjeldahl method [29]
For the purpose of assessment of trophic changes occurring in the analysed lakes, the ratio of total nitrogen to total phosphorus (TN:TP) was calculated for the results obtained in the summer season
Summary
Intensification of agricultural production and global population growth has resulted in the enhanced fertility of aquatic ecosystems. The primary and most efficient method for reducing the eutrophication rate of water reservoirs is to limit or eliminate the sources of biogenic supply to lakes. In the case of highly eutrophic reservoirs, such protective actions prove to be insufficient due to the so-called “internal loading” of reservoirs when biogenic compounds accumulated over the years in the bottom sediments are introduced into the water [1,2,3,4]. Various methods for locking phosphorus compounds in bottom sediments, as well as limiting their solubility in water, have been developed. An example of the former method is the chemical precipitation of bioavailable forms of phosphorus with the compounds of calcium [5,6], Water 2018, 10, 219; doi:10.3390/w10020219 www.mdpi.com/journal/water
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