By Judith Burns
Science Reporter, BBC News
The word "algae" is a generic term which can cover anything from seaweed to microscopic organisms.
Algae by satellite - red shows thickest blooms
The type of seaweed causing problems along the Breton coast of France is called Ulva, often known as sea lettuce.
Along the south coast of the UK there are build ups of another algae called Enteromorpha. This is composed of thin filaments which mat together. It looks a bit like pond weed.
These incidents are being blamed on a process known as '"eutrophication".
After heavy rain, nitrate based fertiliser from arable land and effluent from livestock and from the human sewage system can be washed into streams and rivers.
This 'run-off' ultimately flows into the sea where it boosts the growth of algae.
Dave Lowthion, marine team leader at the UK's Environment Agency said: "The things that make these weeds grow are a supply of nutrients; good light conditions so you are really looking for fairly clear water, the light shines through it to some depth; and the right tidal conditions."
Algal blooms are bad for biodiversity because they squeeze out slower growing plantlife and use up the available oxygen in the water both when they are alive and when they die and start to decompose.
Animals that rely on oxygen such as shrimps and worms will die. This has a knock on effect on the birds and fish that feed on them.
The two types of algae causing problems in Europe are not in themselves toxic. But when they die and rot they give off hydrogen sulphide.
In France, the top surface of the piles of rotting algae is baked hard by the sun and the gas is trapped underneath. When the surface is broken, it can give off enough of the gas to kill a larger animal like a horse.
The beaches along France's northern coast are particularly vulnerable to the build up of algae. This is because recent spring tides have dumped it in piles at the top of beaches. Recent hot weather has led to the algae starting to rot.
Off the coast of the UK and Ireland ocean currents are stronger. This means the algae is likely to be swept away before it can be dumped on shore in large quantities by the tides.
The seaweed building up along the south coast of the UK is in sheltered areas like harbours which are not subject to such high tides. So the algae stays spread out in the water rather than being piled up at the edge.
In the UK, The Environment Agency has warned that wetter summers will make the problem worse as heavier rain will increase run off from farmland into rivers and streams.
Last week, fish were killed by an algal bloom off the coast of Cornwall. This was caused by microscopic form of plankton called Karena mikimotoi.
Satellite images allow scientists to monitor the blooms.