A lake can keep blooming long after its catchment inputs are cut, because the sediment releases the phosphorus it banked over decades. The Nürnberg approach quantifies that internal load from the sediment release rate and the anoxic factor — the analysis that decides whether oxygenation or phosphorus inactivation is the right lever.
Whether internal or external loading dominates changes the whole restoration. If the sediment is the source, catchment controls alone will fail and the levers become oxygenation or phosphorus inactivation. Quantifying the internal load is therefore the step that decides the strategy.
Explore Our ProcessThe internal load is how fast the sediment releases phosphorus, multiplied by how long conditions allow it
Under reducing conditions, iron-bound phosphorus is released as the iron(III) holding it is reduced to soluble iron(II). The areal release rate (mg P/m²·d), measured on intact cores or inferred from hypolimnetic accumulation, sets the intensity of the internal source.
Nürnberg’s anoxic factor (AF) is the number of days per year that an area equal to the lake surface is anoxic at the sediment–water interface. It converts a release rate into an annual load by capturing how long, and over how much area, release is switched on.
Release is a redox switch: hold the interface oxidised and iron retains the phosphorus; let it go anoxic and the bond breaks. This is precisely why hypolimnetic oxygenation can suppress internal loading — and why destratification or inactivation are the alternatives.
The gross internal load is estimated as Lint ≈ RR × AF × A0, where RR is the areal sediment release rate (mg P/m²·d), AF the anoxic factor (days/yr) and A0 the lake surface area. The anoxic factor itself, AF = Σ(ti · ai)/A0, sums the days each sediment area ai spends anoxic, normalised to the whole-lake area — so a deep lake with a small but persistently anoxic hypolimnion and a shallow lake with brief, widespread anoxia can be compared on one scale. Placed alongside the external load from the Vollenweider budget, this gives the internal-to-external ratio that decides the lever: a high ratio says the sediment dominates — oxygenation to hold the redox barrier, or aluminium/lanthanum inactivation to bind the phosphorus permanently — while a low ratio sends the effort back to the catchment. The anoxic factor also links the diagnosis directly to mixing: anything that shortens or shrinks the anoxic period cuts AF, and with it the internal load.
The quantified internal load points to a specific intervention
We weigh the Nürnberg internal load against the external budget — the ratio that determines whether in-lake or catchment measures will work.
A sediment-dominated lake points to hypolimnetic oxygenation or phosphorus inactivation; an externally-loaded one points back to source control.
Where mixing is the chosen route, we set the reduction in anoxic factor needed to bring the internal load below the threshold.
Reynolds & Bauhm quantifies the sediment release rate and anoxic factor, weighs the internal load against the external budget, and selects the lever — oxygenation, inactivation or catchment control — that actually fits.
Our expertise spans multiple industries with sector-specific water treatment solutions.