Harmful algal blooms (HABs) appear more often, abundant, extensive, and toxic than ever. Each year, they affect diverse new water areas around the world. Many of these blooms are triggered by increasing nutrient pollution. The dramatic surge in nutrient flow from industrial, urban, and agricultural activities accelerate the eutrophication of lakes, streams, and coastal waters.
Harmful algal species are quickly adapting to the growing nutrient loads. The timing, amount, and proportions of nutrients are the critical factors for HABs expansion. The disbalance in nutrients can accelerate the toxicity of dinoflagellate and cyanobacterial HABs. Changes in climate, environmental, and abiotic conditions due to human activities also favours algal growth.
The main nutrients affecting the health of aquatic ecosystems are nitrogen (N) and phosphorus (P). Algae and aquatic plants rely on these nutrients for their growth naturally. However, excessive amounts of nitrogen and phosphorus in the water tend to trigger extreme HABs growth.
Sources of Nutrient Pollution
The global production of P has increased 18 times since the 1940s. Meanwhile, the production of N grew over sixfold. Hence, is estimated that the annual flow of P to aquatic systems has tripled, while N has doubled.
Nutrient pollution can occur due to land development, agriculture, aquaculture, and atmospheric nutrient deposition. These increase the amount, alter the proportions and chemical forms of nutrients promoting HABs.
The expanding human population intensifies food production and wastewater discharge. These are the main contributors to nutrient pollution globally. Anthropogenic activities affecting freshwater and climate change accelerate the impacts on HABs further.
Major sources of nutrient pollution include:
Crop production involves chemical fertilizers or animal manure which contain nitrogen and phosphorus. Fertilized soils and livestock operations release many nutrients into the air and waterways.
The production of finfish and mollusks stimulate toxic and fish-killing algal growth. Different pond‐based aquaculture production systems prompt high-biomass bloom-forming algae.
The growing human population demands more concentrated animal feeding operations (CAFOs). Most of the animal manure produced in CAFOs enters rivers directly. It is estimated that in China tens of thousands of CAFOs produce 40 times more N pollution than other industries.
Municipal sewer and septic systems often fail to remove the nitrogen and phosphorus from urban waste. Discharges of untreated or inadequately treated waste in the waterways exacerbate the nitrate phosphorus pollution.
The runoff of rain and snow from roofs, roads, and pavements also carries nitrogen and phosphorus into local waters.
Widely used for manufacturing, transportation, electricity generation, and agriculture. They release unprecedented amounts of nitrogen oxide emissions in the air. A big part of it ends up polluting waters. Industrial operations, airplanes, ships, road vehicles, and coal power plants are significant sources of nitrogen pollution.
Some detergents used for cleaning and laundry can contain nitrogen and phosphorus. Garden fertilizers and inadequately disposed biowaste can add to nutrient pollution of waters.
Nitrogen is also the most abundant element in the air composition. Human activities produce a significant surplus of nitrogen in the air either as nitrogen oxides or ammonia. It ends up deposited back onto land and washed into nearby water bodies. It is estimated that atmospheric deposition over land has increased threefold. Extensive atmospheric N deposition leads to N-saturation of watersheds and exporting nitrate further to streams, lakes, and estuaries. This elevated water nutrient pollution stimulates the spread of HABs. Too much ammonia and low pH creates oxygen-deprived water zones suffocating and intoxicating the aquatic organisms.
Effects of nutrient pollution
The human-caused enrichment of water by nutrients results in eutrophication processes. The growth of algae and higher forms of plant life accelerates. This disturbs the natural balance of organisms living in the water, deteriorating the water quality.
Excess nutrients can dramatically alter the food webs in lakes, rivers, and coastal ecosystems. These systems become phytoplankton-dominated. The benthic microalgal and macrophyte production decreases in the water, which can be critical for fish survival.
In lakes, eutrophication creates hypoxia and biodiversity loss due to a lack of dissolved oxygen. It promotes the proliferation of HABs. Lake eutrophication also increases emissions of greenhouse gases, methane, and nitrous oxide. This contributes to global climate change.
Economic costs of nutrient pollution
The economic losses resulting from nutrient pollution and eutrophication are overwhelming. Eutrophication increases the costs of water purification for municipal and industrial use. Loss of fish and wildlife can compromise the food supply for people and animals. Toxic algae and “dead zones” cause losses of hundreds of millions of dollars for the aquaculture industry. The recreational value of water bodies falls, damaging the hospitality and tourism industry.
The complexity and diversity of the eutrophication effects make it difficult to get a precise cost estimate. Eutrophication of European coastal waters is estimated at over $1 billion per year. For the United States, eutrophication of lakes and streams costs over $2.4 billion annually. These estimates include the loss of lake-front property value (49%) and losses from recreation (24%). The taste and odor problems caused by eutrophication and HABs growth lead to costs for buying bottled water (25%). The costs of endangered species protection were estimated at 2% of the total losses.
However, other factors like the costs of water treatment to remove algal toxins, taste, and odor issues were not included. The lack of drinking water and “dead zone” with low oxygen levels result in considerable losses in public health and wildlife.
Solutions for nutrient pollution
Combating eutrophication requires a combination of comprehensive control and mitigation measures. Governments, businesses, and individuals must take urgent actions to reduce nutrient pollution. The key strategies involve controlling nutrient pollutant sources and restoring damaged ecosystems.
Control of nutrient flow
The governments focus their efforts mostly on controlling pollution from point-sources of nutrients. Efforts to control point sources have proved successful for developed countries. Banning P from laundry detergents and removing it from sewage effluent are efficient control strategies. Yet, developing countries have either very limited nutrient removal technologies, or none at all.
Point source P is also associated with operations of mines, factories, and many urban activities. These sources are much more difficult to control. Even after decades of nutrient input reduction efforts, many water bodies still suffer from eutrophication. Water quality remains very low, and algal bloom continues to spread. This is due to the nonpoint loading of nutrients and atmospheric deposition. Controlling nonpoint pollution from sources like agriculture is much more challenging. The main difficulty is that the sources are very diverse and dispersed. Land use management, landscape management, water management practices must be upgraded. They should focus on reducing nutrient runoff and nutrient losses through leaching.
Businesses can reduce nutrient pollution by managing and reducing their emissions into air and water. Investing in energy efficiency and shifting to renewable energy sources helps reduce pollution from fossil fuels. Farm, field and catchment management can help reduce nutrient runoff into water bodies. Farmers can reduce fertilizer use and minimize nutrient losses from their activities through sustainable nutrient management practices. This involves applying fertilizer and manure in strict amounts at optimal timing. Choosing the safest method and precise placement are critical factors. Planting field buffers and ensuring year-round ground cover help prevent farm soil erosion and reduce nutrient losses into waterways.
Individuals can also contribute. For example, choosing phosphate-free cleaning and laundry detergents, soaps, and shampoos. Conserving energy in the household helps minimize airborne nutrient pollution from fossil fuels. This includes using energy-efficient domestic equipment and green building design. Airplanes, buses, cars, and trucks produce significant amounts of nitrogen oxide emissions. So it is important to decrease their use by minimizing driving and shifting to sustainable transport modes.
Restoration of aquatic ecosystems
Studies show that control of external nutrient sources often does not decrease the nutrient loads and algal blooms in water bodies. Lakes seem to respond very slowly to nutrient control interventions. This is because the nutrients remain in deposits for long term. They replenish algal blooms and trigger further eutrophication.
Ecosystem restoration aims to rehabilitate the damaged water ecosystems. The biological, physical, and chemical functions and properties of water bodies are then recovered. Lake rehabilitation and restoration strategies aim to reduce the P concentrations in the water. A common approach is to trap and remove P from the system. This can be achieved by physicochemical methods like ferric dosing. It involves adding ferric sulfate. Alternatively, physico mechanical methods like flushing and dredging of floor deposits can help reduce P concentrations. However, these methods may disrupt the natural balance of the aquatic ecosystem. These solutions are temporary. As soon as the intervention is stopped, the levels go back to the former level.
A more efficient restoration method is to control algal growth, which helps prevent further accumulation of nutrients in the deposits. The main algae control methods include chemical control, aeration, mixing, and ultrasound. The ultrasonic algae control technology is considered safest and environmentally-friendly solution for eutrophication from nutrient pollution. It is harmless for fish and plants and can be used for lakes and drinking water reservoirs.