I can see how heterotrophs would not be present in 50 cm, as that would probably be a part of the littoral zone of the lake, and the heterotrophs don’t need that much sunlight and oxygen to survive. As for the 250 cm area, which would most likely be the profundal zone, there might have been a lack of discovery and identification for heterotrophs at that depth. Also, that area might not have had enough dissolved oxygen for the heterotrophs to thrive or it probably did not have enough organisms available for consumption. The compensation depth does not really apply to the heterotrophs as much as it does the autotrophs because the amount of sunlight and dissolved oxygen is not as important to them as it is to the survival of the autotrophs.
Generally speaking, weather could have played a large impact on the presence of autotrophs and heterotrophs. If the lake reached certain temperatures, either hot or cold, some of the organisms could have died because the temperature of the water did not meet their survival needs. Anything from the amount of sunlight to precipitation could have altered the temperature of the