The Carbonate/Bicarbonate buffering system in the ocean, although high and seemingly infinite, can still be overcome in small shallow bays and/or areas that have heavy algal growths and poor circulation (borate plays a role as well, but that is another story). As plants/macroalgae photosynthesize, they take up the available CO2 in the water column, reducing (via the buffer system) available H+ (Protons) and driving pH up (more alkaline, usually to a top limit of around 8.5 or 8.6 at the absolute most). However, when the photoperiod ends, plants, symbiotic corals,
coralline algae and misc. macroalgae that normally use CO2 as a carbon source for photosynthesis now produce a net positive amount of CO2, and as the CO2 level increases, carbonic acid is immediately produced (CO2 + H20 <=> carbonic acid <=> bicarbonate + H+ ) which leads to bicarbonate plus a proton, which drives pH down. The presence of bicarbonate acts to resist this downward pH pressure, and does so by taking the carbonate ion part of the buffer system and changing it to bicarbonate, using up the excess proton in the process (it is much more complicated than this, but this is the easy form for now) The problem is that the rate of the buffering reaction is much slower than the carbonic acid forming part of the reaction, so that any increase in dissolved CO2 results in a very rapid corresponding decrease in pH, with a gradual correction back to the 8.2 to 8.3 range if no additional CO2 is produced. The upshot of this is that while creatures are producing CO2 at night without any photosynthesis to take it up, then pH will gradually drop to around 8.0-8.1 by early morning, and once the photoperiod starts and photosynthesis goes into high gear, the pH will gradually rise to its maximum for the photoperiod, usually determined by the alkalinity (buffer capacity) of the tank as well as the number of photosynthesizers available.
Taking the pH of a system with a pH test kit is taking a picture of a bouncing ball: from the photo, you can't really tell how high the ball will bounce, has bounced, or whether it is going up or down. The best determination of pH is with a meter and a time frame, ie pH = 8.2 @hour 1 of the photoperiod, pH = 8.5 @hour 12 of the photoperiod and a corresponding value for alkalinity in its time frame as well. In this manner, you can see trends as they develop, and see what effects your day to day operations have on the pH as well.
