Sewage & Run-Off Pollution

by Jessica Vyvyan-Robinson

When one looks at a world map and sees the vastness of our oceans, it’s easy to think of the effluent we pump into aquatic systems as no more than a literal drop in the sea. And yet, the perception that the ocean is large enough to absorb untold quantities of anthropogenic pollution without consequence is false. Every year, chemical nutrients released into the ocean as sewage or via agricultural and industrial run-off have catastrophic ramifications - including the creation of anoxic dead zones, and the triggering of toxic algal blooms.

The Mechanisms of Chemical Nutrient Pollution

Human sewage includes excrement, waste water from bathing and laundry, and organic fats washed down the sink whilst cooking. In most developed countries, legislation requires sewage to be treated before being discharged into aquatic systems. However, untreated sewage often makes its way into the ocean as a result of faulty equipment, negligence, and overflows caused by heavy rainfall. In many developing countries, there is no infrastructure in place to facilitate sewage treatment, and so it is pumped raw into the ocean.

Sewage Outflow

Sewage pollution is not the only kind of effluent contributing to nutrient and chemical imbalances in our coastal waters. Industrial and agricultural activities generate toxic surface run-off, whereby rainfall washes soil particles laden with carbon, nitrogen, phosphorus and other minerals into aquatic systems. This kind of pollution is particularly prevalent in coastal areas undergoing heavy industrialisation - i.e. the development of new roads, buildings, and ports. 

Surface run-off and untreated sewage both cause increased levels of nitrogen and phosphorus in coastal waters. Marine algae thrives on these nutrients, and consequently begins to flourish in affected areas via a phenomenon known as eutrophication. The resulting algal blooms cause severely depleted levels of oxygen in the water, firstly because they prevent sunlight from filtering through to photosynthesising benthic organisms, and secondly as a result of the decay process that occurs when the algae dies off.

In the majority of cases, algal blooms cause coastal waters to become low in oxygen, or hypoxic. Hypoxic conditions cause animals living in the area to become severely stressed; can trigger decreased levels of biodiversity; and may even see the replacement of naturally-occurring species with more resilient invasive ones. Currently, the World Resources Institute has identified 375 hypoxic coastal zones worldwide.

In extreme situations, algal blooms may deplete an area of oxygen completely, causing it to become anoxic. These conditions promote the growth of anaerobic bacteria, which can render the surrounding environment toxic for all other organisms. These lifeless areas are appropriately known as dead zones. Certain kinds of algae are toxic in their own right, and when these harmful species bloom the ramifications are particularly severe. In many countries, harmful algal blooms are colloquially known as red tides.

Human & Environmental Impacts

As a primary food source on the marine food chain, algae is consumed either directly or indirectly by almost every creature in the ocean. Toxic algal blooms therefore have the power to contaminate all kinds of seafood, from the smallest mollusc to the largest fish. Thanks to their role as filter feeders, shellfish are particularly prone to contamination during a red tide. The most common symptoms of toxic shellfish poisoning include acute nausea, vomiting and diarrhoea - although in extreme cases, consumption may even prove fatal.

Some strains of toxic algae are capable of producing airborne toxins that are released as the bloom crashes upon the shore. Even for those humans that don’t eat seafood, these airborne toxins may cause skin irritations and respiratory problems. The health risks posed by harmful algal blooms often cause the tourism industry to crash in affected areas. Even if visitors are unaware of the potential damage to their health, the blooms are unsightly and cause severely reduced water visibility. In this way, they serve as an aesthetic deterrent to beachgoers, recreational fishermen and scuba divers alike.

Lloyd Edwards Red Tide

Because seafood is rendered unsafe by harmful algal blooms, commercial fisheries are often forced to suspend their activities or move them elsewhere. The effects of the algal bloom are compounded by the dead zones created when the algae begins to die off. Often, the sudden loss of income caused by the collapse of the tourism and fishing industries causes such severe damage that local economies take many years to recover. In the United States, it is estimated that harmful algal blooms cost the national economy a staggering $82 million in revenue every single year.

Hypoxic zones are created whether the algal species in question is harmful or not. In this way, even algae that is benign in its own right has the power to cause untold financial damage to coastal communities. Sewage pollution is associated with other health risks, too, because it often contains disease-causing microbes known as pathogens. Some of these waterborne pathogens infect beachgoers through contact with the skin and eyes, whilst others are ingested either directly or via seafood. These sewage-borne pathogens are the most common cause of beach closures.

Humans are not the only ones to suffer as a result of eutrophic algal blooms. Some strains of harmful algae have a corrosive effect on the skin of many marine animals; whilst others variously secrete toxins capable of causing paralysis, amnesia or respiratory problems. A common symptom of red tides is mass fish die-off, caused when paralysis as a result of exposure to algal toxins prevents fish from being able to breathe. In 2013, a Florida red tide caused the death of 276 manatees after the endangered herbivores consumed seagrass coated with toxic algae.


Ultimately, the most effective solution to sewage and run-off pollution is better human management. Those countries that already have legislation and infrastructure in place to facilitate sewage treatment need to ensure that laws are enforced, and that faulty equipment is repaired as a priority. The restoration of shellfish populations in estuaries saturated with pollutive nutrients has also been suggested as a curative measure, as many bivalve species work to filter nitrogen from the water column.

In countries where sewage treatment does not already occur, the responsibility lies with municipal authorities and national governments to work towards introducing the infrastructure needed to make such treatment possible. In these countries, informed citizens may help to bring about change by putting pressure on their governing bodies, and by raising awareness of the issue within their communities.

Did You Know?

“The sea shapes the character of earth, governs climate and weather, regulates temperature, and comprises much of the biosphere – yet it remains largely unknown. It is not, however, untouched. Everywhere, the changes brought about by humankind are evident.”

Sylvia Earle
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