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.  

measuring light intensity

light meters can be used to measure light intensity. the meter is held at the soil surface and pointed in the direction of the maximum light intensity, and then the meter is read.

errors can be made when measuring light intensity by accidentally shading the light meter. the reliability of the results can be checked by taking many samples.