Which of the following is a consequence of excessive eutrophication?

Excessive eutrophication refers to the over-enrichment of water bodies with nutrients, particularly nitrogen and phosphorus. This can lead to algal blooms, which are rapid increases in the population of algae. When these algae eventually die off, their decomposition consumes a significant amount of dissolved oxygen from the water, resulting in decreased oxygen levels. Low oxygen conditions can create hypoxic environments, which are detrimental to aquatic life, as many fish and other organisms require adequate oxygen to survive. This is a significant consequence of eutrophication that can lead to dead zones where aquatic life cannot thrive, negatively impacting ecosystems and biodiversity.

The other options suggest positive changes (like increased biodiversity, improved water clarity, or enhanced fish populations) that typically do not occur as direct results of excessive eutrophication. Instead, the fallout from it typically results in a decline of healthy aquatic ecosystems.