To solve this problem, we need to identify which statement is NOT correct among the given options. Let's analyze each option:$\newline \bullet$Option 1: The micro-organisms involved in biodegradation of organic matter in a sewage polluted water body consume a lot of oxygen causing the death of aquatic organisms.$\newline$ - This statement is correct. Micro-organisms decompose organic matter, consuming oxygen and potentially leading to oxygen depletion, which can harm aquatic life.$\newline \bullet$Option 2: Algal blooms caused by excess of organic matter in water improve water quality and promote fisheries.$\newline$ - This statement is NOT correct. Algal blooms can lead to oxygen depletion and release toxins, which degrade water quality and harm aquatic organisms, including fish.$\newline \bullet$Option 3: Water hyacinth grows abundantly in eutrophic water bodies and leads to an imbalance in the ecosystem dynamics of the water body.$\newline$ - This statement is correct. Water hyacinth can overgrow in nutrient-rich waters, disrupting ecosystems by blocking sunlight and depleting oxygen.$\newline \bullet$Option 4: The amount of some toxic substances of industrial wastewater increases in the organisms at successive trophic levels.$\newline$ - This statement is correct. This process is known as biomagnification, where toxic substances become more concentrated in organisms at higher trophic levels.$\newline$Therefore, the statement that is NOT correct is Option 2.$\newline$The correct answer is Option 2.