Final answer:
An oligotrophic lake is characterized by low biological productivity due to limited nutrients such as nitrogen and phosphorus, resulting in minimal phytoplankton growth and low overall biological activity. These lakes are not limited to mountainous regions and are defined by their clear water and low nutrient content.
Step-by-step explanation:
The statement that is true for an oligotrophic lake is that the productivity of such a lake is low. Oligotrophic lakes have low levels of nutrients like nitrogen and phosphorus, which are essential for the growth of phytoplankton. Since these nutrients are in limited supply, phytoplankton growth is minimal, leading to lower overall biological productivity.
Water in an oligotrophic lake is characterized by being clear, having a high dissolved oxygen level, and it is generally found in regions with low nutrient runoff. Contrary to one of the mentioned statements, oligotrophic lakes are not restricted to mountainous regions but can be found in various geographical locations, provided the conditions for low nutrient levels are met. The other incorrect statement is that the water in an oligotrophic lake is nutrient-rich; in reality, it is the opposite, as these lakes are nutrient-poor.