Estuaries possess distinct blooms of excessive growth of a pigmented dinoflagellates. These blooms are called.

Answer: (a) red tides

Dense blooms of dinoflagellates colour the water red-brown — “red tides”, often toxic to fish.

The idea

Dinoflagellates are single-celled algae with a reddish-brown pigment. When an estuary is rich in nutrients they multiply so fast that the water itself changes colour. The name “tide” is misleading; it has nothing to do with tides.

Rivers bring nutrients into the estuary → Dinoflagellates multiply into a bloom → Water turns red or brown → Some kinds release toxins → Fish die; shellfish become poisonous to eat

Also know

Oysters and mussels filter the algae and store the toxin, so people who eat them can be poisoned. Some dinoflagellates give off light at night, which makes the sea sparkle.