The pH in a facultative lagoon is highest in the late afternoon , when sunlight drives algae photosynthesis and they remove carbon dioxide from the water, which raises pH.

Why that happens

During the day, algae photosynthesize and consume CO2; with less dissolved CO2, the water becomes less acidic and the pH rises. As the sun goes down and photosynthesis stops, algae respire and release CO2, so pH drops overnight and is usually lowest just before dawn.

Practical takeaway

If you are checking lagoon pH for operations or troubleshooting, the biggest daily swing is usually between pre-dawn low pH and late-afternoon high pH. That’s why time of sampling matters so much in facultative lagoons.

In short: highest pH = late afternoon.