UV effects on the primary productivity of picophytoplankton: biological weighting functions and exposure response curves of Synechococcus

Abstract. 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 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_Emax/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.