This has also reduced oxygen level in the lower depths of the wetland.
Experts say the phenomena are the main cause of the deaths of such fish species as common carp, zander and grass carp in the lagoon.
Fish die in northern Iranian lagoon due to hot weather
A heat wave in the northern Iranian Anzali lagoon, Guilan Province, has led to the growth of a highly aggressive species of fern known as Azolla as well as common water hyacinth across the wide swathe of the wetland, reducing oxygen concentration in the water.
