Global Warming threats on Freshwater Biodiversity

Twelve human-caused emerging threats that affect freshwater biodiversity and the health of our planet.

Emily Moore, Isabella Vegso, Chris Kapeller, and Hannah Muncal

With surface temperatures continuing to rise, many ecosystems and habitats are being threatened. Since water is essential for all life forms, there’s a growing concern for how our freshwater sources are going to be affected by global warming. As a background, only three percent of Earth’s water is freshwater, and only 0.5% of freshwater is available through lakes, streams and rivers. The remaining 2.5% of freshwater is stored in glaciers, ice caps, the atmosphere and the soil. With improper management, our freshwater resources are beginning to decline, posing a threat on human life and diverse aquatic ecosystems.

1. Changing Climate

According to Reid et al., the climate crisis is predicted to threaten approximately 50% of freshwater fish species. Changing climates, extreme weather events, and warmer temperatures are predicted to impact species distribution, life cycle, survival, and disease-- and these effects are already being seen. The World-Wide Fund for Nature (WWF) has reported that the decline in freshwater species fell more steeply than either terrestrial or marine populations from 1970 to 2012 (Reid et al, 2019). The changing climate puts a particular stress on freshwater fish due to their phenology, which describes how the timing of lifecycle stages is triggered by changes in season, temperature, and even elevation. Freshwater fish species tend to use these environmental cues for the seasonal timing of reproduction, making them especially vulnerable to extreme temperature shifts (Krabbenhoft et al. 2014). Ironically, another unique threat to freshwater species results from infrastructure constructed for climate change mitigation, such as the construction of dams to protect against extreme floods, or increased water storage to supplement irrigation capacity during droughts. These mitigation measures drastically alter the habitat of freshwater species, putting them at greater risk of extinction. Global governmental commitments to reduce greenhouse gas emissions, expand freshwater protected areas, and restore existing habitats is essential for the survival of these unique and essential species.

2. E-commerce & invasion

In recent years, the increase in the Global Electronic Commerce (e-commerce) market has resulted in the expansion of the Global Trade of live organisms. This trade has become the primary pathway for the introduction of invasive species, and the most popular fish species sold on the market are currently the most likely to become established in freshwater ecosystems. It has become easier than ever for novice collectors and breeders to access unregulated retail websites, auction sites, and chat rooms for the sale and purchase of exotic species. These unregulated sites are a challenge to the current management and policy regarding live animal trade pathways, and it is becoming increasingly difficult to monitor. It is also predicted that with rising temperatures and drought risk, plant and animal species adapted to warm and dry environments will increase in demand. With many pet owners releasing their new, unwanted pets into natural water bodies, native species are at risk of being out-competed by introduced species. Education to enhance both buyer and seller awareness is essential, and tools to increase the ability to monitor these transactions must be put in place by governments and authorities.

3. Infectious Diseases

Freshwater ecosystems are particularly suited for complex inter-species interactions. Both terrestrial and aquatic species rely on the water for survival, and so concentrated hotspots are created in aquatic ecosystems where many species interact. The complex food webs and inter species interactions concentrated around freshwater ecosystems creates the perfect environment for parasites to flourish. Amplifying this, species that have both terrestrial and aquatic life stages link parasites to all types of species on-land and in water. The likelihood of contact with various potential hosts results in a multitude of parasites relying on freshwater organisms for transmission. Oftentimes, introduced parasites and diseases can devastate an ecosystem, resulting in population declines and even extinction. One particular fungal pathogen that has spread throughout ecosystems globally has already caused the extinction of 200 frog and toad species. Research has only recently emerged in regard to the effects on parasites and diseases in freshwater ecosystems, however some researchers are finding that warmer temperatures exasperate the effects of certain diseases among fish populations (Okamura et al. 2011). Increased surveillance and monitoring to prevent the spread of disease and invasive species could help reduce the prevalence of infectious diseases across ecosystems.

4. Harmful Algal Blooms

Although algae are an integral aspect of the freshwater food web, they have the potential to become a detriment if too much algal biomass is accumulated. An excess accumulation of algal biomass is termed a harmful algal bloom (HAB). Many factors compound to create an environment where HAB’s have the potential to form. Most prominent are climate warming, eutrophication, and hydrological intensification (the process of dry areas becoming drier while wet areas become wetter). Another important factor is the altering of chemical nutrient balances, specifically iron and phosphorus, which can lead to changes in the dominant algal communities in an area. Once established, an HAB can have many negative impacts on a freshwater ecosystem such as reduced oxygen availability (as the algae respirates and decomposes) as well as toxin production. The production of toxins can have a large effect on many freshwater organisms, sometimes resulting in their death. Toxins, such as cyanotoxins, can even have an effect on human populations if fish or the water itself are ingested.

5. Expanding Hydropower

At present, 48% of global river volume is altered by some type of anthropogenic flow regulation. The construction of this infrastructure is detrimental to aquatic ecosystems because they disrupt natural flow patterns and fragment habitat, especially in the context of fish migration. On top of large hydrological plants, there has been a recent surge in the development of small hydropower plants. The growth of this sector has outpaced current ecological knowledge and therefore the effects of the intensification of this type of infrastructure are uncertain. Sediment imbalances associated with the construction of dams also represent a major risk for reservoir aging, resulting in shoreline erosion and channel degradation which can have a negative impact on aquatic biota in the area. Since the building of dams alters the accessibility of fish nurseries and spawning habitats, the way of life of river-dependent peoples is at risk especially in large river systems such as the Amazon Basin (Winemiller et al., 2016).

6. Emerging Contaminants

The most basic and integral aspects of human society can lead to pollution of surface waters such as mining, oil and gas production, agriculture, and the use of pharmaceuticals and personal care products. Due to research and international treaties there has been a significant push to reduce the use of persistent and bioaccumulative toxins, such as DDT, and recent water remediation efforts are focused on reducing the effects of older pollutants and emerging contaminants, rather than on issues of acute toxicity. Emerging contaminants include endocrine disruptors, nanomaterials, newer pesticides, illicit drugs, microplastics, and active pharmaceutical ingredients. All of these contaminants are a major concern as they have the potential to become biologically active in aquatic environments and many also exhibit stability in these environments, allowing them to persist for long periods of time. Endocrine-disrupting chemicals can have a large impact on fish populations by creating intersex males which can contribute to both reduced fitness and lower genetic diversity. As well, antimicrobial chemicals found in pharmaceuticals and personal care products can (predictably) have a negative effect on essential microbial communities in aquatic ecosystems.

7. Engineered nanomaterials

The exciting field of nanomaterials and nanoparticles has opened up a whole host of possibilities in construction, biomedical, and electronic industries. They are hailed for their efficacy in creating stronger, lighter building materials and even self-cleaning surfaces, so it’s no wonder this category of material is garnering attention (Lee et al. 2010). However, when it comes to new materials, having them on the nano scale means it’s much harder to trace their anthropogenic impact on our environment. Current estimates of aquatic health show acute (short-term) toxic effects of some nanoparticles on fish and crustaceans, but long-term effects are largely unknown (Reid et al. 2019). For example, it’s estimated that animals living directly in aquatic substrate (rocks, sand, and plant matter) will be greatly affected by particles that tend to sediment to the bottom of water bodies (Reid et al. 2019). A huge issue in understanding how these materials act in aquatic systems is the huge variance between newly produced products, as well as the inability to detect and trace them one they enter water.

8. Microplastic pollution

A problem characterized by the anthropogenic age is that of microplastics, which can be found in practically every part of the ocean and increasingly so in freshwater systems too. Plastic production has increased from 1.7 to 280 million tonnes in the last 60 years (Lechner et al., 2014) and so the management of keeping plastics out of our water will only become increasingly more challenging. A large part of freshwater pollution is microfibres, contributing to >75% of plastic debris as they are washed from residential washing machines into municipal water treatment facilities (Reid et al. 2019). Microplastics are the culprit for increased mortality among freshwater organisms such as birds and fish through ingestion. In recent years, it has even been found that microplastic concentrations have outnumbered planktonic larval fish concentrations in the Danube River, among the excessive abundance of larger plastics already found there (Lechner et al., 2014).

9. Light and noise

Light and noise are examples of pollutants that are not obvious, but can have harsh negative effects on aquatic life, nonetheless. Artificial light has been linked to the disruption of circadian rhythms in certain species, altering the migration of zooplankton, and even affect how microbial communities’ function at night (Reid et al., 2019). Since there are communities and humans surrounding freshwater systems all around the world, the consideration of what their artificial light does to aquatic life is important to be noted. Excessive noise from boats, aircraft, and even trucks has been linked to an increase in stress hormone responses in certain freshwater fish as well as a reduction in food foraging behaviours, and excess metabolic expenditure, which are all linked to early mortality (Reid et al., 2019). It has also been noted that noise from invasive toads has affected the calling behaviour of native Australian frogs (Reid et al., 2019). If other animals can have this effect, what is the whole of the human population doing to our aquatic life? There is still much to be learned on the elusive impacts of light and noise and how we can mitigate disturbances.

10. Freshwater salinisation

With global warming, salt content in lakes, rivers, and groundwater is expected to increase (Herbert et al., 2015). Additionally, more forests are being cleared out for land use and resources, resulting in higher precipitation on groundwater. This increased exposure poses an imbalance in groundwater aquifers leading to more seawater discharged into coastal lowlands. Many coastal zones are threatened by rising sea levels, which overwhelms these lowland systems resulting in freshwater contamination (Henman & Poulter, 2008). With a lack of regulation, salt contamination may increase by improper disposal of saline wastewater from oil and gas operations (Vengosh et al., 2014), and increased use of salts to de-ice our roads (Kaushal et al., 2018).

The high salt content was found to affect biodiversity within streams and lakes. Freshwater plants become less productive, leading to a drop in invertebrate populations. With habitat and food-loss, many fishes and animals are threatened (Finlayson et al., 2013). There was also an observed shift from freshwater to salt-tolerant organisms, disturbing the overall ecosystem (Radke et al., 2013).

11. Declining Calcium

Suitable calcium concentrations in water bodies are dependent on the slow weathering of bedrock. Humans have been disturbing the calcium nutrient cycle of many soft-water lakes, mainly through acid rain and forestry practices (Jeziorski & Smol, 2017). Acid rain drives the addition of calcium into lakes through bedrock weathering; This is not a problem for places with a high-calcium bedrock however, it is a big problem for many low-calcium bedrock regions where calcium supplies may eventually run out. Since calcium is a necessary nutrient for trees, timber is found to hold large amounts of this nutrient. Forestry practices (such as extracting and planting trees) may act as a giant nutrient extractor, leading to the depletion of calcium in watersheds (Watmough, Anherne & Dillon, 2003).

Many lake invertebrate species such as Daphnia spp. have a high calcium demand. With a decline in calcium concentrations in water bodies, many invertebrates are threatened, affecting lake food webs (Ashforth & Yan, 2008). Since Daphnia’s are essential filter feeders, a decrease in their population gave rise to harmful algal blooms in many water bodies (Korosi et al., 2012).

12. Cumulative stressors

The interaction of many environmental stressors is linked to affect global freshwater ecosystems. The combination of different environmental stressors is potentially severe and can lead to species extinctions and maybe worse. Because of its complexity, there has been an increase in interest in studying cumulative stressors, and here is why:

1. With the rapid increase in the human population, there has been a higher demand for freshwater resources. This issue is also paired with downstream impacts from human activities (Strayer & Dudgeon, 2010).

2. Many human activities collectively affect freshwater systems. This can be through urbanization with industry, agriculture, and invasive species release.

3. Global warming is expected to have many effects on freshwater systems.

These stressors can work together to enhance specific environmental impacts and potentially create new and complex problems that no one has ever dealt with before.

What needs to be done?

Most current threats to freshwater are occurring on a global scale, with the exception of freshwater salinization and declining calcium, which occur in coastal lowlands and soft water lakes, respectively. Mostly dependent on the novelty of the issue, varying levels of understanding exist for each threat. Climate change, algal blooms, expanding hydropower, freshwater salinization, calcium decline, along with light and noise pollution have existed in literature for many years and are therefore well studied. However, it is important to note that while a threat may be well-defined, compounding effects with other emerging threats, especially climate change, may not be. As E-commerce, invasive species invasions, emerging contaminants, infectious diseases, engineered nanomaterials, microplastic pollution, and cumulative stressors are all fairly new stresses on the natural world, they are not well understood, and more research is required to effectively address these threats. For many of the identified threats, the mitigation measures required are similar. For example, many threats would benefit from increased research and surveillance so that we can better understand the current state of ecosystems under these threats, as well as to be able to better predict future effects. With increased research, less harmful alternatives for specific threats can be utilized such as different types of light that could have a reduced effect on the impacted ecosystem or polymers of plastic that will biodegrade rather than accumulate. On smaller scales, tighter municipal regulations on wastewater treatment and discharge locations could help to reduce the stress of contaminants on freshwater systems. Ultimately however, global commitments and government regulations are necessary, as in order to neutralize these threats on freshwater systems, change must happen on larger scales.

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