Colder Water, Lower pH

As autumn deepens, falling water temperature exerts a consistent, though often overlooked, influence on pH. The net effect in most temperate lakes, rivers, and reservoirs is a decline in pH, driven by two interacting sets of processes: one thermodynamic, the other biological.

The thermodynamic pathway is direct. Carbon dioxide is more soluble in cold water than in warm water. As temperature falls, the water column can hold more dissolved CO₂ at equilibrium with the atmosphere, shifting the carbonate system toward higher CO₂ and lower pH. 

This is why cold bottom waters frequently exhibit lower pH than warmer surface waters even before biological activity is considered. In coastal and marine systems, the same principle applies: low temperature enhances CO₂ solubility, and autumn respiration further increases dissolved inorganic carbon and CO₂ while depressing pH.

The biological pathway reinforces this trend. In late autumn, phytoplankton photosynthesis weakens as solar radiation declines and day length shortens, reducing the daytime CO₂ drawdown that ordinarily raises pH. Respiration, by contrast, continues and becomes the dominant metabolic process in many water bodies, releasing CO₂ and pushing pH downward. In stratified lakes, the breakdown of summer layering can also mix CO₂-rich bottom water back into the upper water column, producing a broader pH depression across depths.

It is important to distinguish this seasonal baseline shift from the diel pH cycle. Photosynthesis and respiration still generate day–night pH swings, but the amplitude of these swings typically narrows in late autumn as biological reaction rates slow and the photoperiod contracts. 

The overall trajectory, however, is toward lower pH, because the thermodynamic solubility effect and the respiratory CO₂ accumulation both act in the same direction. Thus, a modest but detectable pH decline is a normal chemical signature of cooling water—an expected seasonal adjustment rather than an automatic indication of contamination or instrument drift.