Animal waste flows into water bodies, causing algal blooms and stagnant water areas! Environmental disasters occur in many places around the world

When nitrogen and phosphorus in animal waste enter water bodies, they cause eutrophication, algal blooms, and anoxic stagnant water areas, seriously damaging aquatic ecosystems and impacting fisheries and human health. Countries are promoting wastewater treatment and agricultural management in response.

Environmental problems such as eutrophication of water bodies, algal blooms and stagnant water areas have received increasing attention. The occurrence of these problems is closely related to the excess nutrients in the water body, especially the content of nitrogen and phosphorus. As one of the important sources of these nutrients, animal excrement has a significant negative impact on aquatic ecosystems after entering water bodies. This report aims to deeply explore how nitrogen, phosphorus and other nutrients in animal waste lead to eutrophication of water bodies, further causing problems such as algal blooms and stagnant water areas, and analyze their ecological and environmental impacts, as well as existing control and prevention methods.

2.1 Definition of eutrophication

Eutrophication refers to the phenomenon of excessive increase in productivity due to the enrichment of excess nutrients (mainly phosphorus and nitrogen) in water bodies.1. The United States Geological Survey (USGS) defines it as a process that occurs when a body of water receives an excessive nutrient load, particularly phosphorus and nitrogen, which often results in excessive growth of algae, which in turn consumes oxygen in the water and kills aquatic animals such as fish.1. The European Environment Agency (EEA) points out that eutrophication is a pollution process that occurs when lakes or rivers are rich in plant nutrients, causing algae and other aquatic plants to overgrow, consume oxygen as they die and decompose, and make the water body less vital.5. It is worth noting that eutrophication is not a simple aging process of lakes. Although lakes will gradually become eutrophic over time under natural conditions, human activities have significantly accelerated this process, which is called cultural eutrophication.6. Eutrophication often results in ecological problems such as harmful algal blooms, stagnant water areas, and fish kills.8. Eutrophication not only affects freshwater systems, but also poses a threat to marine ecosystems4。

2.2 The formation mechanism of eutrophication

The formation of eutrophication is a complex process driven primarily by excess nutrient input. The sources of these nutrients include natural factors, such as rock weathering, atmospheric deposition, and natural erosion.3, but the main driving force is excess nutrients produced by human activities, including agricultural runoff, sewage discharge, industrial activities and urbanization processes, etc.3. When the content of nutrients such as nitrogen and phosphorus in the water body is too high, it will act like fertilizer to promote the rapid growth of algae and aquatic plants, forming a so-called algal bloom.3. These algal blooms form a dense covering layer on the water surface, preventing sunlight from penetrating into deeper water layers, thereby affecting the photosynthesis of underwater plants.3. When algae and plants die and begin to decompose, bacteria break down these organic materials, consuming dissolved oxygen in the water in the process3. Excessive consumption of oxygen leads to a decrease in oxygen content in the water body, forming an anoxic (low oxygen) or anaerobic state, causing harm to fish and other organisms that rely on oxygen for survival, and even leading to death.3. In addition, chemical fertilizers and animal waste used in agricultural production are major causes of cultural eutrophication.5. In freshwater ecosystems, phosphorus is often the key nutrient limiting plant growth, while in marine ecosystems, nitrogen is more important7。

2.3 Impact of eutrophication on aquatic ecosystems

Eutrophication has widespread and far-reaching negative impacts on aquatic ecosystems. The most direct consequences are the formation of harmful algal blooms, stagnant water areas and mass fish deaths.1. Overgrown algae can block sunlight, causing underwater vegetation to die from lack of light.2. The species composition and biomass of aquatic communities will also change, often resulting in a decrease in species diversity and an increase in the number of some organisms that are resistant to low oxygen (such as jellyfish)2. In marine ecosystems, eutrophication can also harm coral reefs because high nutrient levels favor algae growth and inhibit the attachment and growth of coral larvae.2. In addition, a decrease in seawater pH, known as ocean acidification, due to the large amounts of carbon dioxide produced by the decomposition of excess algae and plant matter, slows fish and shellfish growth and prevents shellfish from forming shells8. Eutrophication also causes economic losses to fisheries, tourism and recreation industries2. What’s more serious is that some algae may produce toxins, posing a threat to human health and aquatic life2. Eutrophication also disrupts aquatic food webs and ecosystem services2。

3.1 Definition of algal bloom

An algal bloom is a rapid increase or accumulation of algae populations in fresh or marine water bodies14. Algal blooms can often be identified by the color change in the water due to algal pigments15. There are many types of algae, including large multi-celled organisms such as seaweeds, and microscopic single-celled organisms such as cyanobacteria (also known as blue-green algae). Algal blooms generally refer to the rapid growth of microscopic single-celled algae rather than the explosive growth of large algae15. The frequency, duration and intensity of algal blooms increase due to nutrient pollution14. It’s worth noting that the definition of an algal bloom varies across different scientific fields, ranging from harmless microalgal blooms to large, harmful algal bloom events.15。

3.2 Common algae species (especially those that produce harmful substances – harmful algal blooms, HABs)

Algal blooms can be caused by a variety of algae, but the most common and a threat to human and animal health are harmful algal blooms (HABs). The following are several common HABs algae:

• Cyanobacteria: Also known as blue-green algae, it is the most common cause of HABs in freshwater. They are photosynthetic bacteria that can produce a variety of toxins, such as microcystin, cylindrocystin, anabaena toxin, and saxitoxin.14. Cyanobacteria are more likely to bloom in still waters that are rich in nutrients, have high water temperatures, have plenty of light, and have slow currents.14。
• Dinoflagellates: It is one of the most common causes of HABs in the ocean and the main algae that causes red tides. Certain dinoflagellates can produce a variety of toxins, such as paralytic shellfish toxin, neurotoxic shellfish toxin, diarrheal shellfish toxin, and sigatoxin, which pose a threat to marine life and human health.14。
• Diatoms: It is also a common phytoplankton in the ocean. Some species such as Pseudo-nitzschia can produce domoic acid, which is harmful to marine life and human health.18。

The following table summarizes some common harmful algal bloom species, the toxins they produce, and their effects on humans and wildlife22。

Algae name	Toxins/bioactive compounds	Human Health Effects/Symptoms	Impact on wildlife and/or domestic animals	Destructive effects on ecosystems	economic impact	Affected areas in the United States
Akashiwo blood	surfactant	Suspected respiratory irritant	Death of migratory birds, including protected species	Discoloration of water; foam formation	Rehabilitation of protected bird species	pacific coast
Alexandria; Gymnodinium; Pyrodinium bahamense	Saxitoxin	Respiratory paralysis, death (paralytic shellfish poisoning, PSP)	marine mammal death		Loss of Shellfish Fishing Revenue; Human Illness from Recreational Fishing; Florida Recreational Puffer Fishing Closed	Pacific Coast, including Alaska; Northeast Atlantic Coast; Florida
Anabaena, Aphanizomenon, and Nostoc sp.	Anabaena toxin, saxitoxin	neurotoxin, respiratory paralysis	Livestock, dog, waterfowl and fish disease or death	Effects on fish and waterfowl		Freshwater systems are widespread; Great Lakes region
Aureococcus anophagefferens — Masashio Nagashima	Uncharacterized: Extracellular polysaccharides (EPS); there may be other uncharacterized compounds			Water discoloration; mass die-offs of seagrass and shellfish; hypoxic zones	Loss of shellfish harvest revenue; disruption to recovery	mid atlantic coast
Cylindrospermopsis	Cylindrocystin, Saxitoxin	Hepatotoxicants, kidney damage, neurotoxins	Dog and fish deaths, bird diseases	Effects on fish and waterfowl	Livestock and pet losses	fresh water system
Karenia	Brachybatulin	Neurotoxicity; gastrointestinal and sensory effects (neurotoxic shellfish poisoning, NSP), respiratory effects	Fish kills; manatees, dolphins, sea turtles and birds killed	discoloration of water	Lost tourism revenue; beach cleanups of dead fish, shellfish harvest closures, increased emergency room visits for respiratory and gastrointestinal illnesses	Gulf of Mexico and Atlantic coast extending to Delaware
Lyngbya	Cyanotoxin, Aplysia toxin, debrominated Aplysia toxin	Dermatitis, gastrointestinal inflammation toxins	Domestic animal deaths (horses) that may cause fibropapillomatosis in sea turtles		May impact tourism (beach visitors)	Benthic mats for marine and freshwater systems
Microcystis	Microcystin	Hepatotoxin, may damage kidneys and reproductive system, has carcinogenic potential	Livestock, dog and fish disease and mortality	Effects on fish and waterfowl		fresh water system
Pseudo-nitzschia	Domoic acid	Gastrointestinal and central nervous system effects (amnestic shellfish poisoning, ASP)	Massive die-offs of fish and shellfish, and deaths of seabirds and marine mammals

3.3 Causes of algal bloom formation

The formation of algal blooms requires the combined action of a variety of environmental conditions, the most critical of which is excess nutrients, especially nitrogen and phosphorus.14. These nutrients typically come from agricultural fertilizers, wastewater discharges, and urban stormwater runoff.14. In addition to nutrients, warm water temperatures are also important for algal blooms to occur, as most algae grow faster at higher water temperatures14. Sufficient sunlight provides energy for algae photosynthesis and is also a necessary condition for the formation of algal blooms14. In addition, water bodies with slow or still water flow are also more conducive to the formation of algal blooms, because algae can gather on the surface and obtain more sunlight and nutrients.14. Stable water conditions and less water mixing also favor the development of algal blooms23. Climate change, such as rising temperatures and changing precipitation patterns, may also exacerbate the occurrence of harmful algal blooms16. In marine environments, algal blooms can also be triggered by the upwelling of nutrient-rich deep water15. In short, the formation of algal blooms is the result of a combination of factors, among which excess nutrients are the core driving factor, while environmental factors such as water temperature, light, and water conditions play an important regulatory role.

4.1 Definition of backwater area

Dead water zones, more commonly known as anoxic zones, are areas in a body of water where dissolved oxygen is so low that most aquatic life cannot survive25. Hypoxia is defined as when the dissolved oxygen concentration drops below 2 ml per liter31. These areas are often called “backwaters” because most marine life will die or, if they are mobile creatures such as fish, leave the area28. Hypoxic zones may occur naturally, but scientists are more concerned about areas caused or exacerbated by human activities28. Hypoxia is a common consequence of eutrophication25。

4.2 Formation process of dead water area

The formation of dead water areas is mainly caused by excessive nutrients (mainly nitrogen and phosphorus) entering the water body, usually from agricultural runoff and wastewater discharge.25. These excess nutrients will stimulate the growth of algae and form an algal bloom.25. When the algae die and sink to the bottom, they are broken down by bacteria.25. Bacteria consume dissolved oxygen in the water as they break down organic matter25. In addition, the stratification of water bodies (due to differences in temperature or salinity) will limit the mixing of oxygen-rich water in the surface layer and oxygen-deficient water in the bottom layer, further exacerbating the hypoxic conditions in the bottom water.29. In some freshwater lakes, invasive species such as zebra mussels and Quagga mussels may also contribute to the formation of backwaters by altering nutrient cycling33。

4.3 Impact of backwater areas on marine and freshwater ecosystems

Backwaters have serious negative impacts on marine and freshwater ecosystems. The most direct impact is the death or forced migration of most marine life (such as fish, shellfish and crabs)25. This results in biodiversity loss and habitat degradation25, and disrupt food webs and ecosystem services31. Backwaters also cause economic damage to fisheries and tourism25. Surviving organisms may face increased stress and risk of disease36. Some low-oxygen-tolerant species, such as jellyfish and certain squid, may increase in abundance in stagnant waters31. Even more worrying, low-oxygen sediments may release more greenhouse gases such as nitrous oxide and methane34. The effects of backwaters are not limited to marine ecosystems; lakes and rivers in freshwater ecosystems can also be similarly affected25。

5.1 Nitrogen and phosphorus content in animal excreta

Animal waste is rich in many nutrients needed for plant growth, including nitrogen (N) and phosphorus (P)42. Approximately 70-80% of feed nitrogen and 60-85% of feed phosphorus are excreted in animal waste42. However, the exact levels of nitrogen and phosphorus in animal waste vary depending on a variety of factors, including animal species, feed formulation, amount of bedding, moisture content, and handling methods.43. For example, poultry manure is often higher in nitrogen and phosphorus50, and cow dung also contains significant amounts of these two nutrients56. Therefore, in order to accurately assess the fertility value and potential contamination risk of animal waste, a specific analysis of its nutritional content is crucial.

The table below summarizes the average levels of nitrogen, phosphorus and potassium in some common animal wastes43。

Animal type	Nitrogen (N) (lbs/ton or %)	Phosphate (P₂O₅) (lbs/ton or %)	Potassium Oxide (K₂O) (lbs/ton or %)
Beef cattle feedlot	14.2 lbs/ton (0.7%)	12.8 lbs/ton (0.6%)	17.8 lbs/ton (0.9%)
dairy cow	11.2 lbs/ton (0.6%)	4.6 lbs/ton (0.2%)	12.0 lbs/ton (0.6%)
pig	10.0 lbs/ton (0.5%)	6.4 lbs/ton (0.3%)	9.2 lbs/ton (0.5%)
horse	13.8 lbs/ton (0.7%)	4.6 lbs/ton (0.2%)	14.4 lbs/ton (0.7%)
sheep	28.0 lbs/ton (1.4%)	9.6 lbs/ton (0.5%)	24.0 lbs/ton (1.2%)
Poultry (no litter)	31.2 lbs/ton (1.6%)	18.4 lbs/ton (0.9%)	8.4 lbs/ton (0.4%)
duck	1.1%	1.5%	0.5%
Goose	1.1%	0.6%	0.5%
turkey	1.3%	0.7%	0.5%
rabbit	2.0%	1.3%	1.2%

5.2 Forms and availability of nitrogen and phosphorus in animal excreta

Nitrogen in animal waste exists in two forms: organic (slow-release) and inorganic (mainly ammonium, easily used by plants but easily volatile)43. If inorganic nitrogen (ammonium) is not mixed into the soil in time, it can easily be lost to the atmosphere through volatilization.43. Phosphorus also exists in animal waste in both inorganic (orthophosphate, easily utilized by plants) and organic forms43. Organic phosphorus needs to be mineralized before it can be absorbed and utilized by plants.43. The availability of nitrogen in animal waste is a complex issue that depends on factors such as animal species, storage methods, application methods and weather conditions.43. Typically, not all nitrogen is available to plants for efficient use in the first year after application43. In contrast, phosphorus availability in animal waste is generally higher (80-100%) and comparable to commercial fertilizers43. It is worth noting that the ratio of nitrogen to phosphorus in animal waste often does not match crop needs, which may lead to excessive application of phosphorus and increase the risk of water pollution.43。

6.1 Sources of nutrients from animal excrement entering water bodies

Nutrients in animal waste enter water bodies through various pathways. The main source is agricultural runoff. When rain or irrigation water flows through farmland, nutrients from fertilizers and animal manure on the surface will be carried into nearby rivers, lakes and oceans.36. Animal husbandry wastewater discharge is also an important source. Wastewater produced by intensive livestock farms (CAFOs) and slaughterhouses contains high concentrations of nutrients. If it is directly discharged into water bodies without proper treatment, it will cause serious pollution.66. Aquaculture also produces large amounts of waste (uneaten feed and fish waste), which is released directly into water bodies, causing nutrient pollution71. In addition, ammonia in animal manure will volatilize into the atmosphere and then enter water bodies through atmospheric deposition.62. Livestock entering rivers, lakes and other water bodies directly will also directly bring nutrients in their feces into the water.36. Nutrients from animal manure applied to the land may also find their way into groundwater through seepage63. Stormwater runoff from urban areas may also carry nutrients from animal waste, such as pet feces, into water bodies80。

6.2 Transport mechanism of nutrients in animal excreta

Nutrients in animal excrement are mainly transported to water bodies through two mechanisms: surface runoff and underground seepage. Surface runoff refers to the flow of surface water that carries dissolved nutrients and nutrients (especially phosphorus) attached to soil particles to nearby rivers, lakes and oceans during rainfall or irrigation.60. Underground seepage means that dissolved nutrients (especially highly mobile nitrates) seep into the soil with water, eventually entering groundwater, and may enter surface water bodies through groundwater flow.63. Soil erosion is also an important means of transporting nutrients, especially phosphorus, which is tightly bound to soil particles. Erosion can bring phosphorus-rich soil particles into water bodies.60. Directly discharged wastewater directly transports nutrients into the water body.68. In addition, ammonia deposited from the atmosphere will also dissolve in water and become a source of nitrogen in the water body.76。

7.1 Gulf of Mexico backwaters (USA)

The Gulf of Mexico’s backwater is one of the largest oxygen-deficient zones in the United States, its formation largely attributed to nutrient pollution from the Mississippi River Basin, much of it from agricultural runoff from Midwestern farms8. Excessive nitrogen and phosphorus cause algae to multiply, consume oxygen in the water after death and decomposition, and form huge anoxic areas.83. This has had a severe impact on fisheries and marine life in the bay83. Backwaters can be as large as New Jersey83。

7.2 Chesapeake Bay (USA)

The Chesapeake Bay also has a significant backwater problem caused by excess nitrogen and phosphorus pollution from agriculture (including animal waste and chemical fertilizers), urban runoff, and wastewater discharges36. Excess nutrients lead to massive algae growth, which in turn consumes oxygen in the water and causes harm to aquatic life.36. This also affects the fishing and recreational industries in the area36. However, long-term trends show that the Chesapeake Bay’s backwaters are shrinking due to management measures36。

7.3 Baltic Sea (Europe)

The Baltic Sea is one of the largest backwaters in the world, and its problem is exacerbated by nutrient pollution (mainly nitrates and phosphates) from agriculture and other human activities88. Additionally, overfishing of cod is exacerbating the problem because cod is a predator of herring, which feed on zooplankton, which in turn feeds on algae. Decreased cod numbers lead to increased herring numbers, which in turn lead to increased algae numbers, exacerbating oxygen depletion.88. Algal blooms also further contribute to oxygen depletion88。

7.4 River Wye (UK)

In 2020, the River Wye in the UK experienced a massive algal bloom, which research linked to large amounts of phosphorus being emitted by nearby expanding poultry farms90. Excessive nutrients in animal waste lead to eutrophication of water bodies90, posing a threat to local biodiversity90. This case demonstrates that intensive livestock farming, particularly poultry farming, can cause significant nutrient pollution to freshwater ecosystems.

7.5 Long Island Sound (USA)

Eutrophication caused by excessive nutrient input in Long Island Sound has caused huge economic losses to the commercial shellfish fishery8. It is projected that without intervention, the bay could lose all of its seagrass beds by 2030, and two-thirds of the bay could lack enough oxygen for fish to survive8. However, research shows that oyster farming can effectively reduce nutrients in the bay8。

7.6 Upper Ahala Watershed, Wisconsin (USA)

A study quantifies economic losses from phosphorus runoff from livestock waste into Lake Mendota in Wisconsin’s Upper Ahara watershed91. Excess phosphorus causes harmful algal blooms, which can impact property values, recreational activities, and cleanup costs91. Studies estimate that each additional kilogram of phosphorus lost results in an economic loss of $74.5091。

8.1 Waste management system

The key to controlling and preventing nutrient pollution of water bodies from animal waste is to implement an effective waste management system. This includes proper collection, storage and treatment of animal manure and wastewater69. Anaerobic digestion is a sustainable method that converts animal waste into biogas (a renewable energy source) and nutrient-rich compost94. Composting is another effective way to recycle nutrients and reduce the amount of waste81. For liquid waste, storage and treatment pond systems are available69. More advanced technologies, such as separation and granulation, can be used to recover nutrients from animal waste91. Microalgae-based technologies are also being investigated for nutrient recovery from livestock wastewater96。

8.2 Buffer zone

Placing vegetated buffers along waterways is an important best management practice (BMP) that intercepts pollutants in runoff81. Buffer zones can effectively reduce the entry of sediment, nutrients, pesticides and pathogens into water bodies97. They also help stabilize river banks and provide wildlife habitat97. Riparian buffers should limit the use of chemical and organic fertilizers and pesticides98. The U.S. Department of Agriculture (USDA) has also developed buffer zone guidelines for organic farms to prevent contamination from banned substances101。

8.3 Best Management Practices (BMPs)

In addition to waste management systems and buffer strips, there are many other best management practices (BMPs) that can reduce nutrient pollution from animal waste. These include directing clean runoff away from animal enclosures102, Reduce wastewater discharge into surface water bodies102, minimize the transport of manure to surface water bodies (through appropriate stockpiling, storage and frequent collection)102, protect groundwater (through appropriate facility siting and buffer zones around wells)102, take steps to prevent the movement of fecal pathogens (e.g., extended storage, composting, and appropriate land applications)103, implement a nutrient management plan (applying nutrients at the right time, at the right rate, and in the right place)104, Use conservation tillage to reduce soil erosion104, manage livestock access to streams104. In aquaculture, nutrient emissions can be reduced by improving feed formulation and feeding efficiency71, and uses recirculating aquaculture systems (RAS) and biofiltration technology106。

8.4 Wastewater treatment technology

Wastewater from livestock and aquaculture can be treated using a variety of techniques to remove nutrients and other contaminants. These technologies include physical, chemical and biological treatment methods107, such as sedimentation, filtration and bioreactors. Membrane technologies (such as membrane bioreactors and enhanced biological phosphorus removal) can also be used to remove particulate matter and nutrients from wastewater107. Aerobic and anaerobic activated sludge systems can be used to treat organic matter and nitrogen in wastewater95. Ozone treatment can be used to reduce chemical oxygen demand (COD) in wastewater108. In addition, there are technologies specifically designed to recover water and concentrate nutrients from manure109。

This report explores in detail how nutrients such as nitrogen and phosphorus from animal waste enter water bodies, causing problems such as eutrophication, algal blooms and stagnant water areas. These problems have widespread and severe impacts on aquatic ecosystems, including biodiversity loss, habitat degradation, disruption of food webs, and economic impacts on fisheries and tourism. As an important source of nutrients, animal excrement enters water bodies through agricultural runoff, wastewater discharge and other channels. Multiple case studies around the world, such as backwaters in the Gulf of Mexico, Chesapeake Bay and Baltic Sea, as well as algal bloom events in the Wye River, clearly demonstrate the seriousness of nutrient pollution from animal waste.

Addressing this challenge requires an integrated management strategy that combines waste management systems, buffer zones, best management practices and advanced wastewater treatment technologies. Nutrient emissions can be effectively reduced through improved waste collection, storage and treatment methods. Buffers act as a natural barrier that traps pollutants in runoff. Best management practices cover all aspects of agricultural production and aquaculture and are designed to minimize nutrient losses. At the same time, the development and application of more efficient treatment technologies for wastewater can further reduce the levels of pollutants discharged into water bodies.

Protecting aquatic ecosystems from the threat of nutrient contamination from animal waste requires continued research, policy development and the joint efforts of various stakeholders. Only through a comprehensive approach can we effectively manage and reduce nutrient pollution and protect our precious aquatic environment.

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