ULTRAVIOLET-B RADIATION EFFECTS ON INORGANIC NITROGEN UPTAKE BY NATURAL ASSEMBLAGES OF OCEANIC PLANKTON

Ultraviolet-B radiation (UVBR: 290-320 nm) inhibited ammonium uptake (rho(NH4)) and nitrate uptake (rho(NO3)) in natural plankton assemblages collected during a transect from 37 degrees N to 55 degrees N in the Pacific Ocean. Comparison of responses in rho(NH4) to ambient solar- and lamp-enhanced UVBR spectra allowed calculation of an action spectrum for rho(NH4) inhibition. The slope of the action spectrum for rho(NH4) is half as steep as action spectra for UVBR inhibition of photosynthetic carbon uptake. Consequently, UVBR-induced photoinhibition of rho(NH4) extends to greater depths than inhibition of carbon fixation due to the greater relative effect of longer UVBR wavelengths. Inhibition of rho(NH4) was dependent upon UVBR dose when doses were weighted by the rho(NH4) action spectrum. Dependence of UVBR inhibition of rho(NH4) on dose rate was not apparent. We found that near-surface rho(NH4) and rho(NO3) can be overestimated in excess of 50% when measured using standard incubation vessels made of UVBR-absorbing materials such as polycarbonate.