14 April 2021.

Introduction:

More than 4,000 tonnes of Salmon in Chile have been killed by deadly algae.The algal bloom in the water reduced the amount of oxygen suffocating the salmon fishes.

Highlights:

• More than eighteen salmon fish farms that produce 26% of world salmon have been affected.
• The presence of urea and ammonia in the salmon farms have  led to the algal bloom. Three different types of harmful algae were detected in eighteen farms.
• Chile is the second largest producer of Salmon in the world after Norway.
• It exported more than 4.4 billion USD of fish in 2020. Chile produces more than 26% of world Salmon.

About Algal Bloom:

• Algal Bloom is rapid increase of algal population in marine water or fresh water.
• It occurs due to the nutrients from phosphorous or nitrogen fertilizers run off.
• They create harmful effects such as blocking of sunlight.
• This causes depletion of oxygen level in water.

Dissolved Oxygen in Water

• The Dissolved oxygen in water is the most important water quality factor. A pond supporting an aquatic ecosystem will have more than 10 ppm oxygen dissolved in it.
•  On the other hand, atmosphere has 200,000 ppm of oxygen.
• The oxygen concentrations below 3 ppm will stress the species of fish in the water.
•  The oxygen concentrations below 2 ppm will kill the fishes.
• Oxygen dissolves in water in two ways.
• One is through atmospheric contact that happens at the surface.
• The other primary source of dissolved oxygen is from algae.
• The algae add dissolved oxygen to the water by photosynthesis, thereby supporting lives in the water system.
• In that case, how did the algal bloom in Chile kill thousands of fishes?

How does Algal Bloom kill fishes?

• During day, the algae produce oxygen through photosynthesis.
• While at night, in the absence of sunlight, it stops photosynthesis and starts respiring just like other organisms in the water system.
• Respiration is taking in Oxygen and releasing carbon dioxide.
• Thus, during night the dissolved oxygen drops too low suffocating the fish in the water system.
• Also, as the algal bloom increases, it forms thick blanket over the water surface.
• This prevents sunlight from penetrating the water.
•  Hence the second source of oxygen where the oxygen dissolves in water at the surface contacts is blocked.