Nutrients set the ceiling on algal biomass, but light and the nutrient ratio decide whether a lake reaches it. Euphotic depth from Secchi transparency and the nitrogen-to-phosphorus (Redfield) ratio identify the controlling factor — so a restoration targets the lever that is actually limiting, not the one that is merely abundant.
Reducing phosphorus achieves nothing if nitrogen or light is what limits growth. Our assessment identifies the controlling factor from the nutrient ratio and the light climate, so the restoration acts on the constraint that is actually binding the system.
Explore Our ProcessThree readings separate a nutrient problem from a light problem
The euphotic zone — where enough light reaches for net photosynthesis — extends to roughly Zeu ≈ 4.6/kd, with the attenuation coefficient kd inferred from Secchi depth. A shallow euphotic zone means light, not nutrients, is capping production.
If the mixed layer is much deeper than the euphotic zone, cells spend most of their time in darkness and net growth collapses — the critical-depth principle. This is the basis on which induced mixing can light-limit a bloom.
The nitrogen-to-phosphorus ratio against the Redfield benchmark identifies the limiting nutrient: a high ratio indicates phosphorus limitation, a low ratio nitrogen limitation — and low ratios favour nitrogen-fixing cyanobacteria that bypass a nitrogen shortfall.
Light attenuates exponentially, I(z) = I0e−kdz, and the euphotic depth is conventionally taken where 1% of surface light remains: Zeu ≈ 4.6/kd. The attenuation coefficient follows from transparency as kd ≈ 1.7/SD (Secchi depth in metres), so a single field reading fixes the light climate. Comparing Zeu with the mixed-layer depth Zmix gives the critical-depth test: when Zmix/Zeu is large, the average cell is light-limited and mixing can hold a bloom down. Nutrient limitation is read from the Redfield ratio — the 106 C : 16 N : 1 P molar stoichiometry of healthy phytoplankton, equivalently about 7:1 N:P by mass. A TN:TP well above Redfield means phosphorus runs out first (phosphorus-limited); well below, nitrogen runs out first (nitrogen-limited), and persistent low ratios select for nitrogen-fixing cyanobacteria. Putting the two together tells us which lever bites: if the lake is light-limited, mixing or turbidity control matters more than nutrient cuts; if phosphorus-limited, the phosphorus budget leads; if nitrogen-limited with bloom-formers present, cutting nitrogen alone may simply hand the lake to fixers — an outcome the assessment is designed to avoid.
It decides which lever a restoration should pull
Euphotic depth and the mixing ratio establish whether light is capping production and whether induced mixing can suppress blooms.
The TN:TP ratio against Redfield identifies the controlling nutrient — and flags the risk of selecting for nitrogen-fixers.
The diagnosis aligns the restoration with the binding constraint, so effort is not spent reducing a nutrient that was never limiting.
Reynolds & Bauhm reads the light climate and nutrient ratio, identifies the binding constraint, and aligns the restoration with the factor that actually controls growth.
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