Deb - pH should not be normally used to determine the amount of adjustment needed for carbonate additions (alkalinity /hardness) because pH can fluxuate quite a bit during a 24 hour period, depending on several water quality parameters or influencing factors. For one, the amount of pH fluxuation will depend on alkalinity of the water (buffering) and the amount or density of plant life present, usu phytoplankton. This is especally true in waters with low alkalinity and abundant phytoplankton. Noticable swings of pH can occur in these waters. During daylight (esp more so w/ sunshine) plants absorb CO2 from the water, Hydrogen ions are bound or lost and water pH goes up (losing hydrogen ions). Obviously the density of the phytoplankton at time of measurement can have a big affect with this. It is important to note here that the water turbidity due to non-phytoplankton items is an important factor also in this AND a secchi disk reading may not be measuring total phytoplankton density but it could be easily indicating other suspended solids in the water column. At night or on cloudy days plants can have a net release of CO2 (produce more than they absorb) and this forms carbonic acid; in this reaction H ions are released and pH goes down.
Another cause for pH swings is decompostional processes i.e. denitrification of nitrate to molecular nitrogen and reducion of sulfate to sulfide, both result in a net loss of hydrogen ions and pH goes up. This may be an important role in ponds with heavy feeding and manure production where lots of decompostion needs to occur.
In eutrophic conditions more CO2 is produced due to all the extra biological activity and thus pH can have more rapid changes compared to a pond that is not as nutrient rich or organically loaded.
Tendency and causes for pH to fluxuate can be caused by numerous conditions and using pH allows more room for errors or improper conclusions. Alkalinity is a more stable parameter and it typically fluxuates very little over a 24 hr period, when considering varying weather, time of day or "pond" conditions, thus alkalinity is typically used as the standard to determine amount of carbonate or hydroxide materials needed to adjust the water hardness / alkalinity /buffering capacity.
