Abstract
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.
Highlights
Lake water quality is influenced by many factors, whose source is both in the catchment area and in the aquatic ecosystem
The following were assayed in a volume of 1 cm3 of wet sediment: loosely bound phosphorus (NH4Cl-P), extraction with 1 M NH4Cl for 2 h; phosphorus bound with iron (BD-P), extraction with a mixture (1:1) of 0.11 M NaHCO3 and 0.11 M Na2S2O4 for 2 h; phosphorus bound with aluminum (NaOH-P) and organic matter (NaOH-NRP), extraction with 1.0 M NaOH for 18 h; phosphorus bound with calcium (HClP), extraction with 0.5 M HCl for 18 h and the residue (Res-P), being the difference between total phosphorus (TP) concentration and the sum of the first four fractions
The most commonly expected effect in lakes subjected to restoration treatments is the reduction or elimination of phytoplankton blooms caused by the large numbers and biomass of cyanobacteria, which are often toxic (Søndergaard et al 2002; Kobos et al 2013)
Summary
Lake water quality is influenced by many factors, whose source is both in the catchment area (external loading) and in the aquatic ecosystem (internal loading). Phosphorus (P) in particular, have been considered the most important factor influencing lake water quality. It is a key element in controlling primary production in lakes (Kuczyńska-Kippen and Joniak 2010; Søndergaard et al 2001, 2017) and most of it is accumulated in lake sediments, from which it returns to water column mainly as orthophosphates. Particulate and organic P forms may reload the water column for a short time, e.g., after extensive sediment resuspension events (Markou et al 2007).
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