In aquatic ecosystems, what can be a result of excessive algae growth?

Excessive algae growth in aquatic ecosystems can lead to decreased dissolved oxygen levels, which is a critical factor for the survival of aquatic organisms. When algae proliferate excessively, especially in a process known as eutrophication, the decomposition of these algae consumes large amounts of oxygen from the water, especially when they die and decompose. This can create hypoxic or anoxic conditions, which can be detrimental to fish and other aquatic life that rely on dissolved oxygen for respiration. Additionally, these low oxygen levels can lead to the death of organisms, disrupt food webs, and reduce overall biodiversity in the ecosystem.

The other options do not accurately reflect the consequences of excessive algae growth. Increased sunlight penetration is typically hindered due to algae forming a dense layer on the water's surface. Increased biodiversity is unlikely in conditions where oxygen is depleted, as many species may perish. Lastly, excessive algae can lead to warmer water temperatures due to reduced cooling effects of water evaporation in surface layers, rather than lower temperatures. Thus, decreased dissolved oxygen levels accurately describe a significant consequence of excessive algae growth in aquatic ecosystems.