Threats on Aquatic Ecosystem- Mitigation and Conservation Strategies

Limnology is the study of the structural and functional interrelationships of organisms of inland waters as their dynamic physical, chemical, and biotic environments affect them [1]. The freshwater ecosystem provides services like water for sanitation, agriculture, drinking, and animal husbandry. Of all the water on earth, fresh water accounts for only 3%, nevertheless, the freshwater system is threatened by forces like climate change, overdevelopment, and polluted runoffs [2,3]. 

Thus, understanding the variation in biodiversity trends in response to biotic and abiotic threats and consolidating the available knowledge is crucial in sustaining the services of freshwater ecosystems [4]. 

This decade is providing us with a critical opportunity to take action and influence the freshwater biodiversity on the right path. The United Nations Decade on Biodiversity (2011-2020) has ended, allowing governments all around the world to review and analyze their international agreements including Convention on Biological Diversity (CBD), the Sustainable Development Goals (SDGs), and the UN Framework Convention on Climate Change(UNFCCC). A Global Biodiversity Framework is under development, with a mission to “Halt the loss of species, ecosystems and genetic diversity by 2030’’. 

The purpose of this paper is to list the existing and emerging threats to freshwater biodiversity, research, and reiterate the conservation strategies to be employed to overcome the challenges. The paper also lists the mitigation and conservation ideas to combat existing and emerging threats.

The threats can be categorized as I. Existing threats and II. Emerging threats 

Existing threats due to human activities 

Human activities

Anthropogenic activities like deforestation, waste disposal near ponds and lakes, construction of bridges and dams, and agriculture, domestic, and industrial activities result in contamination of the aquatic environment. Human settlements and industries are found to be the major source of water pollution (Table 1). In developed countries, agriculture is the major factor in the contamination of aquatic ecosystems. In developing countries, municipal and industrial effluents are a major threat [5].

Table 1: List of Existing threats to the freshwater ecosystem. 

Agrochemicals and nutrients 

The unsustainable use of agrochemicals (fertilizers, herbicides, pesticides, and plant hormones) has resulted in a greater amount of pollution in the environment [6]. Along with these, agricultural areas gather an extensive amount of agrochemicals from nearby fields due to surface run-off, direct drift, and leaching [7]. 

When fertilizers are applied at higher concentrations they easily run off and leach into surface water bodies. Even organic manure when used in excess, tends to cause eutrophication and algal blooms, which can cause diseases like blue baby syndrome [5].  Chemical compounds like pesticides, herbicides, insecticides, and fungicides are extensively used in agriculture and are passed through the food chain until they become toxic to humans [6,8]. Accumulation of salts in the soil causes the salinization of freshwater bodies. The majority of water salinity problems have been reported in countries like China, India, Argentina, and Sudan [9]. 

Sewage 

An estimated 58% of wastewater from urban areas and 81% of industrial wastewater are discharged into water bodies with no or inadequate treatment in contamination of approximately 73% of water bodies [10]. Sewage contains industrial waste, Municipal waste, domestic waste like bath water, washing machine, kitchen waste, and fecal matter. Fresh water sources also serve as the best sink for the discharge of this wastewater [11,12].  Sewage entering into water contains pathogenic organisms out of which 1400 species including bacteria, protozoa, fungi, and viruses have been identified by scientists [13-15]. 

Many reports of emerging freshwater infections are linked to at least one invasive species, aquaculture intensification, nutrient, and pollutant runoff, or changing food-web structure [16]. Policy changes and improved surveillance have been advocated to decrease the likelihood of pathogen introduction and maximize opportunities for control [17].  In case of infections involving both wildlife and human hosts, or have parallels in transmission control, freshwater management to limit eutrophication, maintain higher trophic levels (e.g. predators), and prevent invasive species could help regulate infections across a range of host taxa. 

Heavy metals 

Heavy metals like Mercury, Lead, Cadmium, Copper, Zinc, and Nickel are released into the water due to industry and agriculture [18]. Once released in the aquatic environment they bind to particulate matter and settle down into sediments [19]. 

Designing efficient engineered liners will assist in mitigating groundwater contaminants by acting as a hydrochemical barrier for leachates [20]. The photocatalytic degradation mechanism under visible light irradiation can be used to remove other organic contaminants such as Persistent Organic Pollutants (POPs) and is also used widely for the removal of low-concentrated heavy metal and metalloid ions from solutions [21,22]. Phytoremediation is one of the most used techniques to eliminate heavy metal pollution in ecosystems or environments. It uses raw or genetically modified plant species to minimize the toxic effects of pollutants [23-26]. 

Eutrophication 

Freshwater algae occupy a pivotal position in the food web. Periodically, algal species are selected by environmental and ecological forces allowing their bioaccumulation. These accumulated habitats contribute to climate warming [27], hydrological intensification [28,29], and eutrophication [30].  Once established algal species can reduce Dissolved oxygen, and produce cyanotoxins.  These cyano toxins can cause physiological and behavioral impairments in secondary and tertiary consumers [31]. 

Preventative measures include i) Reducing or removing external nutrient loads [32] ii) aerating lake sediments [33]; iii) chemically treatment of lake sediments to suppress internal nutrient recycling [34]. Mitigation measures include chemical controls like algicides or flocculants, physical controls like increasing flows to reduce water residence time and remove cyanobacteria, and biological controls like introducing organisms that consume algal bloom species [35].  

Micro and macro plastics 

The larger plastic particles under the influence of environmental conditions degrade into microplastics, typically smaller than 5mm in diameter [36,37]. Microplastics can be ingested by plankton, and fish to birds and can accumulate throughout the aquatic food web [38]. They are also found in human pathogens like specific members of Vibrio [39]. 

Better management of microplastic pollution in fresh waters requires an understanding of (i) sources, sinks, and fluxes; (ii) factors controlling Spatio-temporal variations in microplastic concentrations; (iii) data on co-transported contaminants; and (iv) routes of uptake and effects on freshwater organisms [40]. Legislation to control microbeads has to be implemented. The science supporting mitigation of emerging contaminants such as microplastics lags behind that of pharmaceuticals and personal-care products. Further research is required on what impacts, if any, these materials are having on freshwater ecosystems. 

Emerging threats 

Changing climate 

Climate change is said to threaten approximately 50% of global fish species [41], it is also found to affect phenology, algal bloom, and interspecific interaction [42]. Other threats include an increase in water temperature [43], increased temperature affect species distribution [44], disease outbreak [45], Phenology, and survival [46]. 

Global government commitments to reduce greenhouse gas emissions [45], expanding freshwater protected areas [47], and mitigation. Habitat restoration for thermal habitat is critical to mitigating the effects of climate change on freshwater biodiversity. 

E-commerce and Invasions 

Invasive species is a primary threat to freshwater biodiversity and modes of species introduction may develop further in the future [48].  The recent surge in e-commerce linked to internet sales of novel invasive species [49] is an expanding link between established and emerging trade partners. Aquatic weeds are sold internationally through the internet [49,50] and more invasive species are available on major online auction websites [51]. 

Managing this threat is challenging. The array of mechanisms that can be used are:

1. Using web crawlers to monitor the internet for the sale of plants and animals [52]
2. Authorities can use Artificial Intelligence algorithms to identify the sellers [53].
3. Focusing on accountability and educating buyers with online warning labels and pop-ups [2]. 

Expanding hydropower 

Hydropower dam construction endangers freshwater biodiversity as dams modify natural flow and thermal regimes and decrease river–floodplain connectivity, aquatic productivity, and fish access to spawning and nursery habitats [54,55]. Even when hydropower projects involve fish passage structures to promote movement through dams, such structures may be ineffective [56]. Another major threat associated with Hydropower is river aging and sediment deposition.  Sedimentation fragments aquatic habitats, impairs fish health and survival, decreases fish production, lowers primary production, and reduces storage capacity. Altered waterfront access impairs the ability of reservoirs to support other human needs (e.g. food safety, flood control, water supply, and navigation) [55,57]. 

Shifting the food security of rural inhabitants from aquatic protein to land-based, livestock-derived protein has its own set of socioeconomic challenges. Potential interactions between hydropower and other factors (e.g. climate change, habitat fragmentation) are quite unclear. In the present scenario, hydropower projects are assessed mainly on site-specific and not on their environmental impact. Thus there is a need for a comprehensive EIA that will be unbiased and independent before starting the projects. 

Emerging contaminants 

Surface waters receive pollution from discharges such as mining, agriculture and aquaculture, pulp and paper production, oil and gas production, and urban runoff. Each of these can impair freshwater biodiversity indirectly by impacting habitat or through direct toxicity. In addition, with improved wastewater treatment across sectors (e.g. municipal effluents) [58], the focus in developed countries is less on addressing acute toxicity (e.g. ammonia) and more on assessing and mitigating longer-term effects from both older legacy and emerging contaminants. More recent studies reveal the effects of other emerging contaminants (e.g. anti-inflammatories, antidepressants) on algal communities [59,60].  Yet, the effects of these individual compounds and their mixtures on aquatic populations and communities, as well as ecosystem function, remain understudied. 

Mitigation of emerging contaminants includes advanced treatment of municipal wastewater and reduction of sources [5] – some emerging contaminants (carbamazepine, triclosan, and diclofenac) are more recalcitrant and require the development of novel interventions [61]. Source reductions are effective and necessary for some emerging contaminants given the lack of treatment options, and gains are being made (e.g. reducing the use of antibiotics in livestock production and microbeads in cosmetics in some jurisdictions).  

Engineered nanomaterials 

Engineered Nanomaterials (ENMs) are manufactured materials (size range 1-100 nm) used in a multitude of industrial, clinical, and consumer applications [62]. ENMs have a high surface-to-volume ratio with unique physical and chemical properties, these properties make them desirable for many applications, however, they can also make these materials bioactivity [63,64]. Nano-pharmaceuticals are an area of intense growth, and the introduction of ENM-enabled drugs or drug-delivery systems into fresh waters warrants careful consideration [65]. Agricultural applications, including fertilizers, herbicides, and pesticides [66], are also a concern. 

A major barrier to understanding the risks of emerging ENMs is the lack of sufficient detection and characterization technologies [67]. Current models require more detailed inputs to estimate ENM burdens accurately and to predict risks to freshwater ecosystems. Most available bioactivity data again derive from acute studies on pelagic species, and there is still considerable uncertainty about long-term risks from even the most common ENMs (e.g. titanium dioxide, zinc oxide, silver). 

Cumulative stressors 

First is the need to resolve whether multiple freshwater stressors simply co-occur, or whether they have interacting effects. Early experimental evidence suggested that some stressor combinations could be synergistic (e.g. high temperature × toxic stress), but in most cases, stressor combinations were less than additive [68]. Data from 88 papers and almost 300 stressor combinations revealed interactions were most commonly antagonistic (41%), rather than synergistic (28%), additive (16%), or reversing (15%) [2]. A second challenge is to develop methods for diagnosing the relative importance of stressors with combinatorial effects. A possible explanation for the dominance of antagonistic interactions is that those with a large impact might mask or override the effects of lessor stressors [69]. The third challenge is to identify pragmatic approaches to managing multiple stressors impacts. 

Riparian solutions offer a smaller-scale alternative, for example, where ‘buffer zones’ simultaneously influence water quality, protect thermal regimes, provide habitat structure and maintain energetic subsidies, although they are not equally effective for all pollutants [33]. Overall, however, there is a pressing need to understand and address multiple-stressor problems, particularly their impacts on freshwater biodiversity (Table 2). 

Table 2: List of emerging threats, sources, effects, and mitigation.

Traditional methods 

1. In ancient times, women were considered the gatekeepers of water ecologies and were responsible for building water bodies like step wells, tanks, and even ponds.
2. Periodic cleaning of water bodies for festivals like Teej and Lasipa (Rajasthan).
3. Tribal practices like irrigation of paddy fields by a network of irrigation canals.
4. Traditional rainwater harvestings techniques like Rajwani and Patali pani.
5. Community ownership of the freshwater ecosystem makes people more accountable for conserving the ecosystem. 

Management tools 

1. Usage of blockchain-based incentivized computing framework for saving water. This framework facilitates decision-makers in creating awareness among people about water saving efficiently [20].
2. Cloud-enabled Internet of Things (IoT)integration and wireless sensor network is proposed for Precision Soil and Water Conservation Agriculture (PSWCA) through machine learning [71].
3. Using remote sensing and development of Probalistic Support Vector Machines (SVM’s) model assisted with the GIS technique to study habitat fragmentation and eutrophication [72].

There are existing and emerging threats to freshwater biodiversity, these threats will increase as the years proceed. To cope with the increasing pressures on water quality and quantity, decision-makers should primarily focus on engineered solutions and their proper implementation. On a local scale, the mechanisms like societal actions (Participation in restoration, dam removal), financial actions (investing in ecosystem services), and fiscal incentives (agri- environmental schemes) need to be taken. A global effort is required to overcome this necessary challenge.

Ms. Thrupthi G.N.

Thrupthi G.N. is a research scholar at the Department of stu the es in Botany, University of Mysore, Mysuru. She is working on a Ph.D. in the field of Limnology. Currently, she is also working, as a faculty for the M.Sc Botany course in Jnana Kaveri P.G.Centre, Kaveri. Her area of research includes Physico-chemical analysis of water bodies, and the study of macrophytes, plankton, and fishes. 

Dr.A.G.Devi Prasad

Dr. A.G. Deviprasad is a Professor of the Department of studies in Environmental Studies in the University of Mysore, Mysuru. His field of expertise includes plant ecology, biodiversity assessment, natural resources conservation, biodiversity monitoring, plant conservation  and conservation biology. He has teaching and research experience of more than 30 years and has guided students with their Ph.D.’s.

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