ABSTRACTUltraviolet‐B radiation (UVBR: 290‐320 nm) inhibited ammonium uptake (pNH4) and nitrate uptake (pNO3) in natural plankton assemblages collected during a transect from 37° N to 55° N in the Pacific Ocean. Comparison of responses in pNH4to ambient solar‐ and lamp‐enhanced UVBR spectra allowed calculation of an action spectrum for pNH4inhibition. The slope of the action spectrum for Pnh4is half as steep as action spectra for UVBR inhibition of photosynthetic carbon uptake. Consequently, WBR‐induced photoinhibition of pNH4extends to greater depths than inhibition of carbon fixation due to the greater relative effect of longer UVBR wavelengths. Inhibition of pNH4was dependent upon UVBR dose when doses were weighted by the pNH4action spectrum. Dependence of WBR inhibition of pNH4on dose rate was not apparent. We found that near‐surface pNH4and pNO3can be overestimated in excess of 50% when measured using standard incubation vessels made of UVBR‐absorbing materials such a