Which of the following is a consequence of excessive eutrophication?

Excessive eutrophication is a process that occurs when water bodies receive an overabundance of nutrients, particularly nitrogen and phosphorus. This nutrient overload can lead to rapid algal blooms, which significantly impacts the aquatic environment.

One of the primary consequences of this process is decreased oxygen levels. As algae grow in excess, they eventually die and sink to the bottom of the water body. The decomposition of this organic matter by bacteria consumes a large amount of dissolved oxygen in the water. This results in hypoxic or anoxic conditions, where oxygen levels drop to a point that can be detrimental to aquatic life, particularly fish and invertebrates that rely on adequate oxygen levels for survival.

The other choices do not accurately reflect the outcomes of excessive eutrophication. While increased biodiversity and enhanced fish populations might seem beneficial at first glance, these conditions often lead to a decline in overall ecosystem health and the dominance of fewer, often harmful species. Furthermore, water clarity typically decreases due to the abundance of algae, which can cause murky water conditions and block sunlight penetration, further disrupting the aquatic ecosystem. Thus, the most pertinent consequence of excessive eutrophication is indeed the decrease in oxygen levels.