Phytoplankton absorb mainly red light in lakes with high chromophoric dissolved organic matter

Abstract The impact of chromophoric dissolved organic matter (CDOM) on the spectral underwater light field within photosynthetically active radiation (PAR), and the related responses of spectral phytoplankton light absorption are poorly documented in the mixed layer of lakes. We examined how CDOM influences the spectrum of lake optical properties, underwater light field, photons absorbed by phytoplankton, photoacclimation, and chromatic adaptation of phytoplankton in 127 boreal lakes. In lakes with increasing CDOM content, light intensity decreased steeply with depth, but mixed layer depth and mean light intensity of the mixed layer decreased only moderately. Increasing CDOM content red‐shifted the central wavelengths of lake optical properties, underwater light fields, and the photons absorbed by phytoplankton. In study lakes with increasing CDOM content, the highest light availability shifted stepwise from 580 to 650 nm and close to 700 nm. The ratio of chlorophyll a concentration to phytoplankton biomass decreased slightly in lakes with higher CDOM content. The absorption coefficient of phytoplankton at blue relative to red wavelengths decreased with increasing CDOM content, indicating decreased blue‐absorbing pigmentation in response to decreased availability of blue light. Surprisingly, the number of red photons (600–700 nm) absorbed by phytoplankton increased with CDOM content below the mid‐point of the euphotic zone. Over the entire water column, red light (600–700 nm) accounted for > 50% of PAR absorption by phytoplankton in 69% of the lakes. The high contribution of red photons to the absorption of PAR by phytoplankton may have photobiological consequences, which are poorly understood and require further study.