Why Does Low Tide Smell?

If you’ve ever been down to the beach on a warm summer’s day, you may have noticed a strange, rotten-egg smell.

While it is there most of the time, it is especially apparent at low water when the tide has gone out.

The “low tide smell” is caused by hydrogen sulphide which is produced by decomposing aquatic organisms. When the tide goes out, these rotting organisms become exposed, releasing gas. As hydrogen sulphide is denser than air, it lingers on the beach, causing a rotting egg smell.

In the image above, you can see different areas that are typically encountered on a seashore. 

• First, there is the beach itself, which is dry and always above water level. 
• Next, there is the inter-tidal area, where all the seaweed is visible. 
• Lastly, you find the low-water level, which is always covered by the tide, even at low-water.

It is the inter-tidal areas that cause the “low water smell”.

At low water, the organic matter within the inter-tidal area becomes exposed, increasing the smell’s strength and giving it the name: “low tide smell”.

The physical cause is a gas called hydrogen sulphide, which is a by-product of the decomposition of the aquatic life found within the inter-tidal area.

Fun Fact: Hydrogen sulphide is also produced by rotting eggs, which is why it is known as a “rotting egg smell”.

When the tide goes out, the gas is free to escape from the blanket of weed where it is produced.

As hydrogen sulphide is denser than air, it cannot rise up into the atmosphere. Instead, it lingers near to the ground where it will stay unless it is dispersed by the wind.

In the summer, however, the predominant wind is a gentle sea breeze, caused by warm air over the land, sucking in the cooler ocean air.

As the breeze blows towards the shore, the hydrogen sulphide produced in the inter-tidal area also travels inland, resulting in the distinctive smell you find at the beach in the summer.

What causes the smell at low tide?

The smell at low tide is caused by hydrogen sulphide, produced by rotting material during the decomposition process.

Also known as “sewer gas”, it occurs naturally as a result of bacteria breaking down organic matter.

Read More: The Illinois Department of Public Health has a good fact sheet on Hydrogen Sulphide.

When bacteria break down organic material deposited in intertidal areas, they naturally produce hydrogen sulphide.

At low water, when these inter-tidal areas are exposed, the gas is free to escape, causing the distinctive “low tide smell”.

Conversely, at high water, the inter-tidal organic material is floating or covered by the tide, reducing the escape of gas.

Once the gas has escaped, it mixes with the atmosphere and is dispersed according to the relative weights of all the gasses that are present.

Hydrogen sulphide is one of the heavier gasses, so it tends to linger near the ground.

In small quantities, it will be barely noticeable, but as the concentration increases, the smell will become more apparent.

It will become especially apparent during the summer after there has been a lot of algae growth and there is less wind to disperse the gas.

On a summer’s day, the sun warms the land, causing air to rise. As it rises, cooler dense air is sucked in from the sea, causing a gentle sea breeze.

The sea breeze catches the hydrogen sulphide that is lingering above decomposing algae and brings it inland towards the beach.

The final result is the “low tide smell” that lingers on the beach at low tide, and is especially noticeable during the summer.

What does low tide smell like?

The typical smell associated with the low tide is a bit like rotten eggs.

It’s the same smell that you might find in places like natural hot springs, or near to some types of industry.

All these smells are similar because they are caused by similar gases, hydrogen sulphide, and dimethyl sulphide.

At the beach, hydrogen sulphide is a result of the decomposition of organic matter, and dimethyl sulphide is a result of the digestion of marine algae by phytoplankton.

The predominant component of both compounds is sulphur, which is why they belong to the same family and both smell faintly of “the sea”.

Pro Tip: If you smell this kind of thing at home, get it checked out because it is similar to the smell added to natural gas.

Does the ocean smell bad?

The ocean (saltwater) does not have any smell, however, organisms within the ocean might.

The most common smell you are likely to encounter in the ocean is caused by dimethyl sulphide.

Dimethyl sulphide is a by-product of the digestive process of marine phytoplankton when they consume marine algae.

It smells similar to the hydrogen sulphide produced by decomposition in intertidal areas, but it is not identical.

While the hydrogen sulphide at the beach smells like rotten eggs, the dimethyl sulphide of the ocean smells more like cooking cabbage.

Dimethyl sulphide is the main component of the smell produced when you cook cabbage.

Is the smell at the beach dangerous?

Hydrogen sulphide in low concentrations is not dangerous, however as the concentration increases, the potential danger does increase.

In most naturally occurring areas, the concentration will not reach dangerous levels, but in some cases, it can.

If there has been excessive algae growth for any reason, the decomposition can cause dangerous levels of hydrogen sulphide to develop.