Carbonate Cycle

The carbonate cycle describes the reversible equilibrium among dissolved carbon dioxide, carbonic acid, bicarbonate (HCO3-), and carbonate (CO3 2-) that governs pH stability in natural and culture waters. Its buffering capacity, quantified as total alkalinity, resists pH change when acids are added by nitrification, respiration, or rainfall. Daytime photosynthesis strips CO2, shifting the equilibrium toward carbonate and pushing pH up; nighttime respiration does the reverse, and the size of the resulting diurnal pH swing depends directly on how much alkalinity is present. In intensive systems each kilogram of ammonia nitrified consumes roughly 7 kg of alkalinity (as CaCO3), so biofilters steadily deplete the buffer unless it is replaced with sodium bicarbonate or lime. Recommended alkalinity for stable culture water is 75-200 mg/L as CaCO3, with shrimp ponds often held at 120-150 mg/L. Understanding the carbonate cycle lets farmers interpret pH readings correctly, predict morning lows, dose buffers before crashes, and protect nitrifying bacteria that depend on bicarbonate as their carbon source. It links directly to alkalinity management and diurnal pH behavior.