Differential responses to UVR by bacterioplankton and phytoplankton from the surface and the base of the mixed layer

Abstract

SUMMARY 1. We tested the influence of ultraviolet radiation (UVR) and shallow stratification on phytoplankton and bacterioplankton from the surface and the base of the mixed layer in two boreal lakes in north‐western Ontario, Canada.2. We measured phytoplankton biomass and production, bacterioplankton production and plankton respiration after transplantation under three solar radiation treatments: ambient radiation (Photosynthetically active radiation (PAR) + ultraviolet‐A (UVA) + ultraviolet‐B (UVB)), minus UVB (PAR + UVA) and PAR only. We repeated this experiment on three occasions in each lake during the summer.3. Solar stress (measured as reduced growth and photoinhibition) was generally only found in the ‘base phytoplankton’ (i.e. originating from the base of the mixed layer). No inhibition of photosynthesis by UVB exposure was found in near‐surface phytoplankton. On the other hand, production of near‐surface bacterioplankton was reduced following a 4‐h UVR exposure but had increased after a 48‐h exposure to both UVA and UVB compared with the PAR only treatment.4. Negative effects of UVR on phytoplankton and bacterioplankton were not ubiquitous. We emphasise the importance of conducting experiments repeatedly, particularly those which test the effects of UVR on different community assemblages from different lakes.