At some point with lower pH it is harder to keep alkalinity at a certain level since the pH will tend to drift up. This is dependent on physical factors such as aeration, but with those factors constant, a lower pH outgasses carbon dioxide at a much faster rate. The following link shows a chart with this info.

CO2.png

The horizontal black "limit" line I drew at 15 is somewhat arbitrary but I have heard from others and have my own personal experience that trying to hold a pH and alkalinity combination above this line seems to be more difficult.

So, a lower pH may require lower alkalinity to prevent an unacceptable rate of upward pH drift, but the lower alkalinity will also make the pH swing more rapidly to changes in pool acid/base equilibrium. A bit of a catch-22 that is mostly avoided at a pH of 7.4 (7.3 or higher).

Richard