All types of dams, ponds, lakes and other waterbodies can experience water stratification. Unfortunately, it can bring about lots of unpleasant factors, like causing death to fish and creating a bad smell that comes from the water. Despite the issues it can trigger, it’s important to know it’s actually a natural process that happens on its own. You just need to manage it before it gets out of hand.
In this blog, we’ll take a closer look at water stratification. We’ll explore what it is, why it happens, and proven ways to fix it.
What Is Water Stratification?
What does water stratification mean, and what exactly is it? When water separates into horizontal layers, each with its own temperature and characteristics, that’s called stratification. It’s especially common during the warmer months in deep or still freshwater like lakes and ponds.
These different layers tend not to mix, causing serious issues with the water’s quality. Near the bottom especially is where the issues occur.
Signs Your Water Might Be Stratified
Does this sound like something that might be happening in your waterbody? Sometimes, it’s hard to tell and issues in the water are caused by something completely different. If you’re not sure, here are some common signs you can look out for:
- Fish gasping for oxygen near the surface: If you notice fish near the surface, in the morning especially, it’s usually a sign that there’s little oxygen in the deeper water.
- A buildup of surface algae or scum: If the water looks unhealthy, it often means the upper layers of water are warm and full of nutrients.
- A bad smell: Stratification can cause a bad, rotten egg smell.
- A lack of surface disturbance or wind mixing: The water will be still, especially in small, sheltered water bodies.
- Different temperatures between top and bottom: If the top of the water is warm and the bottom is cold, your water is likely stratified. Swimmers notice this almost immediately when jumping in.
If you notice these signs, your water has likely developed separate thermal layers and water stratification is taking place.
What Causes Stratification?
What actually causes stratification? Here are some reasons:
Temperature Differences
Temperature differences are the main reason why stratification generally happens. During spring and summer, the sun warms the surface of the water. This makes the surface water lighter whereas the colder water underneath becomes heavier, and sinks down to the bottom.
This causes three layers to form:
- Epilimnion – the warm top layer rich in oxygen
- Metalimnion (or thermocline) – the middle layer where the temperature falls quickly
- Hypolimnion – the cold, dense bottom layer that gradually loses oxygen
Lack of Wind or Water Movement
In calm areas or sheltered spots without enough wind or waves, the water layers stay separated and don’t mix. This lets stratification stay for weeks or even months.
Salinity Differences (mainly in saltwater)
In oceans or salty lakes, denser saltier water sinks beneath fresher water, forming layers that depend on both salt content and temperature.
Density Gradients
Water density depends on both temperature and salt content. When denser water sinks to the bottom, it resists mixing, creating stable but separate layers.
Stratification is Seasonal
Stratification changes with the seasons and isn’t always present, especially in temperate climates. You can expect it a lot more in the warmer months because of the sun heating up the surface water.
Here is a breakdown of each season:
- Spring: As surface water warms, layers begin to form. In this season the wind might still help to mix the water.
- Summer: In summer, stratification is strongest with clear, stable layers that hardly mix with warmer water at the top and cooler water at the bottom.
- Autumn: As the air cools, surface water sinks and breaks up the layers, this is called turnover.
- Winter: In cold areas, water can form layers under ice, with the heaviest water (about 4°C) settling at the bottom and colder water resting above.
Why Does Stratification Matter?
Water stratification is natural most of the time, but if it’s left untreated for too long it can cause a lot of problems. The issues that it brings are especially problematic in small or nutrient-rich water bodies.
Here are some reasons why water stratification is important to tackle:
Low Oxygen at the Bottom
When stratification happens, the bottom layer of the water loses contact with surface oxygen and causes the layers. As organic matter breaks down, oxygen runs out, making the water hard for fish and other creatures to live in.
Sudden Algal Blooms
No pond, dam or lake owner wants algae. When stratification ends, usually in autumn, nutrients trapped at the bottom mix into the upper layers. This can cause strong algal blooms, including harmful blue-green algae.
Mass Fish Kills
The most severe impact comes during seasonal turnover. When the water layers suddenly mix:
- Oxygen-poor water from the bottom spreads throughout the whole dam or lake.
- The previously oxygen-rich top layer is diluted, spreading oxygen too thin to support life.
- Fish suffocate, particularly large species or those sensitive to low oxygen.
After hot summers, when stratification lasts too long, mass fish deaths often happen in small farm dams.
Preventing Stratification: The Role of Aeration
The good news is there are steps you can take to fix and prevent stratification.
Aeration is the most effective long-term solution. It helps by breaking up layers and keeping oxygen levels stable throughout the water column.
How Aeration Helps:
- Mixes the entire water body, preventing thermal layering
- Restores oxygen to deeper zones, especially sub-surface aerators
- Reduces buildup of harmful gases like ammonia and hydrogen sulphide
- Supports fish and other aquatic life across all depths
Types of Aeration Systems
Here are the different types of aeration systems:
1. Diffused Aeration (Bottom Sub-Surface Aerators)
Air is pumped through diffusers positioned on the bottom of the lake or dam. The rising bubbles create circulation, lifting stale, low-oxygen water to the surface and pushing oxygen-rich water downward.
- Ideal for deep water bodies
- Energy-efficient and excellent at destratifying water bodies
2. Surface Aerators
Surface aerators, also known as floating systems, churn the surface, improving oxygen exchange with the atmosphere.
- Best for shallow ponds or small dams
- Also useful in emergencies, like when fish are gasping at the surface, because it adds oxygen to water very quickly. Fish in dire situations can hang out right next to the aerator to get immediate oxygen.
3. Solar-Powered Aerators
Perfect for remote or off-grid locations, solar aerators can be used for both surface and diffused systems.
- Environmentally friendly
- Low running costs
Extra Benefits of Aeration
Beyond preventing stratification, aeration also:
- Improves overall water quality
- Reduces algae and aquatic weeds by limiting nutrient hotspots
- Breaks down bottom sludge, which is fertiliser for algae and weeds
- Eliminates bad smells, caused by “bad” anaerobic bacteria
- Helps “good” beneficial bacteria thrive
Final Thoughts
Feeling equipped to tackle water stratification? Even though it’s a natural process, it’s still important to take steps to stop it from happening. By investing in an effective aeration system, you can keep your waterbody healthy and clear all year round.
If you’re unsure which aerator is right for your pond, lake, or dam, our team of experts can help. We offer free, personalised advice and water treatment plans. No obligation. To get started, contact us today.