Salla Rajala

The effects of water colour, dissolved organic carbon (DOC), iron (Fe) and fetch on the mixing depth ( Z mix ), euphotic depth ( Z eu ) and phytoplankton biomass of lakes were studied to clarify the factors behind changes in lake productivity along a DOC gradient. The study was conducted by field sampling 102 lakes in Finland. The lakes were situated mainly in forested areas. At DOC concentrations below 15 mg/L, light attenuation was explained both by Fe and DOC, but at higher DOC concentrations the effect of Fe faded. For the absorption of different wavelengths of photosynthetic light, both DOC and Fe had a significant effect up to 475 nm, but for longer wavelengths only the effect of DOC was significant. Both Z mix and Z eu decreased logarithmically with increasing DOC and water colour. Water colour was determined by DOC and Fe. Z mix increased along with increasing fetch. Z mix exceeded Z eu when DOC concentration was >14.4 mg/L and water colour >106 mg Pt/L (platinum as chloroplatinate ion 3). Above the 14.4 mg/L DOC threshold, the illuminated part of the mixed layer decreased logarithmically with increasing water colour, leaving an increasing part of the mixed epilimnion below the photosynthetic layer. Chlorophyll‐ a concentration showed a unimodal relationship with DOC, with the highest biomass occurring at DOC concentrations of 15.7–15.9 mg/L. The results suggested that above DOC concentration 14 mg/L and water colour 106 mg Pt/L, both quality and quantity of light in the epilimnion restrict production by phytoplankton. This can explain the observed threshold‐like changes in lake productivity along a DOC gradient.