What is a vegetarian substitute for Bonito Flakes in this recipe?

Thanks for all of the replies! I'll definitely fiddle with the kombu a bit to get some more ocean-y flavor (and add some more shiitake soaking water). Thanks again!

When I run out of Bonito for dashing, I double the kombu. It works well.

Probably just add more kombu and water from shitake mushroom http://www.vrg.org/journal/vj2000jan/2000janmiso.htm ?

Some people substitute konbu, but that's already in the recipe. I think I'd just leave them out.