Effects of UVB radiation on freshwater autotrophic and heterotrophic picoplankton in a subalpine lake

The effects of UVB radiation on the activity of heterotrophic (HPP) and autotrophic (APP) picoplankton (0.2-2 mu m) and of autotrophic assemblages >2 mu m has been measured and compared. Under natural UVB irradiance in a large, deep, oligotrophic subalpine lake (Lago Maggiore, 45 degrees 55'N) with mean dissolved organic carbon (DOC) concentrations of 1 mg C l(-1), the microorganisms of the two size fractions were not significantly photoinhibited in their autotrophic and heterotrophic activities. The vertical attenuation coefficient (K-d) for irradiance at 305 nm ranged from 1.45 to 1.67 during spring and summer. The mixing layer extended to a greater depth than the layer affected by UVB radiation (z(1%) < z(mix)), thus enabling the microorganisms living there to photoadapt. As the assimilation numbers of APP and nanoplankton were higher at 2 m depth than near the surface, we suspected that the influence of longer wavelength (UVA, photosynthetically active radiation) could be stronger than UVB in affecting the photosynthetic efficiency of natural populations. The artificial increase in UVB irradiance had a higher detrimental effect on HPP due to their smaller size, less protection and indirect effects through autotrophic cell inhibition. Picocyanobacteria were percentually more affected by UVB than nanoplankton during April due to the presence of diatoms, which are more resistant than other algal groups to solar UVB irradiance. Furthermore, picocyanobacteria had lower assimilation numbers with respect to larger phytoplankton in the quartz tubes during stratification.