Articles 16 Aug, 2021

Nutrient pollution: Causes and solutions

In short

  • Nutrient pollution is one of the primary causes of harmful algal blooms;
  • Legacy nutrients will impact algae growth for many years to come;
  • Technological solutions can limit nutrient inflow and keep ecosystems healthy.

Many of these blooms are triggered by increasing nutrient pollution. The dramatic surge in nutrient flow from industrial, urban, and agricultural activities accelerates 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 contributing to the proliferation of HABs. The disbalance in nutrients can accelerate the toxicity of diatoms and cyanobacterial HABs. Climate change with environmental and abiotic conditions due to human activities, also favor algal growth.

The main nutrients affecting the health of aquatic ecosystems are nitrogen (N) and phosphorus (P). The natural biogeochemical cycles of nitrogen and phosphorous could be disrupted by increased effects brought on by climate change.

Algae and aquatic plants rely on these nutrients for their growth. However, excessive amounts of N and P trigger extreme HABs.

What are the causes of nutrient pollution?

The global production of P has increased 18 times since the 1940s. Meanwhile, the production of N grew over sixfold. It’s 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 growing human population intensifies food production and wastewater discharge. These are the main contributors to global nutrient pollution. Anthropogenic activities and climate change further accelerate the prevalence and impact of HABs.

Major sources of nutrient pollution include:

Agriculture
Crop production involves chemical fertilizers or animal manure, containing N and P. Fertilized soils and livestock operations release many nutrients into the air and waterways.

Wastewater
Municipal sewer and septic systems often fail to remove the N and P from urban waste. Discharges of untreated or inadequately treated waste in the waterways exacerbate nitrate and phosphorus 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.

Fossil fuels
Widely used for manufacturing, transportation, electricity generation, and agriculture. Release unprecedented amounts of nitrogen oxide emissions into the air. A big part of it pollutes our water bodies. Additionally, industrial operations, airplanes, ships, road vehicles, and coal power plants are significant sources of nitrogen pollution.

Stormwater
Rain and snow runoff from roofs, roads, and pavements also carries nitrogen and phosphorus into local waters.

Nitrogen is also the most abundant element in the air composition. Human activities produce a significant surplus of nitrogen either as nitrogen oxides or ammonia. It ends up deposited back onto land and washed into nearby water bodies.

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.

What are the effects of nutrient pollution?

The human-caused enrichment of water with nutrients fuels the process of eutrophication. It accelerates the growth of algae and other aquatic life. This disturbs the natural balance of aquatic ecosystems and deteriorates the water quality.

Excess nutrients can dramatically alter the food webs in lakes, rivers, and coastal ecosystems. Thus, these systems become dominated by phytoplankton. The benthic microalgal and macrophyte production decreases and can be critical for fish survival.

In lakes, eutrophication creates hypoxia and biodiversity loss due to a lack of dissolved oxygen. Also, it promotes the proliferation of HABs. Lake eutrophication further increases emissions of greenhouse gases, methane, and nitrous oxide, contributing to global climate change.

Economic costs

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 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 total losses.

However, other factors like the costs of water treatment for algal toxins removal, as well as taste and odor issues, were not included. The lack of algae control in drinking water reservoirs and “dead zone” with low oxygen levels results in considerable public health and wildlife losses.

What are the solutions for nutrient pollution?

Combating eutrophication requires a combination of comprehensive control and preventative measures. Governments, businesses, and individuals must take urgent actions to reduce nutrient pollution. Key strategies involve controlling nutrient pollutant sources and restoring damaged ecosystems.

Controlling the nutrient flow

Governments focus their efforts mostly on controlling pollution from the point-sources. Efforts to control point sources have proved successful for developed countries. Banning P from laundry detergents and removing it from sewage effluent are efficient are both efficient control strategies. Yet, developing countries have either minimal nutrient removal technologies or none at all.

Point-source P is also associated with operations of mines, factories, and various other 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 occur. This is due to the non-point loading of nutrients and atmospheric deposition. Controlling non-point 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, and water management practices must be upgraded. These solutions 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 strictly applying fertilizer and manure 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 nitrogen oxide emissions. Therefore, it’s essential to minimize driving and instead, shift to sustainable transportation modes.

How to restore aquatic ecosystems?

Studies show that controlling  external nutrient sources doesn’t always decrease the nutrient loads and, therefore, algal blooms in water bodies. Lakes seem to respond slowly to nutrient control interventions. This is because the nutrients remain in deposits for the long term. They replenish algal blooms and trigger further eutrophication.

Ecosystem restoration includes rehabilitating 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 from the water. A common approach is to trap and remove P from the system. Physicochemical methods like ferric dosing can achieve this. It involves adding ferric sulfate.

Physico-mechanical methods like flushing and dredging of floor deposits can help reduce P concentrations. However, these methods may disrupt the natural balance of aquatic ecosystems. These solutions are temporary. As soon as the intervention stops, levels go back to the former value.

A more efficient restoration method controls algal growth, preventing further accumulation of nutrients in the deposits. The primary algae control methods include chemical control, aeration, mixing, and ultrasound. The ultrasonic algae control technology is considered the safest and most environmentally-friendly solution for eutrophication. It’s safe for fish, plants, and other aquatic organisms, and can be used for lakes and drinking water reservoirs.