What is one of the major problems caused by filamentous algae?

Filamentous algae, often found in freshwater environments, can significantly obstruct sunlight from penetrating the water column. This blockage of sunlight hinders the ability of submerged aquatic plants to perform photosynthesis, which is crucial for their growth and survival. Without adequate sunlight, these plants can weaken, die off, and ultimately disrupt the entire aquatic ecosystem. The competition for light can lead to reductions in biodiversity, impacting both the plants and the animals that depend on them.

In contrast, the other options describe phenomena that are generally beneficial for aquatic ecosystems. While filamentous algae can provide some habitat for certain aquatic organisms, the overall impact of excessive growth is more negative than positive. Improving water clarity and increasing oxygen levels are desirable outcomes typically associated with healthy aquatic plant growth, rather than the type of conditions that filamentous algae create.