At a spot in the ocean , the surface irradiance is 7.8*10^16 photons cm^-2 s^-1, and k=0.14 m^-1. At what depth does the euphotic zone end? (Find definition for euphotic zone and solve for z)

Answer 1

Same idea as the last question but e^-(.14 z)
and Io = 7.8*10^16
and you have to look up the irradiance for light to die out enough.
Once you do that, take the ln of both sides to solve it.

Answer 2

From wikipedia:

"It extends from the atmosphere-water interface downwards to a depth where light intensity falls to 1 percent of that at the surface (also called euphotic depth), "
so where I = .01 Io

or
I/Io = .01 = e^-.14z
ln .01 = -.14 z
-4.6 = -.14 z
solve for z