Photoinhibition of natural phytoplankton assemblages in Lake Erie exposed to solar ultraviolet radiation

Photoinhibition was examined in natural assemblages of phytoplankton from L ake Erie exposed to ambient solar radiation. The impacts on photosynthesis of photosynthetically active radiation (400-700 nm) (PAR), ultraviolet-A ra diation (320-400 nm) (UVA), and ultraviolet-B radiation (295-320 nm) (UVB) were assessed at three sites on the lake using pulse amplitude modulated ch lorophyll fluorescence. Short exposures (less than or equal to 30 min) to s unlight containing UVB (1.8-4.4 mmol.m(-2)) resulted in the rapid loss of u p to 60% of photosystem II efficiency (in the dark-adapted state) (Fv/Fm) a nd quantum yield (in the light-adapted state) (Delta F/F-m'). Exposure to U VA (46-105 mmol.m(-2)) generally diminished Fv/Fm and, to a lesser extent, Delta F-m'. Short exposures to PAR (733-1588 mmol.m(-2)) had no significant effects on electron transport. Recovery from UVA- or UVB-induced photoinhi bition was complete for Fv/Fm and 90% complete for Delta F/F-m' after 2 h i n low light. The results indicate that exposures of phytoplankton to surfac e radiation need only be short in duration to cause substantial UV inhibiti on of photosynthesis. However, depending on the kinetics of mixing of the w ater column, recovery of photosynthesis is possible if there is sufficient time for repair of UV damage. Future elevated levels of solar UVB due to oz one depletion could significantly inhibit primary production in mesotrophic lakes such as Lake Erie.