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The present study compares the Ultraviolet A (UVA: 320 to 400 nm) enhancement of in situ carbon fixation relative to Ultraviolet B (UVB: 280 to 320 nm) inhibition for a community domi- nated by the diatom genus Pseudo-nitzschia with other mixed phytoplankton communities in the Santa Barbara Channel in June 1998. At the time, a widespread bloom of toxic P. australis and toxin- related mortality among marine mammals occurred between San Francisco and the Southern Califor- nia Bight. We sought out the location of the Pseudo-nitzschia bloom during a cruise designed to quan- tify the variability in ultraviolet (UV) effects on primary productivity in diverse phytoplankton communities. Replicate phytoplankton samples were incubated in situ at a depth of 5 m to determine the sensitivity of carbon uptake and pigmentation in 7 distinct spectral light treatments, in which shorter wavelengths of UV radiation are progressively removed. Carbon fixation for the Pseudo- nitzschia-dominated community was enhanced relative to photosynthetically available irradiance (PAR, 400 to 700 nm) in all PAR + UVA treatments and enhancement appeared maximal at 15% (p 318 nm). In other mixed phytoplankton com- munities, UVA enhancement of carbon fixation was only observed for the long-wavelength treat- ments (340 to 400 nm), whereas inclusion of wavelengths between 320 and 340 nm had an inhibitory effect on carbon fixation. UVB photoinhibition relative to maximum production rates was comparable for all 3 communities. Based on our in situ observations, we conclude that Pseudo-nitzschia sp, although susceptible to UVB inhibition, is well adapted to utilizing the entire UVA portion of the spectrum to enhance carbon fixation. This might also enable algal species of the genus Pseudo- nitzschia to occupy shallower portions of the water column, where the full spectrum of UVA and PAR penetrates and inhibits photosynthesis of other mixed algal assemblages.

marine ecology. progress series

UVA enhancement of carbon fixation and resilience to UV inhibition in the genus Pseudo-nitzschia may provide a competitive advantage in high UV surface waters