A group of researchers want to determine the average concentration of mercury in zooplankton in all the lakes in a region. They randomly selected 10 lakes from all lakes that were within 500 m of a road in that region, and measured the concentration of mercury from 5 g of zooplankton collected from the top 2 meters of water from those lakes.

Based on this description, explain what the target population, the sampled population, and the sample are in this example. Note at least one reason that the sampled and target populations might systematically differ in the target variable (mercury concentration).