This zone has the greatest concentration of plankton in a freash water lake ecosystems

 According to an article/websites, the link stated below, plankton dominates the "limnetic" zone of freshwater ecosystems, which make sense since this is the topmost layer. This is the zone that receives the most sunlight, thus the plants can photosynthesize better than those at the lower levels. Plankton will rarely be found in the lowest zone as there is little to no sunlight there. The lower in the lake/freshwater ecosystem you go, the less plankton you'll find because of their lack of ability to photosynthesize. You can refer to the website below and other books/websites like it for more in-depth information on plankton and their role in freshwater ecosystems. Hope this helps.