What effect do filamentous algae blooms have on aquatic ecosystems?

Filamentous algae blooms can significantly impact aquatic ecosystems, primarily by decreasing oxygen levels. When these algae grow excessively, they can cover the water's surface and block sunlight from penetrating to submerged plants and other aquatic organisms. This reduction in sunlight limits photosynthesis, leading to decreased oxygen production in the water.

As the filamentous algae eventually die off, their decomposition by bacteria consumes even more dissolved oxygen in the water column. This process can create hypoxic or anoxic conditions, which are detrimental to fish and other aquatic life that rely on sufficient oxygen to survive. Low oxygen levels can lead to fish kills and a decline in overall biodiversity within the ecosystem, disrupting food webs and ecological balance.

In contrast, the other options do not accurately reflect the typical consequences of filamentous algae blooms. They do not enhance fish spawning, improve water clarity, or attract beneficial insects; instead, they create challenging conditions for the aquatic ecosystem.