Eco-bioengineering tools in ecohydrological assessment of eutrophic water bodies

Eutrophication of water bodies and deterioration of water quality are emerging environmental crises. The root causes and consequences of eutrophication are multidirectional. Thus, they provide a huge scope of risk-analysis and risk-assessment in the domain of remediation studies. However, recent restoration studies reveal a global trend of utilizing traditional restoration methods combined with advanced pioneer innovative techniques developed in the field of science and technology. This review introduces a novel approach to consider ecohydrological assessment of eutrophication by classical biomanipulation practices emphasising on their evolution into innovative ‘eco-bioengineering’ methods. The main objective of this study is to critically analyse and recognize the research gaps in classical biomanipulation and appreciate the reproducibility and efficacy of eco-bioengineering methods at micro- and macrolevel aquatic ecosystems. Comprehensive literature review was conducted on offline and online platforms. Our survey revealed (a) continuation of a historical trend in classical biomanipulation practices (61.64%) and (b) an ascending drift in eco-bioengineering research (38.36%) in the last decade (2010–2021). At a global scale, recent biomanipulation research has a skewed distribution in Europe (41.10%), East Asia (32.88%), North America (10.96%), South Africa (4.11%), South America (2.74%), Middle East (1.37%), Oceania (1.37%), and non-specific regions (5.48%). Finally, this review analysis revealed the comprehensiveness of eco-bioengineering methods and their strong ecological resilience to recurrence of eutrophication and fluctuating environmental flows in the future. Therefore, our review reinforces the supremacy of eco-bioengineering methods as cost-effective green technologies providing sustainable solutions to restore the eutrophic waters at a global scale.


Introduction
Eutrophication causes loss of aquatic habitat, proliferation of algal blooms, and rapid growth of macrophytes affecting nutrient dynamics, environmental flows, and ecological niches of aquatic systems (Carey and Migliaccio 2009); municipal sewage and agricultural runoff are the major players and cause exogenous enrichment of inorganic and organic contaminants (Kumwimba and Meng 2019;Zhang et al. 2014) and ecohydrological imbalances (Paul et al. 2021;Sharpley et al. 2008). Another alarming consequence of eutrophication is the proliferation of harmful algal blooms (HABs), (Smith, Schindler 2009). HABs are visual indicators of eutrophication and are mostly cyanobacterial blooms (Aphanizomenon, Cylindrospermopsis, Dolichospermum, Microcystis, Nodularia, Planktothrix and Trichodesmium etc.) (Wiegand and Pflugmacher 2005;Berdalet et al. 2016;Huisman et al. 2018) HABs deplete oxygen levels in natural water systems, produce cyanotoxins, and hinder penetration of light into the euphotic zone. The HABs not only affect the population of submerged flora and fauna (Anderson 2009) but also release cyanotoxins, resulting acute hepatic, digestive, and neurological disorders in birds and mammals, including humans (Carmichael 2001;Merel et al. 2013). Thus, eutrophication, which has turned out to be a common toxic phenomenon, is acquiring greater geographic distribution of HABs every year (Paul et al. 2021). Upstream flow management may lessen the quantity and toxicity of sewage and effluents; however, it is rather impossible to prevent entry of pollutants into water bodies (Yoon et al. 2015). In the new era of scientific development and technological upgradation of existing water treatment methods, ecohydrology offers an interdisciplinary approach to address all hydrochemical and geochemical aspects of water bodies. This review highlights the role of ecohydrology in treating eutrophication, emphasizing the classical biomanipulation methods and advanced ecological and bioengineering methods to restore eutrophic waters.
Eutrophication management practices involve a variety of physical, chemical, and biological methods for treatment of eutrophic water bodies for reduction, recuperation, and reuse of nutrients and contaminants (Newman et al. 2006;Pärn et al. 2020). For over a century, conventional methods were successfully applied to treat wastewaters including eutrophic waters. However, with the advancement of information, knowledge and technology, newer and innovative techniques have either replaced or modified the conventional methods for better treatment of eutrophic water bodies . Biomanipulation is one such effective ecohydrological tool that modifies aquatic biodiversity to restore eutrophic waters. In the course of time, biomanipulation has become a very promising ecohydrological tool and evolved as an eco-bioengineering method of treatment of eutrophication (Paul et al. 2021;Zhang et al. 2020). The current review introduces a novel approach to identifying the research gaps in classical biomanipulation practices and proposes an interdisciplinary scope for overcoming this discontinuity and prioritizing ecohydrological and ecobioengineering methods. The main goal of this study is to address the evolutionary pathways of classical biomanipulation practices to innovative eco-bioengineering methods and endorses it as one of the best suitable ecohydrological approach to address eutrophication. Accordingly, this study is based on some specific objectives -• Propagation of ecohydrological management and evolution of conventional biological restoration methods.
• Comprehensive study on biomanipulation of eutrophic water bodies.
• Reinforcement of biomanipulation as an ecobioengineering tool for treatment of eutrophic water bodies.
Our study focuses primarily on endorsement of biomanipulation as an innovative eco-bioengineering method of rehabilitation of eutrophic water bodies. Literature review amply revealed that biomanipulation is the most primitive ecological engineering tool and that it evolved over time as an eco-friendly bioengineering method (Mannina et al. 2020). Biomanipulation has been recognized as an eco-bioengineering tool since the early 1990s and reported as an excellent technique to accelerate rehabilitation without resulting in any ecological imbalance in the eutrophic water bodies.

Methodological approach
Our main objective is to critically analyse classical biomanipulation methods and identify the progressive transition pathways into sustainable eco-bioengineering methods to restore eutrophic water bodies. Methodology included offline and online or electronic mode of literature review to obtain relevant information from original research articles, e-reports, archives, and reports published by government agencies, detailed in Fig. 1. An offline literature survey was conducted by collecting information from books, magazines, and scholarly journals in libraries. Online study was conducted by browsing and downloading articles from bibliographic databases like Scopus, Web of Science, Sci-enceDirect, Google Scholar, PubMed and Research Gate following Paul et al. (2021) and Das et al. (2019). Keywords such as 'ecohydrology ', 'classical', 'historical' 'biomanipulation', 'eutrophication', 'ecological', 'engineering' and 'bioengineering' were used to find relevant information based on the objectives of the study. Our literature survey revealed that biomanipulation was the earliest ecological engineering tool, practiced since the early 1990s and progressively evolved over decades as ecobioengineering methods. It was also exposed that classical biomanipulation practices reformed as eco-bioengineering methods within a time span of only three decades. The retrieved information was strategically arranged into various sections and subsections to obtain a meaningful gist of the entire study. However, final drafting of the retrieved information on biomanipulation research was partially justified due to various constraints in offline and online accessibility of articles and privacy policies of bibliographic sources.

Propagation of ecohydrological management and evolution of conventional biological restoration methods
Eutrophication occurs naturally when water bodies age and are inundated with sediment over a long period of time (Carpenter 1981). However, the rate and extent of eutrophication is aggravated by anthropogenic introduction of point source and non-point source pollutants into aquatic ecosystems, causing depletion of multipurpose water resources, fish breeding, and aesthetic activities (Carpenter et al. 1998). In the early 1960s, investigators linked the root causes of algal blooms and nutrient enrichment to major land use activities like agriculture, industrial establishments, and urbanization (Schindler 1974). Eutrophication exacerbates deterioration of water quality by promoting noxious and foul smelling HABs. Dense algal blooms reduce light penetration in littoral zones and affect predators' physiological activities (Lehtiniemi et al. 2005). Algal blooms eventually die and create a hypoxic environment, also known as 'dead zone'. 'Dead zones' are reported during the summer in the Laurentian Great Lakes e.g., the central basin of Lake Erie (Arend et al. 2011). Hypoxia and gradual anoxia result from extreme eutrophication cases termed as hyper-eutrophication (Sebastiá-Frasquet et al. 2020). The effects and causes of eutrophication are represented in Fig. 2. These ecological problems may be collectively addressed by ecohydrological interventions and can be resolved through interdisciplinary approaches to rejuvenate the water bodies facing recurring eutrophication hazards (Paul et al. 2021).
Ecohydrology is an interdisciplinary science that deals with aquatic ecosystems upholding the basic principles of physical, chemical, biological, and engineering sciences. Ecohydrology encompasses an array of operational modes, such as hydrophysical, hydrochemical, hydrobiological and innovative bio-ecoengineering methods (Paul et al. 2021). Ecohydrological history dates to the early 90 s. Zalewski et al. (1997) led a pioneering study on short-term and showing pathways of retrieving information and drafting of review article (adapted from Paul et al. 2021) long-term hydrodynamic processes in relation to ecological systems. For the first time, the International Hydrology and Man and Biosphere programs under UNESCO (2016), familiarized the concept of ecohydrology at a global platform and motivated researchers to address it in ecosystem functions of freshwater bodies (Zalewski et al. 1997). Later, the idea of an ecohydrological approach in managing freshwater ecosystems has been recognized and gradually applied in studying interactions between biotic processes and the hydrological cycle (Nuttle 2002). In modern times, innovative eco-engineering methods of water treatment are observed as new developments under the umbrella of ecohydrology. Ecohydrology provides a sustainable approach to restore eutrophic water bodies including top-down and bottom-up methods in ecosystem management, and it has been successfully implemented in many polluted coastal estuaries (Wolanski and Elliott 2015). Thus, Ecohydrology is a multidisciplinary approach reinstating the conventional methods of water treatment in a productive way (Paul et al. 2021). Ecohydrological treatments are conducted as ex-situ or in-situ processes depending on the requirement, nature, and level of eutrophication. In-situ ecohydrological interventions are considered beneficial over ex-situ methods as it decreases the expense of transportation from the original site of treatment and overcoming the risk of externalities (Paul et al. 2021).

Biomanipulation in eutrophic water bodies
Biomanipulation is a trending area of ecohydrological research, and over the past 30 years it has been appraised for its effectiveness in restoring eutrophic water bodies (Mehner et al. 2002;Arlinghaus et al. 2016). It involves insitu exploration of ecohydrological tools in managing ecologically unstable food-webs in stressed water bodies. Technically, biomanipulation may be defined as a series of manipulation events that modify biotic components or their ecological niches to improve water quality and reduce eutrophication (Shapiro 1990;Shapiro et al. 1975). These events target ecological food-chains by adding or removing some important organisms or key species from aquatic systems. Usually, biomanipulation involves the addition or removal of benthivorous, zoo-planktivorous, and piscivorous fish to manage populations of grazing zooplankton, especially Daphnia sp. . For example, Daphnia sp. actively graze on phytoplankton, especially harmful algal blooms (HABs) in eutrophic water bodies and improve clarity and quality of water (Dionisio Pires et al. 2005). However, the grazing activity of zooplankton is affected by excessive predation (by macroinvertebrates and fishes), insufficient feedstock, and harmful algal toxins (Benndorf et al. 2000). Biomanipulation experiments can be performed in in-vitro conditions to obtain an optimized species composition and extended to field conditions for sustainable and long-term effective results (Fig. 3).
In the late 20th century, a new concept of the trophic cascade hypothesis revolutionized biomanipulation techniques (Wright and Shapiro 1984). It involves "top-down" and "bottom-up" approaches to biomanipulation techniques. Removal of planktivorous fish increases the population of large-sized Daphnia, which feeds on the phytoplankton and gradually decreases cyanobacterial blooms (Lammens 1999;Drenner and Hambright 1999). The hypothesis of atrophic cascade based on "top-down" biomanipulation was confirmed by lab-scale investigations, in secluded artificial aquatic ecosystems (Christoffersen et al. 1993). The trophic cascade is also supplemented by transplantation of submerged macrophytes that prevents resuspension of sediment; this practice is most suitable for restoration of warm shallow lakes (Christoffersen et al. 1993).The trophic cascade hypothesis thus revolves around the removal of species from an   Paul et al. 2021) aquatic system depending on the species composition and stress level of a water body.

Evolution of biomanipulation
Biomanipulation can be performed in several ways by choosing a key species in any aquatic system. In classical biomanipulation methods, the key species is usually a fish; however, biomanipulation evolved over years and researchers are now using newer techniques to maintain a healthy species ratio in aquatic ecosystems. Mehner et al. (2002) showed that successful biomanipulation depends on the food-web structure that affect food-web complexity and success rate of various biomanipulation studies; and revealed a few significant observations -(1) contribution of aquatic organisms to nutrient recycling, (2) role of morphology and physiology of aquatic organisms in food-web interactions and (3) requirement of an equilibrium between aquatic communities like piscivores, planktivores, and benthivores. Dense fish populations rigorously prey upon zooplankton (cladocerans, copepods, and rotifers), reduces algal blooms, and increases transparency of water (Blindow et al. 1993). Some of the frequently practiced classical biomanipulation techniques are discussed below:

Biomanipulation involving planktivores and piscivores
It is evident that proper management of piscivores, zooplanktivores/benthivores and zooplankton in experimental enclosures may help in obtaining a balanced species composition ). Gladyshev et al. (2003) reported that a reduction in the number of planktivorous fish, Carassius auratus significantly lowered cyanobacterial blooms in a small reservoir in Bugach River, Russia. Planktivorous fishes such as crucian carp ingest cyanobacteria while filtering their primary food i.e., zooplankton (Gladyshev et al. 2003). A study showed that bighead carp and silver carp (at a density of 65-75 g/m 3 ) inputs caused a sharp decline in the biomass of total phytoplankton in just 14 days accompanied by disappearance of algal blooms from the culture enclosures (Ping 1996). However, grass carp are not very effective in reducing phytoplankton biomass or controlling water bloom and grazing zooplantivores (Ping 1996). The outcome of this study revealed that the phytoplankton community was dominated by large sized species (>30 µm) in both fish-free and grass carp-stocked enclosures. Ping (1996) reported that in waters stocked with a high density planktivorous fishes (i.e., silver carp and bighead carp), small sized (<30 µm) phytoplankton communities dominated. Another study showed that in culture enclosures stocked with silver carp and bighead carp at various weight ratios (1:0, 1:1, and 1:2), led to significant decrease in chlorophyll-a and total phosphorus levels (Yi et al. 2016). Grass carp (Ctenopharyngodon idella) is a voracious macrophyte feeder, reduces a submerged macrophytes, and increase nutrient concentrations. Similar studies show that planktivorous fishes and rotifers, cladocerans, and overall zooplankton population decreases after macrophyte removal (Maceina et al., 1992). Therefore, the addition of zoo-planktivorous fishes may have a deleterious impact on the zooplankton community.
Experimental models illustrate the pathways of planktivorous fish affecting the viability of blue green algae while passing through fish intestine (McQueen 1990). It has been reported that both colonial and filamentous forms of blue green algae can withstand the digestion process in planktivorous fish guts and eventually increase their primary productivity (Miura and Wang 1985). Reduction in the number of zoo-planktivores in a lake may increase the size and grazing activities of herbivorous zooplankton accompanied by a significant reduction in cyanobacterial abundance and a shift from small-bodied to large-bodied Daphnia sp. Gradual shift from Daphnia ambigua to Daphnia parvula and Daphnia parvula to Daphnia galeata and finally Daphnia pulex is a common outcome observed in biomanipulation experiments. Post-biomanipulation effects appear as decreased concentrations of TP and TN, epilimnion chlorophyll, dominant population of larger, and increased transparency of water. This substantially increases the grazing pressure of newly dominant zooplankton communities; survival rates in even low concentrations of chlorophyll and few algal communities, and presence of high inorganic N and P concentrations .
Shallow eutrophic lakes are often treated with rotenone to curb the populations of planktivorous fish e.g., Coregonus lavaretus (Sanni and Waervågen 1990). This elimination leads to dominance of rotifers and decrease in chlorophyll-a, nutrients, and algal blooms, accompanied by increase in populations of Daphnia galeata and transparency of water (Sanni and Waervågen 1990). These activities further create an anomaly in the ecological food chain and aquatic species composition which affect the zooplankton community leading to an oligotrophic condition. Decrease in chlorophyll-a to P ratio also indicates inconsistent nutrient cycling and increased predation on herbivorous zooplankton that shifts a lake to an oligotrophic state (Karjalainen et al. 1999). Additionally, imbibing piscivores and reducing zoo-planktivores can create an upsurge in zooplankton biomass, and transitions between cladoceran and copepod or rotifer assemblages, following reduction in phytoplankton biomass, and loss of photosynthetic activity (Carpenter et al. 1987). Another experiment conducted in eutrophic Vaeng Lake, Denmark showed that removal of 2.5 tons of bream (Abramis brama) and roach (Rutilus rutilus) during spring season caused a 50% reduction in the total planktivorous/benthivorous fish biomass. This created a remarkable shift in the dominance of the zooplankton community, succeeded by rotifers and larger cladocerans (Sondergaard et al. 2007). In Bautzen Reservoir (Dresden County), introduction of predators like Salmo gairdneri in a predator-free system, greatly reduced the biomass of zooplanktivores, specifically Leucaspius delineates (Benndorf 1995). In the absence of predatory fishes, biomass of zooplankton may increase by 400% and phytoplankton species may reduce significantly (Benndorf et al. 1995). A Secchi disc may be used to determine the zone of light penetration in eutrophic wetlands, depth; it is an efficient mechanism to identify a change in phytoplankton composition relating to the depth of the euphotic zone in a lake (Benndorf 1995). One sign of successful biomanipulation is restoration of ecological niches, equally effective for zoo-planktivores, zooplankton, phytoplankton, and macrophytes (Donk and Bund 2002).

Biomanipulation involving submerged macrophytes
Submerged plants prepare a platform for stocking piscivores that can control populations of rigorous predator fishes and sedimentation activities without affecting the natural ecological balance (Drenner and Hambright 1999). In the presence of submerged macrophytes, piscivorous population will also control zooplankton that rapidly graze on phytoplankton, and minimize oxygen consumption, and prevent P-release from sediments. Therefore, biomanipulation may be enhanced by the introduction of adequate species of submerged plants to restore eutrophic shallow lakes (Carignan and Kalff 1980;Jaynes and Carpenter 1986). Submerged plants improve cleanness of water and promote growth of useful benthic algae by directly taking up nutrients and pollutants, limiting phosphorus (P) release from sediment by oxidation of sediment and increasing the P-binding capacity of sediments and preventing sedimentation of particles in the water column that affects light assimilation by phytoplankton (Carignan and Kalff 1980;Blindow et al. 1993; Barko and James 1998;Zhang et al. 2017). This combined biomanipulation and submerged species treatment also promotes dominance of large-bodied Daphnia magna, showing high densities during spring and summer.
Involvement of submerged macrophytes in biomanipulation experiments showed a positive effect on restoration of water quality and maintaining a healthy ecosystem, especially in temperate lakes in European countries. . In warm lakes, re-introduction of submerged vegetation had a negligible effect on fish and zooplankton abundance. Introduction of macrophyte beds offers shelter and protection of zooplankton from planktivorous fish (Timms and Moss 1984). Macrophyte beds also provide a platform for multiplication and feeding ground for the zooplankton (Barko and James 1998). However, assemblage of zooplankton at the vicinity of macrophytes becomes scanty in the presence of omnivorous fish predators (dominant), and hence predation of zooplankton near the dense vegetation in warm shallow lakes Jeppesen et al. 2007;Liu et al. 2014). Therefore, fish abundance, especially omnivores is apparent in warm shallow lakes in sub-tropical regions than the temperate regions (Zhao et al. 2006). Young omnivorous fishes are dominant in warm lakes that rapidly feed on zooplankton, whereas the adult fish consume the abundant macrophytes (González-Bergonzoni et al. 2012). But submerged macrophytes may sometimes release chemicals, mostly allelopathic substances, that may have adverse impacts on the aquatic communities; however, there is minimal information on the mode and mechanism of action of these allelopathic substances (Donk and Bund 2002). Eichhornia crassipes, Pistia stratiotes, Salvinia molesta, Hydrilla verticillata, Myriophyllum aquaticum, Elodea nuttallii etc. are some of the macrophytes reported in effective biomanipulation techniques.

Biomanipulation involving zooplankton
Zooplankton species are another key in biomanipulation experiments, removal, or addition of which affects the feeding activities of zoo-planktivores. Zooplankton grazing activity facilitates a clear-water period, in most of the mesotrophic and eutrophic lakes, and is measured by Secchi disc transparency i.e., drop in chlorophyll concentration (Lampert et al. 1986). In other words, the rate of zooplankton grazing is directly proportional to the clarity of water. It has been reported that before attainment of a clear water phase, small phytoplankton communities (<35 μm size) contribute to around 88% of the primary productivity; later, zooplankton filter feed on small phytoplankton leaving behind small populations of large size phytoplankton (Lampert et al. 1986). In the presence of actively feeding adult copepod species as Pseudocalanus, Temoralongicornis, Oithonasimilis, and Acartiatonsa (1-15 µ) in Bras d'Or Lake and Morrison's Pond, Cape Breton Island, a skewed daily zooplankton food intake and non-uniform primary production was observed at the depths of maximum photosynthesis (Hargrave and Geen 1970). Abiotic variables like reduced pH, water transparency and total phosphorus levels are linked to primary productivity of the phytoplankton communities (Schoenberg and Carlson 1984). Daphnia are another group that rapidly feed on cyanobacteria (Microcystis), Daphnia longispina can reduce the population of Anabaena flos-aquae (cyanobacteria) by a factor of 350, resulting increased transparency of water in mid-summer (Ger et al. 2016). D. longispina can also reduce the population of Microcystis aeruginosa, and effectively restore eutrophic water bodies (Pogozhev and Gerasimova 2001). However, cladocerans, copepods, and rotifers have differential feeding abilities. Copepods use chemical signals to choose algal feed of a particular size range. Consequently, they are selective slow feeders as compared to cladocerans and rotifers (Bogdan and McNaught 1975). Rotifers prey on small algal cells, for example, Brachionus calyciflorus cleared smaller Microcystis (<20 µm diameter) at a rate of 1.9 times higher than larger Chlamydomonas (>20 µm diameter) (Fulton and Paerl 1987). Such feeding behavior of rotifers may compromise their nutritional requirements with toxin containing biopolymers, because cyanobacteria lack essential lipids. Hence, they are not suitable for fulfilling the nutritional requirements of zooplankton (Dickman et al. 2008). Unlike cyanobacteria, enzymatic digestion constraints are recognizable in nonblue-green algae diets. These include gelatin and mucopolysaccharides in species like Sphaerocyetis, Scenedesmus and diatoms (Porter 1980). Knisely, Geller (1986) reported that, in Lake Constance, cladocerans, Daphnia hyalina and D. galeata grazed upon both coccoid and flagellates but preferred the latter for nutritional requirements. Cladocerans and rotifers mostly feed on filamentous algae; however, their feeding activity is inhibited by a cyanobacterial toxin named microcystin (Hairston et al. 1999). Research investigations reveal that zooplankton can adapt to microcystin-containing cyanobacteria in their diet and develop or produce microcystin assimilation factors in their body (Hairston et al. 1999;Sarnelle and Wilson 2005). In an experiment feeding zooplankton with a microcystin diet, it was demonstrated that microcystin minimized the growth rates of around 70% grazing population without recording any other detrimental effects (Sánchez et al. 2019). Zooplankton, also known as filter feeders, feed more frequently on small chlorophytes and flagellates than cyanobacteria, which limits the toxin assimilating activities of zooplankton such as copepods, cladocerans and rotifers. However, no statistically significant difference was found regarding size and metabolism among the grazing populations feeding on both toxic algae and nontoxic algae (Wilson et al. 2006). Toxic phytoplankton has been intrinsically diverse and often coexists with zooplankton communities (Kerfoot et al. 1985). An efficient mechanism of toxin dilution and compensatory feeding has been reported in a copepod (Acartiaclausi) fed with two strains of a dinoflagellate, Alexandrium minutum. A sympatric population of Acartia clausi when fed with a mixed diet of two varieties (toxic and nontoxic) of A. minutum cells; the toxic cells negatively influenced its oviparity, egg hatching and overall total reproductive performance whereas the non-toxic cells had no impact on its life cycle (Bergkvist et al. 2008). However, in cladocerans such as Daphnia sp., mortality rates increased after feeding on toxin producing cyanobacteria (Lampert 1981). Zooplankton have evolved over time and emerged as highly toxin tolerable species with toxin defence mechanisms (Tillmanns et al. 2008). Ger et al. (2016) described some interesting zooplankton cyanotoxin tolerance traits and categorized depending on two factors -(1) grazing behaviour in relation to ingestion of number of cells containing toxins and (2) digestion and assimilation dynamics after intake of toxic metabolites (Kirk and Gilbert 1992;Pflugmacher et al. 1998). Therefore, zooplankton may effectively reduce algal blooms and turbidity of water and help in transformation of trophic status of eutrophic water bodies. Biomanipulation involving mussels and snails Mussels are important components of pisciculture, and over the past few decades, the culture of mussels has played a key role in economic development in Australian and European countries. Mussels are a key species in biomanipulation studies and have been recognized due to their rapid filter feeding activities. Freshwater mussels remove seston (i.e., a collection of non-living material and living organisms) through its filter feeding activity and significantly reduce phytoplankton biomass to restore eutrophic waters. Echyridella menziesii, a fresh mussel, can restore water clarity by filter feeding on phytoplankton biomass (Ogilvie and Mitchell 1995). Dreissena rostriformis bugensis or Quagga, is abundantly found in the Netherlands in lentic and lotic water bodies; it is a fast-growing bivalve and excellent filter feeder, used as a tool controlling harmful algal blooms in urban ponds (Waajen et al. 2016). Quagga mussels can clean hypertrophic waters and improve clarity of water. Another potential filter feeder, Dreissena polymorpha or zebra mussels, has been reported to clear eutrophic waters in a shallow temperate hypereutrophic lake. Zebra mussels are considered as a potential biomanipulation tool for improving water quality and reducing turbidity in eutrophic lakes (Richter 1986).
Snails can also help clear turbidity from eutrophic waters; they are also known as scrapers because they ingest phytoplankton and organic matter growing on solid substrates (Alfaro et al. 2007). Bellamya aeruginosa, a freshwater snail commonly found in freshwater lakes, rivers, and ponds in China, can significantly reduce algal blooms in wetlands (Chase and Knight 2006;Loman 2001). Stocking of the Lymnaea sp. (pulmonata snail) in eutrophic waters showed a decline in cyanobacterial cell density, especially Microcystis aeruginosa (Jin et al. 2015).

Reinforcement of biomanipulation as an eco-bioengineering tool
Biomanipulation is an emerging practice since the past century, and it has been globally recognized as a pioneering tool in managing eutrophic water bodies. Biomanipulation was reinforced as eco-bioengineering for the first time by Shapiro et al. (1975), as it included modified species composition or rearrangement of species by manipulating fishes, zooplankton, harmful algae and aquatic biomass, and overall reduction in nutrient load. Jones (1986) considered biomanipulation as a relatively new technique performed by removing some fish species and restructuring plankton communities to enhance water transparency in eutrophic lakes. With the advent of new concepts and technological developments in the field of biomanipulation studies, researchers have reintroduced biomanipulation as an ecohydrological approach in treating water bodies under eutrophication and environmental pollution stress. Biomanipulation has evolved over several decades and adapted to innovative ecohydrological practices with remarkably better results than traditional biomanipulation methods. In the immediate decade (2010 to 2020), the flux of bioengineering research is concentrated in two areasclassical biomanipulation studies (61.64%) and non-classical or advanced eco-bioengineering studies (38.36%). A worldwide online literature survey revealed that most of the biomanipulation research is reported from Europe and East Asian regions followed by North America, South America, South Africa, and Oceania (Fig. 4). This signifies the crucial role of environmental factors, climate change and anthropogenic agents in inducing eutrophication of water bodies and the response to risk assessment and risk analysis with a prompt scientific outlook.
Eco-bioengineering methods vis-à-vis non-classical or advanced biomanipulation methods are often conducted in amalgamation with eco-computational models based on current investigations or reconstruction of old age database, and a physical or chemical agent or a bio-engineered agent to obtain sustainable outcomes (Shapiro et al. 1975;He et al. 2011;Hobbs et al. 2012;Lewtas et al. 2015). He et al. (2011) reported a series of ecohydrological inputs including biomanipulation in a three-dimensional eutrophication model to reduce eutrophication in Beijing Guanting Reservoir, China. Researchers are exploring newer ways of treating eutrophic waters by following goals and ideas from successful rehabilitation experiments. Eco-bioengineering enhances classical biomanipulation methods by combining it with technical interventions of nutrient (N, P) retention and prevention of nutrient release from sediments and internal nutrient transfer processes ). In innovative eco-bioengineering methods, nutrient (C, N, and P) and chlorophyll-a are not always considered as key eutrophication drivers. Rather some unique features like algal biomass, sprat spawning stock biomass (SSB), pesticides, organic pollutants, hydrochemical variables drive eutrophication. Additionally, the whole ecological niche may modulate water quality, DO levels, nutrient enrichment, chlorophyll-a levels and exacerbate nutrients and trophic status (Lindegren et al. 2010;Skov et al. 2019, Sharma 2015Sharma et al. 2010;Bernes et al. 2013).
Mid-Infrared radiation (MIR) spectroscopy may prove to be a promising advanced biophysical tool in detecting irregularities in aquatic and terrestrial systems by recording the molecular vibrations at the surface of the recipient. MIR absorption may indicate the characteristics of any substance up to molecular levels which helps to identify water bodies  (Cunningham et al. 2016). MIR has shown excellent results in reconstruction of primitive water salinity scenarios in lakes of Australia, fluctuation of ice or snow cover in the Great Lakes (Cunningham et al. 2016;Dorofy et al. 2016). Molecular biological approaches like environmental DNA (eDNA) metabarcoding are pioneering in the identification or assessment of harmful algal taxa or any other aquatic organisms in eutrophic waters (Liu et al. 2020;Salter et al. 2019;Schadewell and Adams 2021). eDNA metabarcoding can help in exploring the known as well as unknown species under environmental stress in the freshwater or marine water bodies (Schadewell and Adams 2021). It is a very useful method for retrieving unknown genetic material from environmental samples vis-à-vis eutrophic waters and for identifying or tracking unknown biological pollutants by barcoding the 16SrRNA or 18SrRNA regions of the genetic samples. Genetic material sampling depends on the nature of investigation, in nutrient pollution studies. Target genetic material are mostly benthic flora that are responsible for nutrient enrichment and resultant eutrophication. eDNA are retrieved from environmental samples by systematic molecular biological techniques of extraction of DNA, polymerase chain reaction (PCR), and construction of a metabarcode that is unique to an organism. Selection of the DNA region to be barcoded is decided with the help of genetic markers (short nuclear base-pairs) specific for an organism. Mostly ribulose-1,5-bisphosphate carboxylase⁄ oxygenase gene (also known as rbcL) is selected for a wide range of investigations (Ruppert et al. 2019;Harper et al. 2019;Clark et al. 2020). This information helps track the source or origin of the biological agents (useful as well as pathogenic) and prepare suitable treatments or mitigation strategies for rehabilitation of eutrophic water bodies. eDNA metabarcoding in managing nutrient loading is successfully reported in Waimea Estuary (New Zealand), Okinawa coral reefs (Japan), Hood Canal, Washington, and Gulf of Maine (GOM) (USA), Yangtze River, Qinhuai River and Tai Lake (China) and marine ports near University of Tasmania (Australia) (Clark et al. 2020;DiBattista et al. 2020;Jacobs-Palmer et al. 2020;Prabowo 2020;Li et al. 2018;Shaw et al. 2019). Interestingly, these organisms have taxonomic characteristics of diatoms, dinoflagellates, cyanobacteria, and algae associated with freshwater and marine nutrient enrichments. This popular eco-bioengineering tool may provide effective solutions for remediation of eutrophic waters by enlisting the biological causal agents, which facilitate irreversible treatment for a long term (Jacobs- Palmer et al. 2021). Advanced biomanipulation practices involve manipulation of biological agents combined with non-biological agents like addition of biological products (barley straw) and chemicals (alum, rotenone), and physical treatment (draining, dredging), and reconstruction of previous environmental conditions by modelling nutrient dynamics and ecological dynamics (Mustapha 2010;Webber 2014;He et al. 2011).
Eutrophication is monitored by nutrient modelling utilizing Soil and Water Assessment Tool (SWAT) models. These models are extremely helpful in ascertaining the nutrient pollution in relation to the upsurge of cyanobacteria population and take necessary remediation measures for treating eutrophic waters. Point source nutrient pollution monitoring is very popular in monitoring eutrophic water bodies, watersheds, surface waters and large water bodies, models like Modelling Nutrient Emissions in RIver Systems (MONERIS), BATHTUB, AGricultural Non-Point Source (AnnAGNPS), and reservoir eutrophication model etc. (Yazdi and Moridi 2017;Malagó et al. 2017;Xu et al. 2019;) These models efficiently utilize meteorological, experimental, spatiotemporal, and geographical information system (GIS) data to derive meaningful information on the nature, source, status, and mitigation strategies (Gungor et al. 2016;Gregar et al. 2019;. Remote sensing has also been utilized as an innovative technique in combination with bioengineering methods to manage eutrophic water bodies. Research investigations have focused on the derivation of algorithms based on remote sensing for monitoring particulate organic carbon (POC), nutrients, and pollutants that cause eutrophication in water bodies . There are concurrent reports on the role of GIS, remote sensing, and artificial intelligence (AI) in detecting anomalies in water bodies, varied spectral bands of satellite images are coordinated with experimental or meteorological datasets into algorithms or mathematical models (Palmer et al. 2015;Dorofy et al. 2016;Papenfus et al. 2020;Chen et al. 2021;Free et al., 2020). For an instance, WorldView-2 multispectral image in shallow Lake Eymir, Ankara (Turkey) helped to develop a bathymetric map (water body depth) to track eutrophication at various depths; Sentinel-2 Multispectral Imager data, Sentinel 2 surface reflectance (SR) data detects cyanobacterial bloom and fluctuation in concentration of chlorophyll-a in eutrophic lakes (Yuzugullu and Aksoy 2014;Grendaitė et al. 2018;Hussein and Assaf 2020;Borges et al. 2019;Peppa et al. 2020;Alba et al. 2020). Recently eutrophication monitoring in a few lakes in the United States and Canada, elaborates the use of multiple spectral or hyperspectral satellite images obtained from Earth observation (EO) satellites; these images were used to record fluctuations in algal bloom intensity with the help of Medium Resolution Imaging Spectrometer (MERIS) and Ocean and Land Colour Instrument (OLCI) sensors at spatial and temporal scales (Groom et al. 2019;Binding et al. 2021). Ancient reports, baseline information or new findings are helpful in constructing algorithms or models and providing inputs in sensors like OLCI, MERIS, Geostationary Ocean Color Imager (GOCI), Moderate Resolution Imaging Spectroradiometer (MODIS) etc. for eutrophication monitoring (Bresciani et al. 2011;Xiang et al. 2015;Anthony et al. 2016;Xue et al. 2019;Xu et al. 2020;Sharp et al. 2021). Assessment of many eutrophic water bodies is accomplished by application of GIS-remote sensing based artificial neural networks (ANN). ANNs generate massive information on the trophic status (derived from biophysicochemical parametersnitrogen, phosphorus, chlorophyll, and temperature) and other trophic level index (TLI) of numerous inland water bodies (Xiang et al. 2015). Studies on TLI or trophic status index (TSI) are frequently and extensively reported in East Asian, South Asian, and South American eutrophic lakes (Sheela et al. 2011;Mishra and Garg 2011;Xiang et al. 2015;Esfandi et al. 2018;Zhou et al. 2019;Leiva et al. 2019;Sabrina and Sudaryatno 2021;Hu et al. 2021). A list of advanced ecobioengineering approaches and their effective outcomes are detailed in Table 1.

Success rates of biomanipulation and nonrecurrence of eutrophication
In classical methods of biomanipulation, success rates vary depending upon the geomorphic features, duration, and nature of treatment (Gulati et al. 2013;Benndorf 1995). Eutrophication recurrence episodes are common and therefore, upgradation of conventional biomanipulation methods and identification of the loopholes help amplify the success rates of manipulation of biotic species. Jepessen et al. (2012) categorized biomanipulation into seven different ways to highlight successful biomanipulation and non-recurrence of eutrophication in target water bodies. Removal of zooplanktivorous and benthivorous fishes may lead to successful and long-term stability in shallow and warm temperate eutrophic lakes (Jeppesen et al. 2005;Søndergaard et al. 2007). Berg et al. (1997) report that stocking of piscivorous fishes (e.g., pike) also increases the success rates of biomanipulation in Lake Lyng, Denmark, in the absence of some of the other pelagic predator fishes (e.g., perch). Sometimes introduction or removal of fish leads to partial success of biomanipulation with a challenging prospect of reappearance of excessive nutrient load, chlorophyll-a, and turbidity in the water (Xie and Liu 2001). Classical biomanipulation involving removal or introduction of zooplankton and macrophytes also improves viability and long-term effectiveness of a modified food-web structure in aquatic systems (Moore et al. 2019). However, abiotic factors such as nutrient-sediment dynamics and sustainability of biomanipulation experiments indicate whether ecological resilience in response to eutrophication is strong and recurrence is completely prevented ). Zhang et al. (2008) derived that successful biomanipulation depends on a few fundamental factors -(a) number of planktivorous fishes in experimental enclosure, (b) threshold levels of phosphorus, (c) P release from sediment and (d) populations of submerged macrophytes that may increase or decrease clarity of eutrophic waters. These factors unambiguously determine the restoration success rate and sustainability of eco-bioengineering methods worldwide, as reported by most researchers working with advanced and innovative biomanipulation methods (Gulati et al. 2013;Banerjee et al. 2018;Paul et al. 2021). Earth geography is characterized by abroad spectrum of geomorphic features, life forms, and thermal regimes that govern the nature and degree of eutrophication of water bodies in a particular region (Wojtal-Frankiewicz 2012). A series of extrinsic and intrinsic factors influence the environmental flows and regime shifts in water bodies that contribute to eutrophication (Vermaire et al. 2017). Therefore, biomanipulation experiments necessitate both bottomup and top-down controls for water bodies in cold as well as in warm climatic regions; temperature and phosphorus loadings are, however, identified as the major drivers of eutrophication in temperate and tropical regions (Beklioglu et al. 2010). Apparently, global change in temperature regime alone can transform the growth patterns of phytoplankton and zooplankton and challenge the success rates of biomanipulation experiments (Burns et al. 2013). Thus, it may be inferred that a cold temperate lake in Europe will meet successful restoration at a faster pace than a warm tropical lake in Asia. Our review indicates that success rates in eco-bioengineering practices are highly evident in European (59.26%) and Asian countries (22.22%) during 2010-2021 (Fig. 4). It is appraised that no immediate rebounding or recurrence of eutrophication is observed and that the pathways for restoration measures are ubiquitous in eco-bioengineering methods. A larger perspective of eutrophication recurrence rates is reflected by consistency of restoration measures, and classical biomanipulation techniques are being upgraded with an ecohydrological prospect to obtain ecologically balanced systems. It is accentuated that those eco-bioengineering methods would help reduce recurrence of eutrophication and fulfill successful biomanipulation to withstand any future ecological imbalances at temporal and spatial scales.

Cost-benefit analysis of eco-bioengineering methods
Eco-bioengineering methods are based on cost-effective risk assessment and analysis to mitigate the extremes of eutrophication. It has been observed that in-vitro management of biological agents and transfer of the optimized technology to in-vivo conditions will help minimize the cost of direct in-situ biomanipulation undertakings (Håkanson and Bryhn 2010). Size optimization of enclosures or the insitu water bodies for conducting biomanipulation experiments has been found to be effective in reducing the cost of restoration studies (Godwin et al. 2011). Jeppesen et al. (2012) uphold the effectiveness of innovative biomanipulation methods in obtaining both short-term and long-term benefits. These eco-bioengineering methods generally reduce the restoration costs for treatment of eutrophic water bodies. Short-term classical biomanipulation experiments may also be cost-effective, but the main goal (i.e., attainment of permanent and sustainable restoration) may not be enough to justify the costs of biomanipulation (Hobbs et al. 2012;Hansson et al. 1998).
There are instances of very expensive classical biomanipulation projects like the Hartbeespoort Dam Bioremediation Programme (HDBP) in south African reservoirs costing up to R100 millionyr −1 (approximately US$ 8.5 million), realizing the need of a continual and sustainable approach instead of costly therapeutic tools for managing eutrophication (Hart and Harding 2015). Large-scale pilot projects may be managed with optimal costs; Lewtas et al. (2015) conducted a combined classical biomanipulation and eco-bioengineering restoration of Twin Lake, Minnesota at minimal cost and presented a detailed cost-analysis report of for their pilot project. This report motivates researchers to adopt inexpensive combined restoration technologies in the long run and obtain cost-effective results with fundamental infrastructures and eco-friendly bio-physicochemical agents. Successful biomanipulation studies involving biological agents like barley straw showed cost-effective results with a small capital amount for treating eutrophic reservoirs in African provinces (Mustapha 2010).
Comprehensive appraisal of literature on restoration of eutrophic waters shows that in both classical biomanipulation and eco-bioengineering methods, activities like harvesting, dredging, selected fishing, introduction, or removal of aquatic plants, building enclosure, combined biophysicochemical treatment etc. an optimal capital amount is inevitable (Burns et al. 2013;Paul et al. 2021). However, this capital investment in eco-bioengineering techniques is always justified by a viable outcome, i.e., sustainable restoration of polluted or eutrophic water bodies at a low cost and a lesser time-period (Paul et al. 2021). Ecobioengineering tools under the umbrella of ecohydrological assessment are a progressive outlook adopted by different countries in the world. The state-of-the-art scenario of advanced biomanipulation research shows that Europe (59.26%), East Asia (22.22%), North America (14.81%), South Africa (3.70%), Middle East (3.70%), and nonspecific regions (7.41%) are increasingly adopting ecobioengineering methods in treating eutrophic water bodies, considering the parity and multi-dimensional feasibility of these innovative methods. Geomorphic conditions and climatic factors also determine the nature of the most suitable biomanipulation treatment and hence the experimental outlays (Gulati et al. 2013). A positive correlation was observed between extrinsic-intrinsic factors and climatology in relation to eutrophication (Benndorf et al. 2000;Vermaire et al. 2017). Therefore, type of geographic location appreciates the most suitable biomanipulation experiments in cold as well as warm countries (Beklioglu et al. 2010). As we have already discussed, a lake with a temperate climate will result in faster restoration than a lake with a tropical climate, but eco-bioengineering tools will provide a unanimous, cost-effective solution anywhere in the world. A group of researchers working on eutrophication management in the Baltic Sea area reported that cost-effectiveness of nutrient abatement strategies is dependent on nutrient reduction tasks, where ecodynamics models are good tools for assessing cost-benefit analysis of large-scale projects linking massive nutrient reductions in eutrophic waters (Håkanson and Bryhn 2010). At the present era, it is possible to fathom the cost-benefit aspect of ecobioengineering methods because there is a global surge of upcoming research in the field of biomanipulation (Banerjee et al. 2018); this also indicates that the modalities in accomplishing an irreversible eutrophication treatment will be promisingly cost-effective (Paul et al. 2021). Thus, biomanipulation experiments enhanced with ecohydrological approaches will provide a sustainable and positive cost-benefit platform to treat water bodies over classical methods, which often involve costly ex-situ treatment or equipment analysis and other externalities (Jurajda et al. 2016).

Conclusion, research gap, and future prospects
Eutrophication is an emerging environmental crisis, and our review focuses on restoration of eutrophic waters using ecohydrological approaches of classical biomanipulation and innovative eco-bioengineering methods. This paper critically examines the role of biomanipulation in historical as well as current research endeavours i.e., rehabilitation of eutrophic water bodies. Comprehensive literature review showed the pathways of reform of earliest biomanipulation methods into new eco-bioengineering tools (Shapiro et al. 1975;Mannina et al. 2020). Classical biomanipulation methods have been successful over a wide range of environmental and geographic conditions; however, these approaches may not be effective in maintaining long-term stability of restored water bodies or preventing recurrence of eutrophication. It has been realized that the effects of classical biomanipulation on ecosystem function needs extensive and exploratory research with a multidirectional approach . These biomanipulation techniques are improved by blending them with ecofriendly bio-physicochemical agents or by extrapolating databases with the help of ecological models to predict future consequences. The present review emphasises a holistic approach to addresses the research gaps in classical biomanipulation practices and advocates advanced ecohydrological methods infilling these breaches, namely, by promoting eco-bioengineering tools. In the purview of the "state of the art" scenario of biomanipulation techniques, the following research conclusions and future recommendations have been summarized: • At a global scale, classical biomanipulations have been practiced to rehabilitate eutrophic waters. However, success rates and eutrophication recurrence events are uncertain. Eco-bioengineering methods combined with ecological models would help to extrapolate, predict, trace, and modulate the dynamic environmental variables that play a crucial role in accomplishing selfsufficient biomanipulation practice.
• Classical biomanipulations lack comprehensiveness regarding reproducibility and efficiency, while innovative bio-physicochemical amendments facilitate increasing longevity and efficacy of the restoration techniques at micro-and macrolevel ecosystems.
• Eco-bioengineering methods are ubiquitous and costeffective green technologies that may be practiced in any part of the world. These innovative methods will provide a sustainable and positive cost-benefit approach to restore eutrophic water bodies, without compromising the ecological health of aquatic communities.
• Early biomanipulation studies may lack totality in terms of irreversible restoration of affected water bodies. Therefore, innovation of classical biomanipulation techniques will help in actively responding to environmental flows and building a strong ecological resilience to defy recurrence events of eutrophication in future.
• Eco-bioengineering methods involve manipulation of the food chain or food web and mediate changes in the trophic status of aquatic ecosystems. Effective polyculture combinations like planktivores viz. fish, macrophytes and mussels can help in developing an efficient in-situ optimized biomanipulation technology combined with bio-physicochemical and computational support.
• Finally, this review presents a baseline for technical innovations in biomanipulation studies for future endeavours and policy making to encourage exponential research and development in the field of ecohydrology.