What might cause rhythmic pH and dissolved Oxygen level increases durng the day and decreases during the night in a lake?

Natural waterbodies are full of plant and algal life, sometimes you can see it and sometimes you can't but it is always there. During the day, the plant and algal life photosynthesize in response to the availability of light. In the process they consume CO2 and release O2, just like terrestrial plants do. The release of O2 into the water column increases the DO concentration (of course) and the consumption of CO2 reduces the potential for the formation of carbonic acid, thus resulting in a more alkaline (higher) pH.

During the night, when there is no UV light available to support photosynthesis, the plants and algae still have to respire (they are alive) which results in the consumption of O2 and release of CO2 into the water column, having the opposite effect to photosynthesis and resulting in the cycling of pH and DO on a day-night (diurnal) basis.