It was an algal bloom — an excessive increase of algae — that turned the waters of the Periyar downstream of the Pathalam regulator-cum-bridge green last week, reveal laboratory tests.
Water samples collected by the Pollution Control Board (PCB) on March 15 when the green colour was first noticed were given to the Cochin University of Science and Technology’s (Cusat) School of Marine Sciences for microbiological analyses.
Its tests reveal that the green colour was caused by the presence of Platymonas , a marine genus, said Eloor Environmental Engineer Sreelekshmi P.B. “This is a type of green algae that is usually seen in highly eutrophic (when a body of water becomes overly enriched with minerals and nutrients that induce excessive growth of plants and algae) lakes,” said Dr. Bijoy Nandan, Professor in the Department of Marine Biology, Microbiology and Biochemistry at Cusat, who supervised the microbiological tests.
Heavy bloom
“This was a particularly heavy bloom, for algae dominated the samples by almost 90%.”
Algae thrive on the existing dissolved oxygen contents in the water. Dissolved oxygen levels thereby drop, affecting other aquatic organisms such as fish that then need to come to the water surface to gulp in air. Apart from affecting water quality this way, algal blooms can also release toxins and metabolites that could put aquatic organisms in danger if the bloom sustains for long, said Dr. Nandan. The March 15 incident in the Periyar was observed for two days, said Ms. Sreelekshmi.
Such algal blooms are caused by several factors: the accumulation of nutrients (such as nitrates and phosphates) in the water that come through industrial effluents, water stagnation due to low water flow, and increases in the ambient water temperatures that happen during summers.
“Since this was seen only in a localised area, this could be due to the leachates in the water,” said Dr. Nandan.
