  Two atmospheric responses to simulated astrophysical ionizing radiation
events significant to life on Earth are production of odd-nitrogen species,
especially NO2, and subsequent depletion of stratospheric ozone. Ozone
depletion increases incident short-wavelength ultraviolet radiation (UVB,
280-315 nm) and longer ( > 600 nm) wavelengths of photosynthetically available
radiation (PAR, 400 -700 nm). On the other hand, the NO2 haze decreases
atmospheric transmission in the long-wavelength UVA (315-400 nm) and short
wavelength PAR. Here we use the results of previous simulations of incident
spectral irradiance following an ionizing radiation event to predict changes in
Terran productivity focusing on photosynthesis of marine phytoplankton. The
prediction is based on a spectral model of photosynthetic response developed
for the dominant genera in central regions of the ocean (Synechococcus and
Prochlorococcus), and remote-sensing based observations of spectral water
transparency, temperature, wind speed and mixed layer depth. Predicted
productivity declined after a simulated ionizing event, but the effect
integrated over the water column was small. For integrations taking into
account the full depth range of PAR transmission (down to 0.1% of utilizable
PAR), the decrease was at most 2-3% (depending on strain), with larger effects
(5-7%) for integrations just to the depth of the surface mixed layer. The
deeper integrations were most affected by the decreased utilizable PAR at depth
due to the NO2 haze, whereas shallower integrations were most affected by the
increased surface UV.
