How do microscopic algae typically affect water appearance?

Microscopic algae can significantly affect the appearance of water by causing scum and turning it green. This phenomenon occurs when algae bloom in overabundant quantities due to factors like excess nutrients (commonly from fertilizers or wastewater) and favorable environmental conditions such as warm temperatures and sunlight. As these algae multiply, they can create a thick layer on the surface of the water, leading to a green or murky appearance. This not only alters the aesthetic quality of the water but can also have detrimental effects on aquatic ecosystems by reducing light penetration and oxygen levels in the water.

In contrast, options suggesting that microscopic algae make water clearer or have no visible effect do not align with the common observations associated with algal blooms. Sediment buildup, while a potential consequence of some environmental conditions, is not directly caused by the presence of microscopic algae, making the scum and green coloration the most accurate description of their effect on water appearance.