एक झील में अचानक असंख्य मृत मछलियाँ तैरती हुई मिलीं। इसमें कोई विषाक्त पदार्थ नहीं था, परंतु बहुतायत में पाद्प्लवक पाए गए। मछलियों के मरने का कारण बताइए।

The amount of dissolved oxygen present in water is limited. The abundance of phytoplanktons causes depletion of this dissolved oxygen. This is because, phytoplanktons are degraded by bacteria present in water. For their decomposition, they require a large amount of oxygen. Hence, they consume the oxygen dissolved in water. As a result, the $BOD $ level of water drops below $6$ $ppm$, inhibiting the growth of fish and causing excessive fish-kill.