UV effects on the primary productivity of picophytoplankton: biological weighting functions and exposure response curves of Synechococcus
A model that predicts UV effects on marine primary productivity using a biological weighting function (BWF) coupled to the photosynthesis-irradiance response (BWF/P-E model) has been implemented for two strains of the picoplanktonic cyanobacteria Synechococcus, WH7803 and WH8102, which were grown at two irradiances (77 and 174 µmol m −2 s −1 photosynthetically available radiation (PAR)) and two temperatures (20 and 26 • C). The model was fit using photosynthesis measured in a polychromatic
... polychromatic incubator with 12 long-pass filter configurations with 50 % wavelength cutoffs ranging from 291 to 408 nm, giving an effective wavelength range of 280-400 nm. Examination of photosynthetic response vs. weighted exposure revealed that repair rate progressively increases at low exposure but reaches a maximum rate above a threshold exposure ("E max "). Adding E max as a parameter to the BWF/P-E model provided a significantly better fit to Synechococcus data than the existing "E" or "T " models. Sensitivity to UV inhibition varied with growth conditions for both strains, but this was mediated mainly by variations in E max for WH8102 while both the BWF and E max changed for WH7803. Higher growth temperature was associated with a considerable reduction in sensitivity, consistent with an important role of repair in regulating sensitivity to UV. Based on nominal water column conditions (noon, solstice, 23 • latitude, "blue" water), the BWF E max /P-E model estimates that UV + PAR exposure inhibits Synechococcus photosynthesis from 78 to 91 % at 1 m, and integrated productivity to 150 m 17-29 % relative to predicted rates in the absence of inhibition. Published by Copernicus Publications on behalf of the European Geosciences Union.