Warm surface water is less dense and, once wind mixing is weak, sits on top of colder deep water without mixing — the classic summer-lake thermocline. The warm, wind-mixed epilimnion keeps exchanging gas with the atmosphere, but the cold hypolimnion below the thermocline is cut off except for slow diffusion across the boundary. Dead algae mostly sink into the hypolimnion, where bacteria decomposing it consume oxygen that nothing replaces — this is why deep, strongly stratified lakes can go hypoxic at the bottom even while the surface looks healthy.
DOsat(T) ≈ 14.6 − 0.41·T + 0.008·T² (mg/L)
thermoclineDepth ∝ windMixing (more wind → deeper mixed layer)
exchange = k(mix)·(DO_epi − DO_hypo) (diffusion across the thermocline)
DO_epi' = reaeration + photosynthesis − respiration − decomp_epi − exchange
DO_hypo' = exchange·w − decomp_hypo (no reaeration, no photosynthesis reach it)
pH' ∝ −CO2 flux / bufferStrength(hardness)
- Surface temperature — sets the oxygen-saturation ceiling and how much the deep water lags behind it in temperature (bigger gap ⇒ stronger stratification).
- Nutrient load — sets algae's carrying capacity; blooms die off and sink, fuelling hypolimnetic decomposition.
- Wind mixing — deepens the mixed layer and speeds diffusion across the thermocline; low wind is what lets a bottom dead zone form.
- Water hardness — dissolved Ca²⁺/Mg²⁺ (as mg/L CaCO₃) sets the carbonate buffer strength: harder water resists pH swings from the same CO₂ flux far more than soft water.