One reason for pond depth when thinking of 'northern' waters is how thick the ice may get during the winter.

Generally, there will be a thermocline where the water below is not used much by fish, and the water above is used by the fish.


Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."