The spectral sensitivity of phytoplankton communities to ultraviolet radiation‐induced photoinhibition differs among clear and humic temperate lakes

Abstract

We determined the spectral sensitivity of phytoplankton communities to photoinhibition in six temperate lakes spanning a transparency gradient due to variation in dissolved organic carbon (DOC) content. Changes in variable fluorescence (FV : FM) were monitored during experimental irradiance exposures and used to estimate spectral weighting functions for damage by ultraviolet radiation (UVR) and recovery rates. DOC explained a high proportion of the variation in UVR sensitivity, with clear‐water phytoplankton communities showing greater resistance to UVR‐induced photoinhibition than those of browner waters. These differences were greater when assessed in September, after clear‐sky conditions, than in July, after several days of overcast skies, especially in the long‐wavelength UVA spectral region. Surprisingly, the most UVR‐sensitive phytoplankton communities were dominated by filamentous cyanobacteria, a putatively UVR‐resistant taxon, whereas small unicellular eukaryotes were common in the most UVR‐tolerant assemblages. Model estimates of in situ photoinhibition after 2 h exposure to an incident solar spectrum were only slightly higher in the clear lakes than browner ones, despite appreciably higher UVR exposure in the former. Phytoplankton in clear lakes can maintain values of FV : FM comparable to communities protected from UVR by high concentrations of DOC.