Warmer water holds less dissolved oxygen (Henry's law: gas solubility falls as temperature rises), so the oxygen-saturation ceiling drops as the temperature slider increases. Algae photosynthesise by day, adding O₂ and pulling CO₂ out of the water; when nutrient load is high they overgrow, then die off, and bacteria decomposing that dead biomass consume dissolved oxygen faster than reaeration can replace it — the classic eutrophication chain from fertiliser runoff to a fish-kill "dead zone".
CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3- ⇌ 2H+ + CO3(2-)
DOsat(T) ≈ 14.6 − 0.41·T + 0.008·T² (mg/L)
algae' = r·algae·(1−algae/K) − decay·algae
DO' = reaeration·(DOsat−DO) + photosynthesis − decomposition·deadBiomass
- Temperature — sets the oxygen-saturation ceiling and speeds up both algal growth and bacterial decomposition.
- Nutrient load — phosphate/nitrate runoff sets the carrying capacity for algae; past a threshold the bloom outruns its own oxygen supply.
- pH / carbonate buffer — the bicarbonate–carbonate system resists pH swings from CO₂ uptake or release; harder water (more Ca²⁺/Mg²⁺, higher alkalinity) buffers more strongly, so pH drifts less for the same CO₂ change.
- Hardness — dissolved Ca²⁺/Mg²⁺ (as mg/L CaCO₃) shown here is fixed for this water body and sets how strongly the buffer damps pH swings.