After an asteroid impact darkens the whole planet, why might tropical plankton die off more than polar plankton?

Answer: Tropical species are built for bright, warm water, while polar plankton already cope with months of dim light

Tropical species are built for bright, warm water, while polar plankton already cope with months of dim light ✓ — Correct! Plankton that evolved in bright, warm tropical water depend on steady sunlight, so a sky full of impact soot cuts their food supply at the root. Polar communities already live through long dim seasons and stirred-up water, so a planet-wide blackout is closer to a condition they are built for. Ecosystem models of the dinosaur-killing impact reproduce this: darkness and body size explain most of who died, and high-latitude communities come through with smaller losses.

Polar water starts out colder, so blocking the sun changes it less — Wrong. Temperature does change after an impact, but the cold itself is not what sorts survivors by latitude; what matters is how much a community depends on light. A plankton community used to warm, bright water loses its energy source when the sky darkens, whatever the water temperature does.

Tropical seafloors are muddier, and the mud smothers the plankton — Wrong. Mud on the seafloor does not reach open-ocean plankton drifting near the surface; sediments preserve the fossils that record the extinction, they do not cause it. The latitude pattern comes from how light-dependent each community was when the sunlight vanished.