While a big brown seaweed on a rocky seashore is the most common example of an alga, the algae that cause blooms are quite different; they also live in water, but they are small in size, often microscopic, usually green in colour, and they live their lives floating at the surface.

If a lake becomes enriched with nutrients the algae absorb the welcome chemical food, grow rapidly, and reproduce very quickly resulting in a bloom.

The algae in question have the green pigment chlorophyll. A bloom becomes visible to us due to the chlorophyll in the water turning it green, the colour change being the reason the population explosion is called a bloom.

The most frequent nutrients that trigger algal blooms are nitrates and phosphates from sources such as animal dung and urine, human sewage, slurry, leaking septic tanks, agricultural fertilisers, malfunctioning wastewater treatment plants, industry, etc. When farmers spread fertilisers on the land to make crops grow, somewhere between at worst 30% and at best 60% of the fertiliser is absorbed by the crop; the rest ends up in waterbodies.

Algal blooms are unwelcome news for watery environments as they cause pollution in a number of ways. A bloom consumes oxygen at night so if the bloom is significant it can result in a sudden drop in the oxygen content of water suffocating some of the aquatic creatures that need the dissolved gas to survive. A severe and prolonged bloom on the surface can shade plants growing on the bed, starving them of light, and causing mortalities. And, of course, there will be many deaths among the large algal population as individuals reach their end of life.

All of these deaths results in a boom for bacteria that decompose the dead remains of the aquatic creatures, bottom-living plants, and planktonic algae. A tipping point may be reached where the bacterial population explodes consuming more oxygen. Conditions become more foul and other bacteria that can survive without oxygen take over often releasing toxin waste products.

If nutrient inputs continue, they drive a positive feedback loop resulting in a slow deterioration in water quality and eventually a dead waterbody.

The solution is to stop or reduce the nutrient input at source if at all possible, change farming practices, find ways to break the pathway from source to waterbody by creating buffer zones and wetlands, planting trees and other plants along the way to absorb the nutrients, etc.