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

Abstract

Photoinhibition was examined in natural assemblages of phytoplankton from Lake Erie exposed to ambient solar radiation. The impacts on photosynthesis of photosynthetically active radiation (400-700 nm) (PAR), ultraviolet-A radiation (320-400 nm) (UVA), and ultraviolet-B radiation (295-320 nm) (UVB) were assessed at three sites on the lake using pulse amplitude modulated chlorophyll fluorescence. Short exposures (<= 30 min) to sunlight containing UVB (1.8-4.4 mmol·m-2) resulted in the rapid loss of up to 60% of photosystem II efficiency (in the dark-adapted state) (Fv/Fm) and quantum yield (in the light-adapted state) (ΔF/F'm). Exposure to UVA (46-105 mmol·m-2) generally diminished Fv/Fm and, to a lesser extent, ΔF/F'm. Short exposures to PAR (733-1588 mmol·m-2) had no significant effects on electron transport. Recovery from UVA- or UVB-induced photoinhibition was complete for Fv/Fm and 90% complete for ΔF/F'm after 2 h in low light. The results indicate that exposures of phytoplankton to surface radiation need only be short in duration to cause substantial UV inhibition of photosynthesis. However, depending on the kinetics of mixing of the water column, recovery of photosynthesis is possible if there is sufficient time for repair of UV damage. Future elevated levels of solar UVB due to ozone depletion could significantly inhibit primary production in mesotrophic lakes such as Lake Erie.