”Mareld” in shallow coastal waters – a spectacular late summer night phenomenon caused by a toxic algae
A spectacular phenomenon can presently be experienced at night in a shallow sound of the Föglö archipelago in Åland, Finland: when stirred – as for example by a moving motor boat, paddles of a kayak, moving fish, or a swimming person – the water starts to glow in a peculiar turquoise light.
The phenomenon, called “Mareld” in Swedish is caused by blooms of unicellular micro-algae which emit light upon mechanical stimulation, a response also known as bioluminescence. Bioluminescence is thought to be an anti-predator strategy of the algae that disrupts the feeding behavior of grazers by scaring them away through a “flash-bulb” effect, or acts as “burglar alarm” to attract organisms that eat the grazers and save the algae from being eaten.
Globally, most of the toxic algal blooms are caused by dinoflagellates of the genus Alexandrium. The toxins of Alexandrium, saxitoxins and its derivatives (PSTs), affect the nervous system of humans and animals and may lead to paralysis and death.
At the moment the causes of the recent expansion and thriving of Alexandrium ostenfeldiiin the northern Baltic Sea are not well understood. The arrival of the blooms in Åland coincided with the increasing concentration of chemical nutrients in the coastal waters but also with increasingly warm summers. Hence it is likely that the changing environmental conditions play a role in the sudden expansion of the blooms. Interestingly, a recent research revealed, that the Föglö population is genetically more closely related to warm-water populations from the Mediterranean and Western Europe than to its Danish neighbor populations. This could indicate that the blooms may be a result of a recent introduction. Dinoflagellates have been spread around the globe in the ballast water of ships – they form resting cysts under unfavorable conditions which can survive the long passages unharmed and seed new populations after ballast water discharge even at distant locations.
Toxin analyses confirmed that the Föglö population can produce saxitoxins. Even though this may not be of immediate relevance for humans because shellfish is not commercially used in the Baltic Sea, consequences for the ecosystem could be substantial. The coastal areas where the blooms occur are habitat for mussels and clams that feed on the toxic algae and might potentially transfer the toxins to ground fish and sea birds.
Read the full article at The Baltic Sea Portal.
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