Fertilisers are often used in farming, sometimes these fertilisers run-off into nearby water causing an increase in nutrient levels. This causes phytoplankton to grow and reproduce more rapidly, resulting in algal blooms.
This bloom of algae disrupts normal ecosystem functioning and causes many problems.
The algae may use up all the oxygen in the water, leaving none for other marine life. This results in the death of many aquatic organisms such as fish, which need the oxygen in the water to live.
The bloom of algae may also block sunlight from photosynthetic marine plants under the water surface.
Some algae even produce toxins that are harmful to higher forms of life. This can cause problems along the food chain and affect any animal that feeds on them.