Final answer:
The bloom of microscopic organisms in the lake is likely caused by eutrophication, which occurs due to an oversupply of nutrients that leads to oxygen depletion and can harm aquatic life.
Step-by-step explanation:
The process that may have primarily caused the bloom of microscopic organisms in the lake is eutrophication. This biological process occurs when excess nutrients, particularly phosphorus and nitrogen from sources such as fertilizer runoff and sewage, enter a water system.
The abundance of nutrients stimulates the excessive growth of microorganisms, including phytoplankton, which can lead to a depletion of dissolved oxygen in the water. This can cause harm to aquatic life, often resulting in die-offs of fish and other organisms, also known as 'dead zones'.