What are algae blooms?

1 Answer
May 9, 2017

An algal bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems.


Algal blooms are natural phenomenon but their frequency, duration and intensity are increased by nutrient pollution. They occur as a result of excess nutrients from fertilisers, waste water and storm water runoffs, coinciding with lots of sunlight, warm temperatures and shallow slow flowing water.

Algal blooms are often associated with large scale marine mortality events. The thick clumpy blooms block vital sunlight from reaching beneficial underwater plants, considerably damaging the ecosystem. Gradually the overpopulated alga become stressed and die. They are decomposed by bacteria and this leads to reduced levels of dissolved oxygen in the water body. This kills species of marine fauna which have little tolerance to low dissolved oxygen in water. Some algal species can poison fish by producing algal toxins or by clogging their gills.

Depending upon the species, algae can be considered to be blooming at concentrations of tens of thousands to millions of cells per millilitre of water. The photosynthetic pigments in the algal cells determine the colour of the algal bloom. They are mostly greenish in colour but there can be a wide variety of colours ranging from yellows to browns and reds, depending upon the species. Harmful algal blooms are usually brown or red in colour and are called as red tides.