Effects of continuous nitrate treatment of hypolimnion in a small, hypereutrophic lake.

Continuous nitrate treatment is an innovative, nature-based and cost-effective restoration method that has been implemented in a small, hypereutrophic lake by redirecting the nitrate-rich waters of the lake's natural tributaries into its hypolimnion. The aim of this treatment is to increase the redox potential at the sediment-water interface in order to provide conditions for efficient phosphorus binding. To assess the effects of this treatment, studies of the physico-chemical and biological parameters of the lake waters were carried out before its application (PreNT - years 2005-2007), during its application (FullNT - 2008-July 2019), and in the period when installation was partly clogged (LimNT- August 2019-2021). The results show effective improvement of oxygenation of the hypolimnion during the treatment followed by a decrease of phosphorus concentration - a proxy of the reduced internal loading. Water quality gradually improved during the treatment. The periods of cyanobacterial blooms shortened in some years, and ceased in other years, and it was also manifested by the increase in the range of Secchi depth and a decrease in chlorophyll-a concentration. The share of eukaryotic algae in the phytoplankton increased. The method showed high resistance to disturbances in its functioning, because during two years of operation of one pipeline, only a slight deterioration of water quality was observed. This enables effective repair of the hypolimnion's supply of nitrates, without a sudden return to the pre-restoration state. Finally, the benefits and limitations of hypolimnetic nitrate treatment (with nitrate from natural sources) were discussed to facilitate the application of this method to other lakes.

Bottom sediments as an indicator of the restoration potential of lakes-a case study of a small, shallow lake under significant tourism pressure.

The study covered a small, shallow lake, intensively used for recreation (sailing, tourist services and port infrastructure). This study aimed to determine the spatial differentiation of bottom sediments and the potential for phosphorus release in five zones, differing mainly in the type of recreation, depth, direct catchment management, shoreline management and macrophyte presence. The results were used to propose protective and restoration measures to improve the water quality of the studied lake. The innovation in the study was the detailed analysis of bottom sediments, which can be a significant source of pollution besides the external load from the catchment and tourist pressure, in the planned management of this ecosystem. Examination of the physicochemical properties of the bottom sediments showed a clear variation in both composition and potential for internal phosphorus loading. The sediments from the profundal zone, where the most boating activity was observed, together with the sediments from the shallow zone where the boats dock (mooring zone), had the highest potential to supply phosphorus to the bottom waters. This fact was demonstrated by the highest total phosphorus (TP) concentrations in sediments (up to 1.32 mgPg-1 DW) and the content of the most mobile fractions (up to 33%). The other zones associated with the marina, fuel zone, tributary and canal were not significant sources of phosphorus to the ecosystem. Based on the above results, a restoration method involving the removal of bottom sediments from the bottom zone was proposed, supported, of course, by protective measures in the catchment (maintaining a buffer zone around the lake and limiting the inflow of pollutants with tributary waters). The proposed measures with sustainable tourist pressure should improve water quality and thus contribute to protecting this valuable natural landscape.

Spatio-temporal variations in water quality of a river-lake system during restoration treatments.

To fill the knowledge gap about the functioning of the lake-river system subjected to restoration treatments, two tributaries, a shallow, restored lake and its outflow, were examined. The quality of water inflows, lake and outflow was compared before (BR), during sustainable (SR, deep water aeration, phosphorus inactivation and biomanipulation for 3 years) and limited lake restoration (LR, only aeration for 2 years). Physico-chemical parameters were analysed monthly at five stations. The nutrient concentrations at the inflows decreased over the years due to the improvement of water and sewage management in the catchment (in Mielcuch from 18.0 to 8.0 mgN L-1 and 1.0 to 0.6 mgP L-1). The decline at the outflow was the result of a better quality of water at the tributaries and SR in the lake. During LR, decrease of phosphorus concentration still occurred (0.11 mgP L-1), but nitrogen concentration slightly increased (3.9 mgN L-1). Although the outflowing waters still transported a high content of chlorophyll a and suspended solids during SR, their amount was lower (34.5 µg L-1 and 17 mg L-1, respectively) than that during BR and LR. During restoration, it is significant to monitor the water quality not only in the lake but also at the outflow. The slow deterioration of water quality at the outflow indicated that introducing changes in the applied restoration methods must be done carefully because the previously achieved effect may be lost. Hence, restoration of the upstream lake and good quality of its tributaries are of great importance for water bodies located downstream.

Internal Phosphorus Loading from the Bottom Sediments of a Dimictic Lake During Its Sustainable Restoration.

The ribbon type Lake Durowskie (Western Poland) is currently undergoing a sustainable restoration process due to water quality deterioration, manifested in water blooms, low transparency, and oxygen deficits near the bottom sediments. Three restoration methods were applied: (i) hypolimnion aeration using two wind-driven pulverizing aerators installed at the deepest places, (ii) phosphorus inactivation using small doses of iron sulfate and magnesium chloride several times a year, and (iii) biomanipulation using pike fry stocking. Research on the exchange of phosphorus in the sediment-water interphase was conducted in the years 2009-2016 to determine the multiannual changes of internal phosphorus loading from bottom sediments during the restoration process. The sustainable approach resulted in a decrease of internal phosphorus loading and a gradual increase in the sorption capacity of bottom sediments, particularly noticeable in the last 2 years. The content of phosphorus in the sediment and in the interstitial water fluctuated, showing an increase during the first years of restoration and then a gradual decrease. It was proved that the process of sustainable restoration is cheap and does not interfere strongly with the ecosystem, although it is long-lasting. It should be continued for many years, especially in the case of continuous external loading of the lake with nutrients from the catchment area, i.e., until the water quality in the main tributary improves and the lake ecosystem stabilizes.

The influence of restoration measures on phosphorus internal loading from the sediments of a hypereutrophic lake.

Uzarzewskie Lake was a hypereutrophic, dimictic lake characterized by low water transparency, high chlorophyll-a concentration and intense phytoplankton blooms; thus, restoration treatment was started. A chemical treatment, based on phosphorus inactivation with small doses of iron sulphate, was undertaken in 2006-2007. Nitrate-rich groundwater flowing from seepage springs was directed into the lake hypolimnion to increase redox potential since 2008. Phosphorus internal loading was reduced by more than 80% as a result of restoration treatment. In the profundal zone, where P release from the bottom sediments prevailed throughout the year, it decreased from 13.02 mg P m-2 day-1 in 2010 to 2.81 mg P m-2 day-1 in 2015. Meanwhile, in the littoral zone, P accumulation in bottom sediments predominated; hence, the mean value of internal loading was increasing from 2.61 mg P m-2 day-1 in 2011 to 10.24 mg P m-2 day-1 in 2015. The annual P load from the bottom sediments in the profundal zone was much higher than from the littoral zone as a result of (i) a higher P release in most years and (ii) the greater area of this zone (82% of the lake's area). The fraction of residual phosphorus (Res-P) has the largest share, and in recent years, this has shown a tendency to decrease. The sum of bioavailable fractions was low (average 5.7%) showing a downward trend in recent years. Sustainable restoration of the lake resulted in a slow but steady decreasing trend in internal phosphorus loading.

Spatial and seasonal variability of pore water phosphorus concentration in shallow Lake Swarzedzkie, Poland.

Pore waters play an important role in phosphorus dynamics in aquatic ecosystems. Phosphorus concentrations in pore waters are much higher than above the bottom. This is confirmed by the results of this study concerning the hypereutrophic lake. Pore water was analyzed at 11 sampling stations in the upper layer of bottom sediments. This water was separated by centrifugation and phosphorus level was measured spectrophotometrically with ascorbic acid as a reducer. Total phosphorus concentration in pore waters ranged from 0.5 to 8.1 mgP l(-1) (mean 3.2 mgP l(-1)). Mean phosphorus concentration in pore water samples of this lake was the highest in summer and the lowest in winter. High concentrations were observed in samples from the pelagial and low from the macrophyte zone.