9 Algae
Invasive and proliferating algae
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Algal blooms
When newly arrived algae settle in at the expense of local species, we speak of invasive algae. This is the case of Japanese wireweed (Sargassum muticum), and also of the oyster thief (Colpomenia peregrina), both of which appeared on the Breton coast in 1970 with the growth of oyster farming.
But local algae can proliferate too, when conditions become too favourable to their growth. This is known as eutrophication. The phenomenon is amplified by particular weather conditions:
- More sunshine: more light and rising temperatures.
- Heavy rainfall: causing excessive run-off into the sea of nutrients found on land, such as phosphorus and nitrogen.
In Brittany, eutrophication occurs regularly and leads to green tides. As stranded algae decompose, they produce a toxic gas: hydrogen sulphide (H₂S).
Making use of stranded seaweed
Attempts are sometimes made to put stranded seaweed to good use. In southern Brittany, trials have been carried out using the red tides of Solieria chordalis. Held in AlgoBox® units, the algae break down and serve as fertiliser for dune vegetation, helping to limit coastal erosion.