Which freshwater ecosystem is least productive apex?

The "bog freshwater" ecosystem is least productive.

Explanation:

The bog freshwater ecosystem is least productive because of the stability or we can say stagnant type. This type of ecosystem comprise of the evergreen shrubs, and trees and also a floor which will be covered with a thick carpet of the sphagnum moss. The animals like carnivore species will be found. The animal population will be very less. It have, red deer, dragonflies and birds such as grouse and plover will be there. The depth of this type of freshwater is about 6-10 feet it can extend up to 16 feet.  

your body is trying to warm itself up. or more precisely, sensors in your skin send messages to your brain to generate heat to your body.