Why did freshwater animals survive K-Pg darkness better than land animals?

Answer: Detritus food base

Detritus food base ✓ — Detritus food base is the surprise. When darkness throttled living plant growth, land food chains that needed fresh primary production were exposed. Streams and lakes could keep running longer on dead organic matter washed in from land, so the surviving food web was less dependent on today's sunlight.

Clearer lake water — Clearer lake water would help photosynthesis, but the survival pattern is not mainly a visibility story. The useful contrast is what the food web eats when new plant growth stalls. Clear water still needs light and living producers; detritus can feed insects, worms, snails and fish after the sky goes dim.

Warmer river mud — Warmer river mud sounds like a refuge, but the K-Pg survival split is not just temperature. Freshwater systems buffered animals because they could use stored dead material. That is why crocodile-like and turtle-like survivors make more sense as members of detritus-linked food webs than as lucky heat seekers.