What characteristic is increased in surface waters with high algae concentrations during the night?

In surface waters with high algae concentrations during the night, the characteristic that is typically increased is the concentration of carbon dioxide (CO2). This is largely due to the process of respiration that occurs during the night when photosynthesis does not happen. Algae, like all aerobic organisms, consume oxygen and release carbon dioxide when they respire. Since photosynthesis, which uses CO2 and produces oxygen, occurs only during the day, the balance shifts at night. This results in the accumulation of CO2 in the water as respiration continues without being offset by photosynthesis.

The other options refer to changes that may occur with varying concentrations of oxygen and carbon dioxide, but the key characteristic marked by an increase at night, specifically in high-algae environments, is the carbon dioxide concentration due to their respiratory activity.