Fish Kills and Algae Blooms

Why Fish Kills Occur

Fish kills are rarely random events. In most ponds, lakes and dams, they occur when dissolved oxygen levels fall below what fish can tolerate.

This oxygen depletion is usually driven by:

• Excess algal growth
• High organic matter loads
• Warm temperatures
• Still or stratified water conditions

Algae play a key role in this process — but they are not the direct cause of fish deaths.

The Link Between Algae and Oxygen Depletion

Algae influence oxygen levels in two critical ways:

1. At night, algae consume oxygen through respiration.  Suspended algae have the greatest influence, compared to biofilm based algae.
2. When algae die, bacteria break them down, using large amounts of oxygen.

During periods of heavy algal growth, oxygen levels can drop rapidly — particularly overnight or during calm, warm conditions. When oxygen falls too low, fish and other aquatic organisms suffocate.

This is why fish kills often occur suddenly, even when water looked “fine” the day before.

Why Algae Blooms Increase Fish Kill Risk

Dense algae blooms:

• Increase daily oxygen swings
• Add large amounts of organic matter to the system
• Make oxygen levels unstable and unpredictable
• Increase the risk of sudden oxygen crashes

The larger and more persistent the bloom, the greater the risk — especially in nutrient-rich waterbodies.

Why Killing Algae Can Make Things Worse

Many common responses to algae problems involve:

• Chemical algaecides
• Copper treatments
• Rapid mechanical removal

While these may reduce visible algae, they can increase fish kill risk by:

• Causing large amounts of algae to die at once
• Triggering sudden oxygen depletion during decomposition
• Shocking the biological balance of the waterbody

This is why fish kills often follow aggressive algae treatment.

Preventing Fish Kills Requires Stability

The key to preventing fish kills is oxygen stability, not just algae removal.

Stable systems:

• Avoid extreme algal blooms
• Maintain balanced nutrient cycling
• Support biological processes that regulate oxygen naturally

This requires managing the underlying nutrient and biological drivers, not just reacting to symptoms.

The Role of Beneficial Biology

Healthy waterbodies rely on beneficial organisms to:

• Absorb excess nutrients
• Limit algal overgrowth
• Support stable oxygen dynamics
• Build resilient food webs

One of the most important groups in this process are beneficial diatoms — algae that efficiently take up nutrients and support stable ecosystem function.

When diatoms are supported, nuisance algae are less likely to dominate and oxygen conditions become more predictable.