  On planets near M dwarfs, photosynthesis (PS) will occur with an
effectiveness which depends on the supply of visible photons with wavelengths
between 400 and 700 nm. In this paper, we quantify the effectiveness of PS in
two contexts which are relevant for M dwarfs. First, using photons from an M
dwarf in its quiescent non-flaring state, we find that PS on an M dwarf planet
in the HZ of its parent star is less effective than on Earth by a factor of 10
for a flare star with mid-M spectral type. For a flare star with late-M
spectral type, PS effectiveness is smaller than on Earth by a factor of 100 or
more. Second, using photons which are incident on the HZ planet during flares,
we find that PS effectiveness can increase by factors of 5-20 above the
quiescent values. In the case of a flare star with mid-M spectral type, we find
that the PS effectiveness during a flare can increase up to as much as 50-60
percent of the values on Earth. However, for a late-M flare star, even during
flares, the PS effectiveness remains almost one order of magnitude smaller than
on Earth. We suggest that for biological processes on M dwarf planets, the
stellar activity cycle may replace the orbital period as the year.
