If ice was denser than liquid water, the organisms would die because the ice would sink to the bottom on the container, allowing the water at the top to freeze.
Basically, the lake or pond would freeze solid, killing its occupants. When the lake freezes it insulates the rest of the water beneath it, not allowing it to freeze.
Fun fact: water is one of the only substances whose solid state is denser than its liquid state! Pretty cool huh? Funny how huge ecosystems exist only because of the universes strange quirks! I hope this helps!
-AP Chem student
