The "green water stratification" phenomenon—characterized by a dense green surface layer over dark, murky bottom water—is primarily caused by an imbalance in algae distribution driven by light and temperature gradients.
During summer, intense sunlight triggers explosive growth of floating algae (e.g., Chlorella) at the surface, forming a thick "green blanket" that blocks light penetration to deeper water. Simultaneously, the sun-heated surface water becomes warmer (>30°C) and less dense, creating a stable thermal barrier (temperature difference >5°C) over the cooler bottom water (<22°C). This barrier acts like an insulating lid, preventing vertical mixing.
This stratification creates three critical problems:
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Deep-Water Anoxia: Algae in the dark lower layers die and decompose, consuming massive amounts of oxygen. Measurements often show dissolved oxygen (DO) levels at the bottom plummeting below 1 mg/L, creating a lethal, oxygen-depleted zone.
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Toxic Sediment Layer: The decomposing algae release potent algal toxins (e.g., microcystins) and hydrogen sulfide, which accumulate in the stagnant bottom layer, forming a poisonous "dead zone."
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Biological Filtration Collapse: Nitrifying bacteria, essential for processing fish waste, become largely inactive in the cold, oxygen-starved bottom water. This halts the breakdown of ammonia and nitrite, allowing toxins to build up.
Addressing this requires breaking the stratification through increased bottom aeration, partial water changes, or careful circulation to mix the water column, alongside long-term algae control measures.
