Dear Colleagues,

This Special Issue of Sustainability will be focused on novel methods for restoring eutrophicated aquatic ecosystems. Among all threats that impair water quality (e.g., acidification, salinization, and contaminantion with xenobiotics), eutrophication has become one of the most common affecting lakes and reservoirs from all over the world. Enrichment of phosphorus (P) in water bodies, which is the main growthlimiting nutrient of primary productivity in freshwater bodies, constitutes the primary cause of eutrophication. Fertilizer run-off coming from agricultural areas and insufficient wastewater treatment are mainly responsible for the increase of P inputs to the aquatic ecosystems leading to, among others, biodiversity loss, toxic cyanobacteria blooms, and changes in the function of the system.

The use of lake restoration techniques in order to meet the requirements set by environmental policies has been increasing in the last 50 years. In Europe, the water framework directive (WFD) has been established as a legal requirement to achieve a "good status" in all water bodies by 2027. Recently, approaches that aim at reducing P availability in the water column have been differentiated into three categories: (a) reduction of external P inputs to the system, (b) increasing P retention by the system, and (c) increasing P exports from the system. Although a multi-dimensional approach is often required, the keystone of a lake restoration project is the reduction of external P load, which can be accomplished by sewage diversion, natural/constructed wetlands, or phosphorous elimination plants (PEPs) situated at the main inflow of the lake. After a reduction of external P load, biological resilience and P release from sediment (internal P load) may be responsible for a delay in lake recovery. In such cases, the application of alum, iron (Fe), and calcium salts (P inactivation methods) is recommended to increase P retention in the sediment. Nitrate addition, hypolimnetic aeration, and sediment capping are other known techniques to reduce internal P load. Among the techniques intended to increase P export from the system can be highlighted hypolimnetic withdrawal, external elimination of P-enriched hypolimnetic waters, and sediment dredging. The manipulation of the physical and also the biological structure of the aquatic ecosystem constitute other types of techniques to improve water quality.

Nowadays, there is not a panacea in lake restoration since all the available restoration techniques present both advantages and disadvantages depending on the characteristics of the system, but without any doubt, a successful restoration strategy requires substantial knowledge of the history of the lake and also of the system’s ecology in order to anticipate possible changes or responses. Due to the difficulties that present some P inactivation methods—redox sensitivity, biotoxicity in highly acidic or alkaline media, acidification of lake water due to floc formation, and reduction of P adsorption capacity with ageing of the adsorbent—the development of new tools to combat eutrophication and improve water quality is of special interest for water resources management.

Magnetic nano and microparticles, mostly based on Fe, have attracted special attention in areas such as biomedicine, industry, environmental remediation, and the one that concerns the present issue: lake restoration. The reason why their use has become widespread in the last decades for adsorbing pollutants from aqueous media stems from their unique properties: fast adsorption kinetics due to their high surface reactivity; high adsorption capacity due to their high surface area; and their magnetism, which makes them different to other adsorbents. The latest property implies that magnetic particles can be removed from the media by applying a magnetic separation gradient once the target contaminant is adsorbed, thus minimizing contact time with biota and enabling their recovery for further reuse.

One of the main advantages of using magnetic particles as adsorbents is the possibility of surface functionalization in order to increase affinity for the adsorbate, reduce biotoxicity, or modify particle density for specific purposes.

Papers comprising this Special Issue should be original contributions focused on different methods for restoring inland aquatic ecosystems. Those papers that focus on case studies for identifying advantages and disadvantages of each technique are particularly welcome.

Prof. Inmaculada de Vicente Álvarez-Manzaneda
Prof. Luis Cruz Pizarro
Guest Editors

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• lake restoration
• eutrophication
• phosphorus
• internal load
• inactivation