What are tiny organisms that float near the surface of freshwater and marine environments?

These tiny creatures are referred to as "plankton."

Plankton includes plants and animals that float along at the mercy of the sea's tides and currents. Their name comes from the Greek meaning "drifter" or "wanderer."

the answer is lactic acid from anaerobic respiration. the acid in lactic acid is what creates sore and burning muscles. if you think about how acid burns that will give you a clue for questions like this.