Final answer:
Freshwater protozoans are affected by temperature fluctuations, light exposure, oxygen concentration, and changes in pH, with temperature and oxygen concentration being critical for their survival and metabolic processes.
Step-by-step explanation:
Freshwater protozoans primarily react to changes in their environment, including variables like temperature fluctuation, light exposure, oxygen concentration, and changes in pH levels. These factors can significantly affect the survival and metabolic processes of freshwater protozoans. Alterations in pH can affect metabolic reactions, temperature changes can affect enzyme activity, the amount of dissolved oxygen relates to their respiratory needs, and light levels can influence the photosynthetic organisms they may depend upon. For instance, Paramecium species maintain their water balance through the use of a contractile vacuole, expelling excess water from their cells in response to osmotic pressure. Additionally, changes in light can affect the population of phytoplankton, which serve as a food source for various aquatic organisms and contribute to the aquatic food web.