Algae term includes a diverse and large group of photosynthetic eukaryotic organisms.
They make their food through photosynthesis, produce oxygen, hold nutrients, bind heavy metals, benefitting the waterbody, and are a good source of nutrition for the fishes.
But, excess growth of algae is harmful because, in the absence of sunlight, they respire at a very high rate, using lots of oxygen and sugar, and releasing carbon dioxide, which depletes the oxygen level in the water causing all the fishes and other aquatic organisms to die.
Blue-green algae are very toxic to dogs, swimming or ingesting a small amount can cause severe neurological or liver damage.