Sure! They're tied to the shallow marine ecosystem. Most marine productivity is within the photoic zone, as well. Marine ecosystems are typically not tightly ti

Sure! They're tied to the shallow marine ecosystem. Most marine productivity is within the photoic zone, as well. Marine ecosystems are typically not tightly tied to land life, though. (Sediment input, etc, is a different story, but while it's biologically mediated, it's not controlled by biologic factors.) However, even a fraction of a meter of water will form a very effective barrier to gamma radiation (One meter reduces it to ~2e-17 times it's original intensity). Even if it were to kill off everything in the upper few tens of centimeters of water, it wouldn't decimate marine productivity. Most of the productivity in the oceans is tied to the ecosystem in the photoic zone, but not to the upper few centimeters of the water column. Edit: I just realized they're mostly referring to the effect of increased UVB radiation _after_ the gamma ray burst removes the ozone layer, rather than the effect of the gamma ray burst itself. Because UV radiation is much longer wavelength, it would have an effect on the upper few meters of the water column, rather than the upper few centimeters, which could be significant. I'm still unconvinced that it's a "good" explanation for mass extinctions, given that we have several others (e.g. long-term sea level fluctuations), but it makes more sense than I thought at first.