Will the Oxygen-Phosphorus Paradigm Persist? - Expert Views of the Future of Management and Restoration of Eutrophic Lakes

Epipelon biomass responses to different restoration techniques in a eutrophic environment

Eutrophication is a worldwide problem. In eutrophic lakes, phosphorus release from stored sediment hinders restoration processes. The epipelon is a community that grows attached to the sediment surface and has the potential to help phosphorus retention by autotrophic organisms. This study evaluated epipelon responses to four lake restoration techniques. The responses of abiotic variables and phytoplankton biomass were also evaluated. Four simultaneous mesocosm experiments were performed in a shallow eutrophic lake. The applied techniques were aeration, flocculant, floating macrophytes, and periphyton bioreactor. Water and epipelon samples were taken on days 3, 10, 17, 27, and 60. The aeration treatment and macrophytes decreased light availability in the epipelon, which had a predominance of heterotrophic components. Flocculant and periphyton bioreactor treatments favored epipelon growth with a higher contribution of autotrophic components. Therefore, some techniques may favor the epipelon growth, while others may harm the community, resulting in less efficient restoration processes. For the complete restoration of a lacustrine ecosystem, the choice of techniques to be applied must consider the restoration and maintenance of the benthic environment.

Control and remediation methods for eutrophic lakes in the past 30 years

Accelerated eutrophication, which is harmful and difficult to repair, is one of the most obvious and pervasive water pollution problems in the world. In the past three decades, the management of eutrophication has undergone a transformation from simple directed algal killing, reducing endogenous nutrient concentration to multiple technologies for the restoration of lake ecosystems. This article describes the development and revolution of three remediation methods in application, namely physical, chemical, and biological methods, and it outlines their possible improvements and future directions. Physical and chemical methods have obvious and quick effects to purify water in the short term and are more suitable for small-scale lakes. However, these two methods cannot fundamentally solve the eutrophic water phenomenon due to costly and incomplete removal results. Without a sound treatment system, the chemical method easily produces secondary pollution and residues and is usually used for emergency situations. The biological method is cost-effective and sustainable, but needs a long-term period. A combination of these three management techniques can be used to synthesize short-term and long-term management strategies that control current cyanobacterial blooms and restore the ecosystem. In addition, the development and application of new technologies, such as big data and machine learning, are promising approaches.

Absorption characteristics of CDOM in treated and non-treated urban lakes in Changchun, China

In urban settings, one may find (i) lakes that are non-treated (NT) and impacted by recurrent discharges of pollutants and nutrients, and (ii) lakes that, through restoration measures and active management, are treated (T) from external inputs. The optical properties of chromophoric dissolved organic matter (CDOM) have been used to assess the anthropogenic impact on lakes ecology, but their application in comparative assessments of urban lakes has not been attempted. For 2 years, we measured nutrients and CDOM properties in water samples collected from NT and T lakes in the city of Changchun, China. Significant differences in CDOM properties were found between the two types of lakes, and these results were supported by redundancy analysis. The NT lakes were eutrophic while the T lakes were mesotrophic, with mean trophic status index (TSI) of 74.2 and 50.3, respectively. The CDOM absorption coefficient at 350 nm, a(350), was 2-fold higher in NT than in T lakes (6.59 vs 3.21 m^-1). In the NT lakes, CDOM components predominantly comprised large molecular weight (MW > 1000-Da) humus-like substances of allochthonous origin, whereas in the T lakes CDOM was dominated by low MW (<1000-Da) substances from autochthonous production. Seasonal fluctuation has a great influence on the CDOM concentration, but a little influence on its molecular composition. The CDOM concentration were higher in summer than in other seasons. Weather conditions (rainfall, temperature) and biophysical processes (biodegradation, photo-bleaching) likely contributed to these variations. We found the water quality of the treated lakes was getting better from 2016 to 2018. In summary, the study results, not only revealed seasonal effects, but most importantly documented the impact of human activities on the characteristics of CDOM in urban lakes. Most specifically, the sharp difference between the lakes in regard to a(350) (2-fold lower in T than in NT lakes) demonstrated the suitability CDOM absorption coefficient as an early indicator of the impact of treatment measures on the hydrochemistry of DOM in urban lakes.

Performance of physical and chemical methods in the co-reduction of internal phosphorus and nitrogen loading from the sediment of a black odorous river

The continuous release of nutrients from sediment is a major barrier to the remediation of black odorous rivers. This study used a long-term laboratory incubation experiment to investigate the effectiveness of sediment dredging, intermittent aeration, and in situ inactivation with modified clays to reduce the internal loading of sediment from a seriously polluted river. The results indicated that intermittent aeration and in situ inactivation were effective in reducing the TN and NH₄⁺ concentrations in the water column. However, sediment dredging did not consistently reduce the TN and NH₄⁺ concentrations in the water column. In contrast, the three methods were all effective in controlling the TP and PO₄^3- concentrations in the water column. Except for dredging, >30% of NH₄⁺ and 40% of PO₄^3- fluxes from sediment were reduced when compared with a control sample after 120 days of remediation. Dredging induced a significant release of NH₄⁺ from sediment. Dredging and aeration made nearly no change to the amount of extractable nitrogen in the sediment. However, inactivation may increase sediment-extractable ammonium in deep sediment layers with time due to vertical transportation of clay by intensive bioturbation. Dredging is the most effective way to reduce surface mobile phosphorus over time while the transported clays can reduce a large percentage of the mobile phosphorus in deeper sediment. The relative abundance of Nitrospira in the surface sediment increased significantly with each remediation measure, creating favorable conditions for the reduction of the ammonium released from sediment. Altogether, the results of this study indicated that clay inactivation is the best method for controlling the internal loading of both phosphorus and nitrogen in seriously polluted river sediment.

Long-Term Water Quality Changes as a Result of a Sustainable Restoration—A Case Study of Dimictic Lake Durowskie

Nature-based solutions in lake restoration enable gradual ecosystem reconstruction without drastic and expensive intervention. Sustainable lake restoration involves limited external interference strong enough to initiate and maintain positive changes in the ecosystem. It was introduced in Lake Durowskie, an urban, flow-through lake situated in Western Poland, using hypolimnetic aeration, phosphorus precipitation with small doses of chemicals and biomanipulation in 2009, and is continued until today. Oxygen conditions in the lake hypolimnion after initial deterioration were gradually improved, and finally a shortening of the duration and range of oxygen deficits was observed. Nitrogen transformations were induced in the hypolimnion by water aeration as well, reducing ammonium N (30% during 2013–2017 in comparison to 2008) and increasing nitrates (90% in 2013–2017 in comparison to 2008). Phosphorus content was diminished (19% during 2015–2017 in relation to 2008 for SRP) due to effective iron-binding and a smaller amount of fresh organic matter being decomposed. Its reduction was related to lower phytoplankton biomass, expressed in a decrease of chlorophyll-a concentrations (55% reduction during 2013–2017 in comparison to 2008) and an increase in water transparency (two-fold during 2013–2017 in relation to 2008) throughout the nine years of treatment. A long-term restoration program, based on non-aggressive, multiple in-lake techniques was applied and, despite the lack of a reduction in total external loading, was able to suppress progressive eutrophication.