ULTRAVIOLET-B RADIATION EFFECTS ON INORGANIC NITROGEN UPTAKE BY NATURAL ASSEMBLAGES OF OCEANIC PLANKTON1

Ultraviolet-B radiation (UVBR: 290-320 nm) inhibited ammonium uptake (p_NH4) and nitrate uptake (p_NO3) in natural plankton assemblages collected during a transect from 37° N to 55° N in the Pacific Ocean. Comparison of responses in p_NH4 to ambient solar- and lamp-enhanced UVBR spectra allowed calculation of an action spectrum for p_NH4 inhibition. The slope of the action spectrum for Pnh₄ is half as steep as action spectra for UVBR inhibition of photosynthetic carbon uptake. Consequently, WBR-induced photoinhibition of p_NH4 extends to greater depths than inhibition of carbon fixation due to the greater relative effect of longer UVBR wavelengths. Inhibition of p_NH4 was dependent upon UVBR dose when doses were weighted by the p_NH4 action spectrum. Dependence of WBR inhibition of p_NH4 on dose rate was not apparent. We found that near-surface p_NH4 and p_NO3 can be overestimated in excess of 50% when measured using standard incubation vessels made of UVBR-absorbing materials such as polycarbonate.